Carpentry - BC Trades Modules Line J1 — 1 Learning Task 1 Notes Describe the Forces Acting on the Building Structure All buildings are subject to loads created by gravity, weather,

Carpentry

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Contents

Learning Task 1: Describe the Forces Acting on the Building Structure . . . . . . . . . . . . . . . . . . . . . . . . . 1

Learning Task 2: Describe Methods of Controlling Forces Acting on the Building Structure . . . . . . . . 12

Learning Task 3: Describe Forces Acting on the Building Envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Learning Task 4: Describe Methods of Controlling the Forces Acting on the Building Envelope Below Grade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Learning Task 5: Describe Wood Frame Seismic Applications and Related Hardware . . . . . . . . . . . . . 41

Carpentry LineJ1—1

NotesLearning Task 1Describe the Forces Acting on the Building Structure

All buildings are subject to loads created by gravity, weather, and in extreme cases, earthquakes. The forces that these loads create cause stresses that a building’s frame and envelope must be designed and built to withstand.

Building loads are divided into two main categories: live loads and dead loads.

In this Learning Task, you will gain an understanding of each of these categories of loads and the stresses they generate on buildings. You will then be shown how these loads are transferred through the building systems, and finally the results of these loads on the building components themselves.

Live LoadsLive loads are variable, non-permanent loads that act on a building structure. Live loads may be described as static, repetitive, or dynamic.

StaticLoadA static load is an external force that is applied and held in a fixed position for a specific amount of time. An example of a static load caused by gravity would be the weight of a piece of furniture. This is classified as a live load because furniture can be easily moved or replaced.

repetitiveLoadA repetitive load is one that repeats itself. A repetitive load may be caused by gravity or the momentum of an object. For example, cars crossing a bridge would place temporary repetitive gravitational loads on the bridge.

DynamicLoadA dynamic load is a load that can change over time. An example of a dynamic load would be the repetitive action of cars braking and accelerating on a bridge.

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People andfurniture

Rain

Wind

Snow

Live LoadsFigure 1.

Environmental Live Loads

WindWind can create both static and dynamic live loads. Strong gusts of wind associated with hurricanes or tornados would place extreme dynamic loads on a building, whereas a steady breeze would place a static live load on the building.

Wind loads can also create forces that the building envelope must deal with. Wind loads create pressure differentials across a building that can draw heated air out of a building and rain water into it. Building envelopes must be designed and built to prevent this from happening.

Wind sway in a tall building

Lateralmovement

Wind flow past a building

Uplift

Wind

Pres

sure

Suction

Effects of Wind Live LoadsFigure 2.


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rain and Snow Rain and snow create static live loads. These gravitational loads are transferred vertically through a building’s structure.

Rain on its own creates very little load on a building unless it is allowed to accumulate on a flat roof where the roof drains are not functioning. Rain has a much greater effect, though, when it falls on a snow-covered roof. The snow absorbs the rain, which increases the weight and load, on the roof. The BC Building Code (BCBC) recognizes this effect by including a region’s rain load in the equation to calculate its snow load. This information helps determine the minimum size roof joists or rafters that are required for a building.

Calculating Specified Snow LoadThe specified snow load formula S = Cb Ss +Sr, provided in B.9.4.2.2 of the BCBC, will help you determine which span tables to use when sizing your roof joists or rafters.

To calculate the specified snow load, first find your town in Table C-2 of Div B, Appendix C of the BCBC. Find the Ss and Sr values for your area. The Cb value depends on the size of the roof. Cb will be either 0.45 or 0.55. This means that the specified snow load will be 45%–55% of the ground snow load. The roof snow load will not be as great as the ground snow load because of the slope of the roof and that snow is blown off the roof.

The formula takes into account that the larger the tributary area of the roof the larger the effect the snow load will have on the roof’s support structure. Roofs up to 4.3 metres in width use 45% of the ground snow load in the calculation while roofs over 4.3 metres uses 55% of the ground snow load.

The impact of rain on the overall weight of the snow is taken into account as Sr in the calculation. This is an important factor as some regions are much drier than others and as a result the snow is much lighter. For example Osoyoos, which is Canada’s only recognized desert region, has an Sr of 0.1. Youbou, which is in the west coast rainforest region, has an Sr of 0.7. Regardless of the specified snow load, no roof shall be designed to support less than 1 kPa.

OccupantLiveLoadsOccupant live loads include furnishings, equipment, and people. Occupancies are usually designed to support specific loads. For example, the BCBC requires that residential occupancies be designed to support 1.9 kPa, which is about 40 pounds per square foot, while dance floors are designed to support 4.8 kPa, or about 100 pounds per square foot. Occupant loads are considered to be dynamic because they change as people move about.

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earthquake LoadsEarthquakes exert a dynamic load on buildings that can be catastrophic. The direction of earthquake forces (seismic loading) is usually horizontal but vertical earthquake movements are not uncommon. The shaking of structures can cause huge loads throughout the building frame and cause serious damage.

The Lower Mainland and Vancouver Island are in a very active seismic zone. Measures must be taken to ensure that buildings in these regions are designed and constructed to resist earthquake loads.

6-Storey Shake TestFigure 3.


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Dead LoadsDead loads are caused by gravity acting on all of the building materials. In a typical wood-framed house this would include the wood framing members, concrete foundation, windows and doors, interior and exterior finish materials like drywall and siding, and all permanently attached fixtures such as kitchen cabinets. A dead load is constant although it may shift, move, or change directions should a structure settle or be affected by an earthquake.

Dead LoadsFigure 4.

gravityGravity is so constant that it is often forgotten. It is the force of gravity that gives objects their weight.

The force of gravity acts towards the centre of the Earth. A hand level containing tubular level vials, and a plumb bob, use gravity to create plumb lines.

A tubular level vial is a curved glass tube filled with alcohol that has a bubble of air inside. The air bubble will move to the highest point of the curve. If the vial is tipped either left or right, the air bubble will move toward the higher end.

A plumb bob is used to test plumb. Gravity pulls the suspended mass of the bob towards the centre of the Earth.

Plumb lines radiate from the centre of the Earth and are not parallel with each other. For example, a plumb line in Vancouver is not parallel with a plumb line in Toronto.

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Plumb line inVancouver

Plumb line inToronto

Gravity is always directedat the centre of the earth

Gravity acts towards the center of the earthFigure 5.

Calculating Loads

Units of MeasurementThe units for mass, acceleration, force, and pressure are:

Unit Metric Imperial

Mass kilogram (kg) slug (slg) *

Acceleration metres per second per second (m/s2)

feet per second per second (ft./s2)

Acceleration due to gravity

9.8 m/s2 32 ft./sec2

Force Newton (N) pounds (Lbs.)

Pressure Pascals (Pa) pounds per square inch (psi or lbs./in2)

* The pound is not the unit of mass in the imperial system. When comparing kilograms to pounds the correct wording should be “A mass of 1 Kilogram exerts a force of 2.2 Pounds (on Earth)” and “A force of 2.2 lbs. has a mass of 1 kilogram (on Earth) A slug is the unit of mass that is accelerated at the rate of one foot per second per second when acted on by a force of one pound.

prefixesThe Newton and the Pascal are very small units. We generally use kN (kilo Newton) and kPa (kilo Pascal). The prefix k for “kilo” stands for 1000 of the unit.

The symbol M for “mega” stands for a million of the unit and the symbol G stands for a billion of the unit. For example, the BCBC requires a minimum compressive strength of unreinforced concrete for foundation walls to be 15,000,000 Pascals or 15 MPa.


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gravityIt is the force of gravity that gives objects their weight. Acceleration due to gravity is a constant on Earth at 9.8m/s2. This means that any falling object, excluding the effect of air resistance, will increase its speed at a rate of 9.8 metres per second every second. At the end of one second it will be travelling at a speed of 9.8 m/sec., at the end of two seconds 19.6 m/sec., at the end of 3 seconds 29.6 m/sec. etc. This rate of acceleration continues until air resistance prevents the object from accelerating further and the object is said to have reached terminal velocity.

Calculating Loads: An ExampleIn the BC Building Code, loads are typically expressed as pressures in Kilopascals (kPa). They are first calculated as a force in Newtons (N). Newtons represent an overall force, which can be expressed as a pressure once we establish the area over which the force is being applied.

Using these formulae, let’s calculate an example of a load:

MASS = Volume (m3) × Density (kg/m3)FORCE (Newtons) = Mass × Acceleration (Gravity = 9.8 m/sec2)PRESSURE (Pascals) = Force (Newtons) / Area (m2)

example: What is the dead load created by 4 m3 of concrete (density = 2400 kg/m3) on a 600mm × 600mm square pad footing?

Stepa:Calculate the total mass of the concrete 2400 kg/m3 × 4 m3 = 9600 kg

StepB:Calculate the force, in Newtons Force = Mass × Acceleration = 9600 kg × 9.8 m/s2 = 94,080 N = 94.08kN

StepC:Calculate the pressure, in Pascals Pressure = Force / Area (m2)

The area of the footing is 0.6 m × 0.6 m = 0.36 m2

So 94,080 Newtons / .36M2 = 261,333 Pascals = 261.333kPa

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StressesLive or dead loads applied to a building structure result in stresses. Left unchecked, these stresses may deform the building structure. The suitability of a building component is dependent upon its ability to resist these deformations and maintain its structural integrity.

The four most common stresses acting on a building structure are compression, tension, torsion, and shear.

CompressionCompression is the force that is acting to shorten an object. Columns, for example, are under compression from the loads above. Building components designed to resist compression must be installed plumb, as uneven loading will induce other stresses that the component may not have been designed to resist. Point loads are often subjected to extreme compressive stress.

Load

Objectbecomesshorter

Load

Effect of CompressionFigure 6.

TensionTension is the pulling force that acts to lengthen an object. For example, a rope that is used to hoist an object is under tension. The rope stretches and becomes thinner. If too much tension is applied, the rope may fail.


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Another example of tension is the stress occurring at the bottom of a beam or joist spanning between two supports.

Load Load

Object becomes longer

Effects of TensionFigure 7.

TorsionTorsion is the force caused when one or both ends of an object are twisted (torqued) in opposite directions. Extreme winds can result in torsion stresses being applied to a building, as they act against wall faces that are structurally anchored to the building’s foundation. Dramatic torsion is experienced during an earthquake.

Symmetry (balance), in general, reduces torsion. If a building is designed to be symmetrical, it is less likely to suffer ill effects from torsion. A balance between stiff and ductile (elastic) building materials also helps ease the stresses of torsion.

Effects of TorsionFigure 8.

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ShearShear stress occurs when there are pushing or pulling forces acting in opposite directions on a solid component or multiple components that are in contact with one another. The direction of shear can be horizontal, vertical, or diagonal.

Load Load

Shear point

Shear on a BoltFigure 9.

Horizontal shearHorizontal shear occurs when the pushing or pulling forces are acting on a horizontal plane. This is often the result of deflection and vertical loading.

No shear due tocentre support

Horizontal shear as theindividual boards bendand slide across each other

Horizontal ShearFigure 10.

Figure 10 depicts horizontal shear between boards in a stack. A solid piece of lumber the same size would be stronger than the multiple boards.


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VerticalshearWhere vertical shear occurs you have loads, such as those in a beam, acting on a supporting member like a column. The stress on the beam at the edge of the column is vertical shear.

Vertical ShearFigure 11.

Diagonal ShearBuilding components that are under both horizontal and vertical shear stresses are experiencing diagonal shear. An overloaded beam under a diagonal shear will fail along a diagonal line of approximately 45°.

Diagonaltension

Beam

Wall

Diagonal Shear 45°Figure 12.

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Learning Task 2Describe Methods of Controlling Forces Acting on the Building StructureAll buildings must resist the loads and stresses applied to them. The BC Building Code (BCBC) provides standards designed to accomplish this. Builders recognize that these are the minimum requirements and that there are a number of ways that we can increase the structural integrity of our buildings that exceed those specified in the BCBC.

The basic question to understand is: “Why do buildings stand up”? In this learning task, you will learn the concepts of load transfer and how specific properties of the individual building systems support and resist the various loads and stresses imposed upon them.

Load PathsAll loads on a building result from forces acting on the building. These loads are first controlled by the individual building systems they are acting directly upon. These systems then transfer that load to the building system below and eventually to the bottom or foundation of the building. This is referred to as a load path. Load paths must be continuous to the foundation, as it is the backbone of the structure. A building’s ability to resist deformation as a result of loads is directly proportional to the structural integrity of these systems.

Transfer of LoadsBuildings are designed to transfer live and dead loads through their structures to the foundation. Exactly how the loads are transferred depends on the type of building. In all cases the loads will follow a load path.

Cantileveredfloor

Cripple ortrimmer

Lintel

Example of load transferFigure 1.


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An example of the load transfer through a building is shown in Fig. 1. The dead load of the roof truss, sheathing, and shingles, plus the live load of snow and rain, is transferred to the lintel. The lintel then transfers this load to the cripples. The point load at the base of the cripples pushes down on the cantilevered floor joists. The floor joists must extend into the building six times the cantilevered distance to resist the roof load. The floor joists bear on the foundation. The foundation transfers the load to the footings and on to the ground.

The load paths are one of the first checks that a building inspector makes during the framing inspection.

Load Transfer Through Triangles All buildings use triangles to make their frame rigid. Sometimes the triangles are hidden, but they still exist. The wall below demonstrates these hidden triangles.

Wood frame wall sheathed with plywood or OSBFigure 2.

RoofsRoof dead loads, and the live loads of snow/rain, must be transferred downward. Roofs behave like sloped floors. Large tributary areas focus loads to the supporting members below. Roofs must also resist uplift and lateral loading created by the wind.

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Dead LoadsSpecial consideration must be given to buildings that are being re-roofed; old shingles should be stripped off before new shingles are installed. The increase in dead load caused by roofing over old shingles can create serious structural issues particularly on old hand cut roofs that may have 2×4 rafters over very long spans. Roofs that are to be covered with clay or concrete tile will have very large dead loads and will require both the roof structure and supporting structures below to be stronger.

Wind LoadsThe nailing requirements of the BCBC provided in Table 9.23.3.4 require that roof rafters, roof joists and roof trusses be toe nailed to the top plate of the wall with a minimum of 3 – 82 mm (3¼”) nails. The shear strength of the nails will resist lateral loads imposed by wind loads and earthquakes. The angle on the nails created by toe nailing causes a mechanical attachment between the roof member and the top plate increasing the withdrawal strength of the nails. This helps to counteract uplift forces created by the wind.

In some cases where wind loads are known to be very dynamic, special “High wind resistant” fastening hardware can be utilized. Simpson Strong-Tie™ provides a number of products to meet these requirements and they are categorized as seismic and hurricane ties.

H2A Hurricane Tie 1 DETAL Truss Anchor 10A Hurricane Tie

Truss/rafter tie downsFigure 3.


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The hardware used is also effective against the dynamic loads imposed by earthquakes and can work independently as the photographs above demonstrate, or they can be part of an integrated system that extends continuously to the foundation. Simpson refers to this as a continuous load path.

roof TrussesTrusses use triangles to support the live and dead loads imposed upon it. Triangles called panels transfer the loads they support to the outside walls of the building. The panels are created by using special gang-nail connectors that have very high shear strength at the panel points where the web members meet. This makes the whole truss act as one member. Figure 4 shows the tension and compression forces acting on the members of a typical W, or Fink, truss.

Tension webs

Compression web Compression web

Bottom chord - tension

Top chord - compression Top chord - compression

Strut Strut

Panel points

Fink TrussFigure 4.

The tension webs are connected to the panel point at the ridge. They support the bottom chord from sagging. The panel point formed by the tension web and the bottom chord provides support for the bottom of the compression web (strut). The strut holds up the midpoint of the top chord. The bottom chord supports the peak by preventing the top chords from spreading at the base.

Trusses clear span the building from one side to the other. All roof and ceiling loads are concentrated on the outside walls of the building. This allows architects a great deal of freedom when designing the top floor of a building with a truss roof, as there are no internal bearing walls that must be located in a specific position.

As trusses are engineered, the loads they impose on the structure below can exceed those allowed in Part 9 of the BC Building Code. Lintels over large openings in the outside wall may have to be engineered. Girder trusses that support very large tributary areas may be included in the roof design and result in the creation of a very large point load. The point load will have to be transferred all the way through the building frame to the foundation. Columns of built-up studs will have to be framed into the wall under the girder to transfer the point load to the floor. These columns should be framed with solid as opposed to finger-jointed studs. Solid blocking must be included in the floor frame under the columns to transfer the load between

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floors. The grain direction in this blocking should be vertical as blocking installed with its grain horizontal is not as strong. Truss designers may specify that Douglas fir wall top plates or a metal bearing plate be placed under the girder truss to prevent the wall top plate from being crushed by the point load.

Hand Cut roofs Without ridge SupportAs the live and dead loads press down on the roof, the rafters tend to want to push outwards. To resist this outward push and force the loads vertically downward onto the wall, the rafter tails of opposing rafters are tied together with ceiling joists. The integrity of this triangle is paramount to the stability of the roof. The ceiling joist prevents the rafters from spreading at the base in the same way that the bottom chord in a truss prevents the top chords from spreading.

Snowload

Snowload

Snowload

Snowload

Reaction fromthe snow load

Reaction fromthe snow load

Reactions of a snow load on a standard gable roofFigure 5.

The ceiling joist is under tension and acts as a continuous tie between opposing rafters. The amount of tension depends upon the snow load and the slope of the roof. Table 9.23.13.8. in the BCBC provides the minimum nailing requirements at the rafter/ceiling joist connection for a roof that is unsupported at its ridge. The number of nails at this connection is critical because it is the total shear strength of all the nails that transfer the tension load from the rafter to the joist. If the ceiling joists are not continuous across the span of the building, then they will be required to have one more nail at the joist splice than is required for the rafter/ joist connection. This connection must occur over a support such as a bearing wall, beam or lintel. The structural member that carries the ceiling load must be the beginning of a load path that carries the ceiling load through the interior of the building.


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Although snow loads would be the same for the 2 roofs in Fig. 6, regardless of slope, the BC Building Code recognizes that there is less tension in the steeper sloping roof by requiring fewer nails at its rafter/joist connection. The steeper the slope of the roof the closer the rafters get to vertical and the less outward force is generated at the rafter/ joist connection. Higher snow loads and lower slopes require more nails than the lower snow loads and the steeper sloped roofs. At roof slopes of less than 1:3 (4:12) the outward forces at the rafter-joist connection become so great that the number of nails required to carry the load would cause the wood to split and the connection would fail. This is why the code requires roof slopes of less than 1:3 to have ridge support.

30 ft. 30 ft.

Snow loadsFigure 6.

ridge SupportIf a ridge beam is used to support the rafters, the outward push is eliminated and the number of nails used to attach the ceiling joists is not an issue. When a vaulted ceiling, as shown in Figure 6, is being framed, ridge support is always required because there are no ceiling joists to support the roof. The ridge beam and roof joists deal with the loads they carry in exactly the same way as floor beams and floor joists. The point loads created by the columns supporting the ridge beam must be carried though the building to the foundation.

Snowload

Snowload

Snowload

Snowload

Vaulted ceiling with roofjoists supporting both roof

load and ceiling finish.

Ridge beam supports roof joists sothere is no outward push from the rafters

onto the walls.

Vaulted ceiling requires ridge supportFigure 7.

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Ridge beams in Table A-12 require a minimum of 76mm (3”) of end bearing and are sized to support rafters or joists with a maximum span of 4.9m. The 4.9m is half the span of the rafter or joist on either side of the beam or the supported joist length. The BCBC allows an increase in the ridge beam spans based on a decreasing supported rafter/joist length. This is because as the supported joist length decreases so does the tributary area and correspondingly the load on the beam.

Collar TiesCollar ties are 2×4s installed between opposing rafters. As snow accumulates on the roof the collar tie prevents the rafters from deflecting as it is compressed between the snow loads on opposite sides of the roof. The advantage of collar ties is that no intermediate bearing walls are required. All the snow load is transferred to the outside walls. Once the collar ties are 2.4 m or more in length they must be supported at mid-span by a 1×4 ribbon board. This mid-span support reduces the unsupported length of the collar tie and prevents it from deflecting under load.

Ridge

Common rafter

Collar tie

Ceiling joist

Roof rafters with collar tiesFigure 8.

WallsWalls can be classified as bearing walls or curtain walls. Bearing walls transfer the gravitational loads from above to the support structures below. Curtain walls keep the weather out or divide the interior of the house into rooms.

In load-bearing walls vertical loads are transferred to the studs through double top plates. The double top plates act like small beams between the studs and are capable of supporting any joist, rafter or truss that lands in between the studs. Larger openings such as those for windows or doors are bridged by lintels. The lintel tables in division B appendix A allow lintels to be sized based on the number of floors they support and the design snow load for the roof. These tables permit a maximum span for floor joists or rafters of 4.9m or a truss span of 9.6m.


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Loads on WallsWalls are required to support the live and dead loads from above and the lateral loads caused by wind, earthquakes and the movement of people within the building.

These stresses, if extreme enough, will cause a structure to rack. Once the wall begins to rack and the weight of the structure above the wall no longer bears plumb, the weight begins to contribute to the racking force. The further the bearing angle moves away from being plumb the more the weight contributes to the failure of the wall. Figure 9 shows the imminent failure of an unsheathed wall.

Effects of lateral loading on wallsFigure 9.

Lateral LoadingThe triangle is the primary form utilized to minimize and prevent damage as a result of lateral loading or racking. A wall that is being subjected to lateral loads as a result of wind or earthquake is under both compression and tension. Fig. 10 shows the compressive and tensile stresses in a wall that is subjected to lateral loading from an earthquake.

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End stud incompression

Resisting forcefrom connection

Earthquake force

Earthquake force

C

C

T

T

End stud intension

Walls under tension and compression during lateral loadingFigure 10.

Although the BCBC recognizes various interior finishes and exterior panel type cladding as adequate for bracing and lateral support, it is good practice to provide more support.

Walls can be significantly reinforced against racking by applying minimum 19 × 89 (1 × 4) boards at approximately a 45° angle extending continuously from the bottom of the wall to the top of the wall or by installing exterior-grade panel sheathing meeting the requirements of BCBC Table 9.23.16.2 A.

Plywood and OSB should be installed with the face grain perpendicular to the studs. The maximum spacing of the fasteners is 150mm o.c. around the perimeter and 300mm o.c. in the interior of the panels as specified in the BCBC. Nails are preferred to staples as staples are more prone to corrosion due to their thinner construction and their lower withdrawal resistance.

VerticalLoadingThe vertical live and dead loads are supported by bearing walls and columns. The bearing walls assume the loads are uniformly distributed while columns are designed to support the point loads. Columns supporting point loads should be constructed of solid lumber or natural (non-finger-joint) built-up dimensional lumber.


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Truss plans often require that top plates consist of Douglas fir lumber or metal bearing plates to minimize the crushing that can occur at point load locations such as those present under the girders.

SlendernessratioWall sheathing not only resists lateral loads but also helps studs and columns to support vertical loads by reducing their unsupported length. Columns and studs will bow out of plumb if too heavy a load is applied to them. To see this bowing action, take a long thin piece of wood and place one end on the floor and push down on the top. If the wood is thin and long, it will easily bow. Now have someone support the stick at its midpoint while you push down again. It will take significantly more force to bow the stick and the bowing will create an “S” shape in the stick. Try it with two points of support.

Support

Column failureFigure 11.

The slenderness ratio for permanent wood columns should not exceed 1:40 where the 1 is proportional to the minimum unsupported cross sectional dimension and the 40 is the unsupported height of the column.

note: The 1:40 ratio is for wood columns only.

For columns that are used for temporary support, as in support for formwork, the slenderness ratio can be increased to 1:50.

example: Using the slenderness ratio, what is the minimum size of wall stud that could be used to frame a wall that is 8’ tall?

The unsupported height is 96 inches. The minimum cross sectional

dimension would be 96/40 or 2.4”. This means that a 38 × 64 (2 × 3) wall stud could be used, as long as the 1½” thickness is supported continuously by the wall sheathing. For unsheathed studs, the maximum height is 1½” × 40 = 60 inches, or five feet.

Checking Table 9.23.10.1. in the BCBC, you will find that 38 × 64 (2 × 3) wall studs are approved for use in load-bearing walls that are not more than 8 feet tall (2.4 m).

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FloorsFloor loads are assumed to be uniform over the entire surface of the floor. In practice, loads are often concentrated in some areas and spread out in others. If the concentrated loads are significant, they must be allowed for.

example: If a large bronze sculpture is to be positioned in a new residential building, the normal 1.9KPa floor loading will not be adequate to support its load. Decreasing the spacing of the floor joists in the area of the sculpture is the best way to allow for the extra load.

Joist spacingreduced to 8” o.c.

Concentrated loadsFigure 12.

Load Transfer Through Tributary AreaTributary area is the area of floor or roof that is supported by a specific structural component, like a column, beam, bearing wall, or lintel. The tributary area of a structural element contributes to the total load that a structural member must carry or support. The tributary area’s boundary is usually the mid-point between one supporting element and the next. The tributary area of a beam supporting a floor is shown in Figure 13.

Supportedjoist length

Beam clear span

Beam tributary areaFigure 13.


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The beam supports the floor joists halfway from the beam to the wall on each side of the beam. This is called the supported joist length. The other dimension of the tributary area is the clear span of the beam. If the supported joist length was 3.6m and the beam clear span was 3.9 m, then the tributary area for the beam would be:

3.6m × 3.9m = 14.04m2

The BCBC states that the design live load for a residential floor is 1.9 Kpa or 1.9 KN/m2. The total live load that the beam is responsible for would then be:

1.9 KN/m2 × 14.04 m2 = 26.676 KN or about 6,000 lbs.

The BCBC does not talk about dead loads but they are built into the span tables that you will learn to use in H2. The beam would also be responsible for the dead loads of the floor structure and any walls, cabinets, plumbing fixtures etc. that are situated within the tributary area. The dead load on the main floor of a house is approximately the same as the live load so if the beam were fully loaded it would have to support about 53.4 KN or 12,000 lbs. and deflect only 1/360 of its length.

The columns supporting the beam would have a slightly larger tributary area than the beam as the column also supports the area directly above the column while the beam does not.

Supportedjoist length

Beam clear span+ a post width

Column tributary areaFigure 14.

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The column focuses the load from its tributary area into the very small cross-sectional area of the column. If the assumption is made that the column is a code minimum 6×6, 140mm × 140mm and the load it supports is approximately the same as the beam, then the pressure created by the point load at the base of the column can be calculated:

.140 m × .140 m = 0.0196 m2

53.4 KN / 0.0196 m2 = 2,725 KN/m2 or 2,725 Kpa

This converts to about 395 lbs/in2

Floor SheathingSubfloor sheathing supports the loads that exist between floor joists; along with solid blocking or cross bridging, it also helps transfer loads between adjacent joists. To provide sufficient support to the floor system, subfloor sheathing must be installed at 90° to the floor joists and the ends must be staggered. Minimum subfloor thicknesses are relative to the material being used and the on-center spacing of the floor joist themselves. The thicker the floor sheathing the greater the strength and stability to the floor system as a whole.

Floor JoistsTable 4.1.5.3 in the BCBC provides the design limits for uniformly distributed live loads based on the areas intended use. This, along with the maximum deflection allowed, are the criteria used to determine the maximum spans for floor joists in Tables A–1 and A–2 of Division B, Appendix A of the BCBC.

Column

Beam

Joist

Foundationwall

Transfer of loads

Floor joists transferring loadsFigure 15.


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A floor in a single-family dwelling must support a minimum of 1.9kPa. This means that the floor is designed to support 1.9 kN/m2. A room that was 8m2 would be able to support a uniformly distributed load of 15.2 KN. There is a safety factor built into these tables, and it is occasionally tested when a building is loaded with drywall prior to boarding the walls. A large stack of drywall creates a load that is not uniformly distributed but concentrated in a small area of the floor.

Floor joists must support the live and dead loads from above without significant deflection.

Table 9.4.3.1 of the BCBC provides you with the maximum allowable deflections for wood frame structural members. A floor joist, for example, has a maximum allowable deflection of 1/360. This means that for every 360mm in length, the joist is allowed 1mm of deflection. A 2880mm floor joist is allowed a maximum deflection of 8mm.

BeamsBeams support uniformly-distributed loads and concentrate them as point loads. Tables A-8 to A-11 in the BCBC provide the maximum spans for built-up and glue-laminated beams. Beam spans are affected by the number of floors being supported and the species and grade of lumber being used, as well as the supported joist length.

DeflectioninHorizontalMembersWhen spanning members such as joists, beams, rafters, or lintels are loaded, they will sag. The amount of sag is referred to as the deflection. The BC Building Code outlines the maximum allowable deflection of structural building components in table 9.4.3.1.

Deflection

DeflectionFigure 16.

Continuing to add weight to a horizontal member will cause it to continue to deflect until it breaks.

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There are two basic positions for building components: plumb and level. Wall studs and columns are installed plumb and are subjected to vertical compression stresses. Floor joists, beams, girders, or slabs are installed level and are subjected to various shear stresses.

Compression and Tension in Horizontal Spanning Members Compression and tension can exist within the same structural member at the same time. In a beam, the top is under compression while the bottom is under tension. These forces are greatest closest to the edges of the beam.

Load

Compression

Compression

Tension

Tension

Neutralaxis

Forces acting

Neutral

min

min

max

max

Beam in compression and tensionFigure 17.

The BCBC takes these factors into consideration when setting the limitations for the notching and drilling of framing members. For example holes cannot be drilled within 50 mm of the edges of rafters, ceiling joist or floor joists as this is the area with the highest compression and tension stresses. Likewise a notch must be kept one half the joist depth from the edge of bearing in order to keep the notch out of the area of the joist with the highest shear stress.

CantileverBeams used in single spans deflect more than beams that are continuous over two or more spans. The three beams shown in Figure 18 are all single spans. They are joined on the posts. In Figures 18, 19, and 20 the span, the beam size and the load are all the same.


Notes

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Single span beamsFigure 18.

Using the principle of the cantilever will reduce the deflection and increase the stiffness of a beam. As a cantilevered beam deflects between its supports, the cantilevered portion of the beam rises. This up-lift can be used to support the load of adjacent spans.

Cantilevered spanFigure 19.

Continuous beam over three spansFigure 20.

The beam in Figure 20 is continuous over all three spans; it deflects the least of the three examples. The decrease in deflection is due to the cantilever.

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The Quarter-Point Beam, as explained in H-line, utilizes the cantilever principle to provide a strong beam with low deflection.

No cantilever hereNo joint allowed

Compression and tension cancel each other,creating a neutral quater point zone suitable

for a joint. No cantilever hereNo joint allowed

1/4 point 1/4 point 1/4 point 1/4 point

Compression

Tension

¼ point beamFigure 21.

FoundationRegardless of whether the foundation is cast-in-place concrete or an alternative foundation such as permanent wood, the loads and stresses are the same for all. The foundation is subjected to vertical loading and horizontal loading.

There are a number of factors that affect the foundation’s ability to resist these loads such as:

thickness of the foundation walls•

materials used to construct the foundation•

length of unsupported wall•

height of finished grade above finished floor•

amount of soil lateral pressure against the wall•

size and number of openings in the wall•

lateral support at the top and bottom of the wall•


Notes

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Lateral LoadingA foundation wall must resist the pressure of the backfill; the higher the fill the greater the pressure.

Lateral support

Soil pressure(lateral pressure)

Lateral support

Foundation wallFigure 22.

Part 9 of the BCBC recognizes concrete foundation walls as either laterally supported or unsupported at the top. Walls that are laterally unsupported at the top are limited to 1.5m in unsupported height while a laterally supported wall may extend 2.3m from support to support. This provides adequate clearance for an 8-foot high basement.

Until a floor system is installed, a foundation can be subjected to dynamic loading as backfill is being placed. If the wall is required to be laterally supported at the top, then the sill plates, floor joists and subfloor sheathing must be installed before backfilling. The plates, joists and subfloor act together as a diaphragm to support the foundation walls.

Notes


VerticalLoadingThe live and dead loads are transferred continuously through the structure to the foundation walls and footings where they are distributed to the ground. The soil bearing capacity must not be exceeded if BCBC Part 9.15 foundation requirements are to be used.

Foundation walls of 150mm–300mm (6”–12”) are deemed sufficient to support all vertical loads as defined by Part 9 of the BCBC. Loads that exceed these parameters require an engineer to analyze and design support that meets the expanded requirements of Part 4 of the BCBC.


Notes

LearningtaSk3

Learning Task 3Describe Forces Acting on the Building Envelope

The Building EnvelopeAs a mailing envelope surrounds and protects a letter, the building envelope surrounds and protects the building and prevents rain, wind, groundwater, and soil gasses from entering the building.

The building envelope includes all the components that make up the shell or skin of the building. Elements such as polyethylene vapour barrier and building paper, although not technically a part of the exterior skin of the building, are included when considering building envelope design.

The building envelope is designed to perform four basic functions above and below grade. It adds structural support to the building, controls moisture and humidity, regulates temperature, and controls air pressure changes. This learning task will introduce you to the forces and the effect those forces have on the building envelope above and below grade.

People andfurniture

Rain

Wind

Snow

Live loadsFigure 1.

Below GradeForces that affect the building envelope below grade include hydrostatic forces, frost jacking/adfreezing, and soil gas pressure. These forces need to be considered and accounted for when constructing the building envelope.

Notes


LearningtaSk3

HydrostaticForcesHydrostatic forces may have to be considered whenever we build below grade. Hydrostatic pressure exists when the depth of the excavation is below the water table. The hydrostatic pressure will force water into the excavation. The excavation will fill with water until it is at the same level as the surrounding water table. (Figure 2)

0.75

1.0

Water tableelevation

Excavating below the water tableFigure 2.

The elevation of the water table changes with the seasons. Generally it is highest in the spring and lowest in the fall. During the digging of an excavation, if water is coming out of the surrounding ground, hydrostatic pressure exists. Small streams of water will seep out of the ground and form puddles at the bottom. This ground water can cause the sides of the excavation to slump or cave in and makes working in the excavation difficult and dangerous. It will have to be dealt with immediately. If the excavation is dry, there may still be hydrostatic pressure at another time of the year when the water table is higher.

Damage may result to the inside of a building if the foundation is in direct contact with water.

FrostJacking/adfreezingAnother problem that can occur below grade is frost jacking/adfreezing. Frost jacking/adfreezing may result in uplift or exert lateral pressure on foundation walls. Frost causes the ground to expand. The pressure is exerted in the direction of the heat flow, from cold to warm. This pressure is usually upwards but it can also be lateral (e.g., in the case of an unheated basement or crawlspace).

Soil gas pressureSoil gas enters the building through cracks in the foundation walls and basement floor slabs. As soil gas enters the building it brings with it water vapour and pollutants with it. The primary pollutants are radon and methane. Radon is found in varying amounts throughout Canada. Radon is a colourless, odourless, radioactive gas. It occurs naturally as a result of the decay of radium. Exposure to radon can lead to an increased risk of lung cancer.


Notes

LearningtaSk3

The potential for high levels of radon infiltration is very difficult to evaluate prior to construction. Often the presence of radon is only determined after the building has been constructed. For this reason, it is best to take measures to prevent the entry of soil gas in all cases.

Above GradeThe portion of the building envelope above grade is also subject to damaging forces. Particularly damaging are elements like sun, wind, and rain. Temperature and air pressure differentials are also significant factors.

Sun: The rays of energy from the Sun provide the energy for life on Earth. They also create stresses and loads on buildings that can be very damaging.

UVrays: The ultra-violet rays from the Sun will destroy unprotected wood surfaces in a short time. The colour of plastics and metal finishes fades and oxidizes from continual exposure to the Sun.

thermalgain: The infrared rays from the Sun carry significant heat energy. The heating and cooling systems in a building must be designed to control heat gain from the Sun. Incorrect installation of insulated glass panels can cause them to fail within one season.

expansion/Contraction: Heat gain causes building materials to expand, and cooling causes them to shrink. This continual movement puts significant stress on components of the building’s envelope.

WindWind can cause a number of issues for a structure, including uplift and pressure differentials.

StormConditions: Wind speeds at the building site must be considered in the design of buildings. In British Columbia hurricane-force winds are not uncommon. Wind speeds in excess of 100 km/h are an annual occurrence in many locations.

The force of the wind is proportional to the square of its speed. As the wind speed doubles the force from the wind increases four times. If it triples the force increases by nine times.

Storm force winds can tear roofing and siding materials off buildings. Flat roofs are particularly susceptible to up-lift as a strong wind blows over the top and creates a Venturi effect.

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LearningtaSk3

prevailingWinds: Storm winds can cause catastrophic damage, but a steady prevailing wind can also harm a building.

Prevailing winds create a positive (high) pressure on the windward side and a negative (low) pressure on the leeward side. This pressure differential creates drafts and heat loss if the building envelope is not well sealed.

Wind pressureFigure 3. Pr

essu

re

Suction

Wind flow past a buildingFigure 4.

Wind-DrivenrainandSnow: If there is wind as well as rain or snow, the negative pressure on the leeward side or in walls and attics will tend to draw the rainwater or snow into the building, as the pressures tend to equalize.

Fine dry snow will build up in attics if there are openings that will allow it in. Rainwater running down the exterior finish will be drawn into the interior of the wall by the pressure difference caused by the wind.


NotesTemperature and Pressure DifferentialThere is a natural tendency for differences in temperature, moisture level, and air pressure to even out, or equalize, over time. For example, if ice is added to a bowl of warm water, the ice will melt and the water will cool down. Eventually the water from the melted ice and the water in the bowl will become the same temperature.

Figure 5 shows a typical example of the temperature and moisture differences found in a building located in the interior of BC.

The greater the difference, the greater the tendency is to even out. The amount of difference is referred to as the “differential”, and the tendency to equalize is the “pressure”.

Heat and moisture tryingto get out causes a pressure

Warm kitchentemp. 22˚Chumidity 70%

Winter coldtemp. -15˚Chumidity 6%

Thermal and moisture pressuresFigure 5.

High pressures will occur if there is a large differential in temperature, moisture level, or air pressure.

A low pressure area can act as a vacuum drawing air from a higher pressure area. Condensation created by the warmer air mixing with the colder air can lead to problems, such as moisture building up in the insulation wall cavities or attic spaces.

Notes


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Learning Task 4Describe Methods of Controlling the Forces Acting on the Building Envelope Below Grade

The various components of the foundation, wall, and roof systems serve multiple roles in the successful performance of a building in controlling the forces acting upon it.

The building envelope must be completely sealed below grade to prevent moisture, water and soil gasses from entering the building. The above grade walls must protect the building from the entry of rainwater while still allowing the wall system to breathe. A weatherproof cap to the building is provided by the roofing system.

In this learning task you will learn how the BC Building Code, various building jurisdictions, and good building practices regulate methods of controlling the forces acting on buildings below grade.

Methods of controlling forces acting on buildings above grade is dealt with in learning guides J-2, “Control Heat and Sound Transmission” and J-3, “Control Air and Moisture Movement in Buildings”.

Below GradeThe forces acting on a building below grade are just as critical as those above grade, and due to the inaccessibility resulting from backfill problems, they can be much more expensive to rectify.

Test HolesDigging test holes is common in commercial construction; the cores provide an opportunity to evaluate the sub-soil conditions. This process is uncommon in residential construction, though, so talk to the neighbors and the local building authority during the planning stages of a project. These sources can provide local knowledge and may even provide information that a test hole may not reveal. The local building authority can provide specific information and requirements that will save time and money in the long run.


Notes

LeArnIng TASk 4

Watertableelevation

Organicmaterial

Sandyloam

Hardpan

Sandygravel

Test holeFigure 1.

HydrostaticForcesHydrostatic forces occur when there is a high water table present immediately adjacent to a buildings foundation. The builders’ task is to prevent the water from entering the structure.

There are several ways hydrostatic forces can be controlled to protect the building envelope below grade. One is proper waterproofing and another is proper drainage.

WaterproofingThe BCBC requires that foundation walls be waterproofed when hydrostatic pressures are present. There are many suitable products on the market to accomplish this. Figure 2 shows two examples. The one on the left is a self-adhering membrane consisting of rubberized asphalt compound; the one on the right is a sheet of dimple-patterned plastic.

Self-adheringmembrane

Foundation

Dimpled membranewith smooth molding

Permanentbarrier

Drainage space

Bridgeswall cracks

Drainstone

Concrete or ICFfoundation

Internalmoisture

Waterproofing membranes below gradeFigure 2.

Notes


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Any sub-grade waterproofing system will require that all penetrations be carefully sealed to ensure there are no gaps. In extreme cases, water stop may also be installed between the footing, wall, and slab.

Water stopFigure 3.

DrainageDrainage consists of a multitude of components.

The RoofThe roof is a huge catchment area. For example, a 1500ft2 roof subjected to 1” of rainfall will shed about 1000 gallons of water (3800 litres). This water must be directed as far away from the foundation as possible. A closed pipe system that directs the water to a public storm drain or to a drainage area on the low side of the building is desired. In most cases, the rainwater system should not be connected to the perimeter drainage (ground water) system until it is downhill from the building. Sometimes a back flow prevention valve is installed to prevent the rainwater from backing up into the perimeter drains.

The GroundThe ground immediately adjacent to the building must slope away from the building for a minimum of 600mm (2’). The greater the distance you can slope the ground away, the better. Special considerations, such as compacting the backfill, must be taken to prevent fill dirt surrounding new foundations from settling over time. This settling can result in “reverse grading” which will direct water back against the foundation wall.

BackfillThe backfill directly in contact with the building should be of a granular nature to allow water to pass through it quickly into the perimeter drainage system.

Drain RockDrain rock at a depth of 200mm - 300mm (8” - 12”) should be placed over the perimeter drains and mounded against the foundation wall. The drain rock should be crushed or round stone of a consistent size, usually ¾” but up to 1 ½”, is not uncommon.


Notes

LeArnIng TASk 4

The drain rock should have a continuous covering of filter cloth. This cloth prevents the finer soils from settling into the drain rock and eventually restricting the easy flow of water to the perimeter drains.

Water control systems below gradeFigure 4.

Perimeter DrainageThe perimeter drain system should consist of minimum 100mm (4”) diameter, thick walled, perforated PVC pipe. Use solid lengths with two 450 fittings to turn corners, not 90° fittings. Avoid the use of continuous flexible pipe. Follow the footing at every wall of the building and provide a minimum of slope. A slope of 1%-2% is optimal. Steeper sections should consist of solid pipe. The top of the perimeter drain must never be higher than the bottom of the basement slab or ground seal.

Soil gasIn enclosed portions of buildings and areas where it can be shown that soil gases pose a hazard, some method of controlling them is required. An exception to this is in the case of garages. The BCBC outlines two methods of controlling soil gases.

Barrier sealThe damp proofing or waterproofing membranes applied to foundation walls, required by the BCBC, are sufficient to prevent the ingress of soil gases at this location. Keep in mind that all penetrations through the foundation wall, such as control joints and water and sewer lines, must be adequately sealed to provide a continuous membrane.

The areas requiring special attention are the floors, junctions of floors with walls, and penetrations in the floors. A variety of building components combined can provide an effective system against soil gas ingress.

PolyethyleneFloors on ground (concrete slabs) use 0.15mm (6mil) U.V. resistant polyethylene sheet plastic to act as the soil gas barrier. For this membrane to be effective, it must be in direct contact with the underside of the slab, the edges must be lapped a minimum of 300mm (12”) and it should lap up the edges of the slab so it can be sealed with caulking to the wall slab connection.

Notes


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Damp prooffoundation wall

Drainagematerial

Drainage pipe

Seal withcaulking

Seal poly tofoundation

Seal around drain

Self priming trap

Sealing out soil gasFigure 5.

Quality concretingCracking as a result of shrinkage in the concrete slab may allow the passage of soil gases, especially if the polyethylene membrane is compromised at all. Use concrete mixes with a low water/cement ratio to minimize shrinkage. The addition of water-reducing admixtures will help keep the water/cement ratio low and still maintain a workable mixture. Use mechanical vibrators to consolidate the concrete in the slab. A well prepared sub-grade of compacted fill is required to prevent cracking. Steel reinforcing may be required if heavy live loads, such as shop equipment, will be placed on the slab.

Cracking also occurs at the corners of openings and penetrations in the slab or wall. Adding reinforcement at the corners, as shown in the plan view in Fig. 5, will help reduce this.

Cracks will often occur atthe corners of openings

Reinforce each cornerwith two 15m bars

Cracks in foundation wallsFigure 6.

The use of a polyethylene barrier prevents excess water in the concrete from draining away into the sub-grade. This can increase the amount of bleed water seen on the surface of the slab after placing. Do not begin the finishing of the concrete until the bleed water has evaporated from the surface of the concrete slab, as this can cause serious defects.


NotesConcrete sealing products can be applied to the surface of the slab shortly after it is placed. This will aid in the curing of the concrete, making it stronger and more airtight, as well as providing another physical barrier to the soil gas.

Additional ConsiderationsTo prevent sewer gases from entering buildings, floor drains must be connected directly to the perimeter drain system. This poses a problem if soil gas is an issue because the floor drains are seldom used and the self-priming traps often dry out. When this happens, a direct link is created between the ground outside the building and the interior of the building. Some form of stopper will be required in these circumstances. Another option is to use floor drains that have a primer line connected to a nearby water line. Every time a certain tap is used, a small amount of water goes to the floor drain and keeps the trap full. This type of floor drain is common in commercial construction.

Like foundation walls, all floor penetrations, particularly block-outs left for shower drain hookups, need to be effectively sealed to maintain the continuity of the soil gas membrane.

Subfloor DepressurizationSubfloor depressurization systems are recommended for areas that are prone to high levels of radon or other ground pollutants. These systems are only permitted in buildings containing a single dwelling unit.

The term “subfloor depressurization” refers to the act of depressurizing the area under the ground floor. A positively pressurized living space will prevent the ingress of soil gases, but it can often lead to exfiltration; this causes condensation problems in the building envelope. Therefore, subfloor depressurization is not the most practical method for achieving an outward pressure difference. Remember, air moves from high pressure zones to low pressure zones. If you are not pressurizing the living space, you must be depressurizing the outside space, in this case, the area immediately below the basement slab.

The system requires a minimum 100mm (4”) granular layer, as per Part 9.16.2.1 of the BCBC to be placed beneath the basement slab. A pipe is cast through the center of the basement slab with its end open into the gravel fill under the slab. The pipe is capped and is used to test for soil gas contaminants.

If the test results show the presence of radon gas, the pipe is then connected to a ventilation system that draws air from below the slab and exhausts it outside the building. To prevent the re-entry of the contaminated air the exhaust from the vent should be above the roof level.

Notes


LearningtaSk5

Learning Task 5Describe Wood Frame Seismic Applications and Related Hardware

At this time, there are no specified earthquake or wind loads in the BC Building Code for Part 9 wood frame buildings. The many redundancies in our construction practices are generally seen as adequate for dealing with these loads. However, changes to the seismic design requirements of Part 9 wood-frame buildings are being proposed for the next National Building Code, on which the BCBC is based.

Vancouver Island and the Lower Mainland are in a very active seismic zone. For this reason, it is important as a carpenter to understand the forces resulting from an earthquake, how the requirements of the BCBC resist those forces, and the “above code” building techniques that will not only minimize the loss of life as a result of a catastrophic earthquake but also maintain the structural integrity of homes to provide safe shelter during the event and to provide a means to exit the building following the earthquake. Particular attention should be made to entrance ways to ensure that non-structural elements are well secured.

This learning task will provide you with an understanding of seismic loading and how it affects Part 9 wood frame buildings, as defined by the BCBC. Basic earthquake resistance measures and concepts will be detailed and defined. The remainder of this learning task will be broken up into the individual building sections: foundations, floor construction, walls, and roof ceiling systems, for more detailed examination.

Each of the individual building sections identified above will be examined in three ways. First you will look at the specific loads imposed by an earthquake on the individual building components and what BCBC requirements support those loads. Second, you will examine strategies (“above code”) that can be under taken to improve upon the BCBC requirements and thus “over build” those components to provide greater resistance against earthquake loads. And third, you will be introduced to a variety of seismic hardware designed for these specific applications.

Seismic Loading and a Building’s ResponseThe forces imposed on a building by an earthquake generally occur as a lateral or sideways motion. There is also a vertical component associated with earthquakes that is considered to be about 30% of the horizontal forces. For most structures, the gravity design can accommodate these forces. It is important to ensure that all connections between components, such as the floor system resting on a wall assembly, be connected in a manner that can resist uplift forces.


Notes

LearningtaSk5

These lateral loads are then transferred throughout the structure by means of load paths. Load paths provide a continuous transfer of loads to the foundation. At the very least, a wood frame building has individual studs; each of these would provide a load path, with the load transferred from the top of the building down to the bottom of the structure. The strength of all of the studs acting independently of each other would be low and the structure would likely fail under low load conditions.

The dynamic forces of an earthquake move the foundation of the building horizontally. As the foundation moves, the upper portions of the building want to remain static or still. This static nature is often referred to as inertia, and is directly proportional to the building’s mass. What this means is that the heavier the building is, the greater its inertia or resistance to movement, but also the higher the earthquake loads.

BEFOREEARTHQUAKE

Roof motion followsground motion after

overcoming initialinertia force.

Roof motion opposite toground motion after

ground motion reverses.

Ground motion reverses.

Inertia force resistingchange in state of roof.

Inertia force resistingmotion induced by

ground acceleration.ground

acceleration

groundacceleration

groundacceleration

groundacceleration

Roof motionbraked by inertia.

Ground motionreverses.

groundacceleration

groundmotionstopped

Roof motion continuesin direction of ground

motion.

Roof motion slowedand �nally stopped byinertia.

EARTHQUAKEOVER

Inertia force resistingmotion induced byground acceleration.

Roof motion followingground motion.

Roof motion braked byinertia.

Inertia force resistingmotion of roof.

Inertia force resistingmotion induced byground motion.

Forces induced in a house due to earthquake ground motionFigure 1.

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LearningtaSk5

BuildingDriftBuilding drift refers to the degree of lateral (side to side) movement of a building in relation to its stationary foundation.

The greater the percentage of drift, the greater the damage a building will suffer during an earthquake. Constructing a building with greater stiffness will reduce the percentage of drift, and thus the amount of damage that is caused.

Building driftFigure 2.

Damage in a building is a manifestation of the building’s ability to absorb the energy from an earthquake. For large earthquakes, some damage is expected and necessary.

DuctilityDuctility refers to the extent to which a material can bend or twist without breaking. Different building materials have different degrees of ductility. A wood frame structure, for example, is inherently more ductile than a concrete one. This means that a wood frame structure can withstand a great deal of flexing and deformation under load (such as an earthquake) without collapsing.

Fortunately, a building’s ductility does not have to take away from its stiffness. This is in essence what we are accomplishing when we use wood in platform framing. A floor system, for example, benefits from the ductility of wood but gains substantial stiffness from the sheathing. This flexing can be an efficient means of transferring loads.


Notes

LearningtaSk5

Basic Earthquake Resistance Measures There are a number of factors that contribute to a building’s ability to successfully withstand the effects of an earthquake. Some of these factors include proximity and geological conditions, while others are more specifically related to the building design, shape, and construction.

SoilConditionsChoosing a site based on the soil conditions isn’t always practical, but understanding that soil conditions will affect the severity of an earthquake is important.

Areas that have high water tables can be susceptible to soil liquefaction during an earthquake. The soil acts like a liquid, sometimes causing buildings to lean, tip over, or sink several feet.

Denser soils and solid rock are more desirable, as softer soils will amplify shear waves and magnify the effects of an earthquake.

proximitytotheFaultLineThe closer a structure is to the fault line, the more severe the forces of an earthquake will be.

WeightLimitationsEarthquake loads are proportional to the weight of the house; the heavier the building, the greater the stresses as a result of an earthquake. The relative light weight of wood frame construction and its high weight-to-strength ratio is one of the reasons it can successfully survive the loads imposed by earthquakes.

BuildingConfigurationsAn ideal house would have a simple rectangular configuration with braced bearing walls directly above and below on each floor, evenly and symmetrically distributed throughout and increasing in length in the lower floors. This configuration results in loads and deformations being uniformly distributed throughout the building, allowing all of the buildings structural components to contribute equally.

Notes


LearningtaSk5

Load

Resistance

DiaphragmRotation

Increased Displacement

ITorsion stresses due to earthquake loadingFigure 3.

Building irregularities can result in torsion stresses as the building reacts to the uneven loading.

One thing that does have to be considered is that unbalanced loading (e.g., floors that are heavier than others) will result in disproportionate loading and uneven stresses on the building. When this occurs, it is often important to use an engineered design to compensate.

exteriorVeneersExterior veneers generally add more weight and reduce the ductility of a wall, making it more susceptible to failure. The brittleness of the veneer works against the ductility of the wall system itself and becomes a liability. There are many cases where a veneered wall has pulled away from the framing and become a falling hazard during an earthquake. This becomes a much greater threat the higher the veneer extends up the wall.

In some cases systems such as stucco can actually improve a structure’s performance in an earthquake. The steel mesh surrounds the entire structure and helps distribute earthquake forces throughout the structure.

Building HeightThe greater the building height, the more susceptible the building is to the lateral loads imposed by both earthquakes and wind which threaten to overturn the building itself. Wood frame buildings have traditionally been limited to four stories in height as a precaution against these loads. However, on April 6 2009, BCBC amendments were put into effect which now allow wood frame buildings up to six stories in height. Buildings extending to six stories must be engineered, but the BCBC recognizes that wood-frame buildings are inherently effective in surviving earthquake loads.


Notes

LearningtaSk5

Load PathsCritical to understanding the measures that must be taken to counteract earthquake loads is an understanding of load paths themselves.

A building is a series of interconnected structural systems. These systems consist of the roof-ceiling, the braced walls, the floor, and the foundation. In a multi- floor building, there may be a series of braced walls connected to floor systems. Each of these systems work to resist the loads applied and transfers them to the “system” immediately below them through structural connections. The loads at the bottom of a building are substantially greater than those at the top because the loads become concentrated as they are transferred to the foundation.

The key load transfer points in a load pathFigure 4.

A building’s ability to resist earthquake loads is directly proportional to the strength of the individual systems and the connections between them.

Notes


LearningtaSk5

Building SystemsAll buildings consist of a series of interrelated systems. These systems create structural redundancies and provide load paths that can successfully mitigate the effects of an earthquake.

FoundationsandFoundationWallsThe primary function of a building’s foundation is to support the vertical loads resulting from the live and dead loads above. The foundation must also resist the horizontal sliding and vertical loads caused by earthquakes.

Sliding and overturning resisted by foundationFigure 5.

For a house to survive an earthquake, its foundation must:

Provide continued vertical support.1.

BCBC requirements: Part 9 of the BCBC requires a continuous strip footing • under foundation walls, as well as pad footings to support any point loads.

Above code techniques: One way to increase the structural integrity of a • foundation against earthquake loads is to provide a continuous footing through openings such as those occurring at garage doors. Also, rather than using a series of independent footings when a series of columns are used to support internal beams, place the footings as a continuous strip footing and anchor the tops of the piers to the slab itself. Another consideration is to place strip footings and foundation walls beneath non-load-bearing walls and anchor them using 12.7mm (1/2”) bolts, as you would with a load-bearing wall.

Provide friction and passive bearing at the soil-to-foundation interface to 2. minimize movement and damage.

BCBC requirements: The footing size is relative to the number of supported • floors and must be a minimum of 100mm (4”) or the width of the projection of the footing beyond the supported element. This footing, buried beneath the backfill and resting upon firm, undisturbed soil, provides a long, wide surface area.

Above code techniques: Increasing the width and depth of the foundation • footing will increase its friction and passive bearing capabilities. Keep in mind, however, that a corresponding increase in steel reinforcing may be required.


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Provide anchorage at the foundation-to-house interface to minimize 3. movement and damage.

BCBC requirements: 12.7mm bolts are required at a spacing of not more than • 2400mm (8’). These bolts must be embedded a minimum of 100mm (4”) into the top of the foundation wall and must be secured to a sill plate with nuts and washers.

Above code techniques: •

Reduce anchor bolt spacing to at least 1800mm (6’) or even 1200mm (4’) o.c. ·Anchor bolts should never be placed greater than 300mm (12”) or nearer than 100mm (4”) from the ends of walls.

Use square washers to minimize the splitting of sill plates caused by uplift. ·These washers have a greater withdrawal resistance than round washers. Square washers will also help prevent the sill plate from pulling through the smaller diameter round washers.

Avoid wet setting of anchor bolts (the placement of anchor bolts while the ·concrete is still “plastic”) since this can create voids adjacent to the bolts, lowering their withdrawal resistance. This will involve securing the bolts and placing the concrete around them.

Use toe-nails for the perimeter joist to sill plate connection. This will assist ·the toe-nailing of the floor joists to the sill plate.

Provide strength and stiffness sufficient to resist both horizontal loads 4. resulting from racking and overturning of bracing walls within the building.

BCBC requirements: Foundations are designed to resist the lateral loads of the • soils placed against them; as such, the thickness of the concrete in a concrete wall is determined by the height of the backfill. The backfill itself also acts to support the walls during an earthquake. The BCBC toe-nailing requirements for a floor joist to plate help secure the floor system to the top of the foundation wall, providing added lateral support.

Above code techniques:•

Increase the depth of the foundation wall, as well as its thickness (with no ·corresponding increase in basement depth). The added mass will add to the overall strength of the foundation. The increased backfill will also contribute to greater resistance to horizontal loads caused by an earthquake, as well as overturning resistance.

Earthquake loads subject all building components to cycles of compression ·and tension. Since concrete is much stronger in compression than tension, additional steel reinforcing in a concrete footing and wall will greatly increase its ability to withstand these cyclic loads. Installing steel in the lower portion of the footing and using vertical dowels with an “L” shape tied to the footing rebar and extending up into the foundation wall and a continuous lateral band of steel around the perimeter of the foundation wall near the top and tied in to restrict the movement of the anchor bolts is an easy way to connect the foundation as an integrated system.

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12 inchesmin.

Foundationsill plate

Stemwall

Constructionjoint

Footing

18 inches ondowel bars

No. 4 horizontal bartop and bottom

Foundation to floor connectionsFigure 6.

Seismic Hardware Seismic hardware isn’t typically designed to assist the foundation but to use the foundation to anchor and receive the loads transferred from above. Simpson strong-tieTM provides special anchor bolts systems such as the Titen HD ® with square bearing plates to provide the strongest possible mechanical connection between the foundation and the framing above.

Straps embedded in the concrete foundation can be utilized in addition to the anchor bolts to assist in the transfer of loads from above.

Supplementary strapsFigure 7.


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Floor Construction

earthquakeLoadsandHowtheBCBCrequirementsacttoMitigatethemWoodframe floor systems form a horizontal diaphragm at each level where they occur. They also transfer earthquake lateral loads to braced walls below that floor level or directly to the foundation when the lowest floor is supported on a foundation. The lateral loads that a floor system resists will include half of the walls above and below that floor. If the floor sits directly on the foundation then it is only half of the walls from above that are contributing to the load supported by the floor.

The lateral loads resulting from earthquakes attempt to roll the floor joists. It is this rolling or rotational load that can compromise a floor system and its ability to transfer loads effectively.

A typical floor system consists of wooden joists set at a regular spacing, typically 300mm, 400mm, 500mm or 600mm (12”, 16”, 19.2” or 24”) sheathed with a minimum 15.5mm (5/8”) tongue and groove panel type subfloor installed at 900 to the joists and secured with 51mm (2”) common nails at 150mm (6”) along the edges and 300mm (12”) along intermediate supports. When spans require it, placing blocking or bridging at mid span helps transfer loads between adjacent joists. Staggering the ends of adjacent sheets of sub flooring causes them to interlock and makes the whole floor work as a unit. These elements contribute to provide a very strong floor effectively transferring lateral loads through the appropriate load path.

It is important to understand that floor systems often include other structural elements such as built up and engineered beams, and headers. These may be engineered and will have an effect on the floor’s ability to resist the loads imposed by an earthquake.

Design irregularities such as cantilevered floors can create uneven loading that may require an engineer if excessive. As mentioned previously, design irregularities should be avoided if possible due to the potentially negative impacts they can have on a buildings overall strength.

aboveCodetechniquesforMitigatingearthquakeLoadsMost floor designs involve finding the minimum-sized joist for the maximum span and largest on centre spacing. Increasing the size of the floor joists and decreasing the on centre spacing are a couple of simple ways to increase the overall structural integrity of the floor system.

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The primary lateral load transfer in a floor system occurs through the rim or box end joists and through blocking that is parallel to braced walls or foundation sill plates.

Placing blocking between floor joists immediately inside the perimeter or box end joists provides a couple of advantages. First, it takes some stress off the nailing that is securing the floor joists to the sill or top plate below and the nails from the perimeter joist to the ends of the floor joists themselves. Second, it provides more nailing area for the walls above to be anchored to.

Reducing the on-centre spacing of the intermediate blocking from 2100mm will increase the floor’s resistance to rotational loads.

Use full depth blocking beneath all bearing walls to fully transfer the load to the bearing below.

Blocking between joistsFigure 8.

Interior non load bearing walls running parallel with the joists could be supported by double floor joists rather than blocking at 1200mm (4’) o.c.

Since floor sheathing is such a critical component for the transfer of lateral loads simply increasing the floor sheathing thickness will improve the floors lateral resistance. Larger diameter nails will provide greater lateral capacity because the lateral capacity of a nail is directly proportional to its diameter. Reducing the o.c. spacing of the nails around the edges of the sheets from 150mm (6”) to 100mm or 75mm (4” or 3”) will also help and ensure that the nails are not over driven. Using screws and gluing the floor sheathing to the floor joists adds a considerable amount of increased stiffness to the floor system as long as the screws are designed for the increased shear loads. It is very important to remember that not all decking screws have a lot of shear strength since their primary function is typically withdrawal resistance, so be sure to choose the correct screws for the job.


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The floor system built to the BCBC minimum produces what is referred to as an unblocked diaphragm. Although an engineer is required to design what would be referred to as a fully blocked diaphragm, the concept is simple enough and the benefits substantial enough that utilizing it as a matter of course will create a floor system that is far superior to an unblocked diaphragm. A fully blocked diaphragm requires that all panel sheathing edges that are not supported by a joist will be supported by blocking. The fasteners along the blocked edges means that the shear loads will be transferred from one sheet to the next far more effectively. This holds the floor together better and allows the supporting walls below to resist the loads more as a system than as a individual walls.

Fully blocked diaphragm Figure 9.

SeismicHardwareUsing mechanical connectors to tie the floor system to the walls above and below provides a better connection that is less reliant on the holding power of the fasteners. Many of these connections utilize susceptibly weak toe-nailing of the various components, so the use of the Simpson A35 framing angle would significantly improve these connections providing a better transfer of loads and ensuring the continuity of the load path itself.

Large openings in floor systems exhibit the same potential problems as they do in walls. They should be avoided at the design stage, or kept to a minimum, as in the case of stairs. Metal galvanized straps across the headers and onto the trimmer joists at each end of the opening will help to hold it together while under load.

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Various Simpson strong-tie connectorsFigure 10.

Reinforcing around floor openingsFigure 11.


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Wall Construction

earthquakeLoadsandHowtheBcbcrequirementsacttoMitigatethemIn typical residential construction, the walls provide the primary lateral resistance to wind and earthquake loads. The walls are also the primary element for transferring loads from the upper portions of the building to the lower portions. Like the foundation, the walls must resist the sliding, overturning, and racking action resulting from earthquake loads.

End stud incompression

Resisting forcefrom connection

Earthquake force

Earthquake force

C

C

T

T

End stud intension

Racking forces on a wallFigure 12.

The members in walls perpendicular to the lateral loads imposed by an earthquake cycle between compression and tension as the forces generated transfer through them. As the bottom of the wall is moved laterally by the motion of the earthquake, the top of the wall remains static as the inertia of the building resists the dynamic movement. This creates tension at the end of the wall in the direction in which the wall is moving and compression at the end of the wall in which it is moving away from. These are the primary reasons why the ends of the walls are so critical in

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resisting these forces. This is also why, as discussed in the “foundations” section of this learning task, anchor bolts should be no more than 12” and no less than 4” from the ends of all sill plates.

Walls must be nailed into the floor joists, the perimeter joists, or the blocking provided directly under if they are non-load bearing and parallel with the floor joist system. These nails are the one means required by the BCBC for transferring the loads from a wall into a floor

The BCBC requires some form of bracing or lateral support for all exterior walls and interior walls are typically finished in panel type drywall. These two elements, if installed correctly, substantially contribute to the lateral strength of the wall. The bracing must be secured to the bottom or sill plate and the double top plate of the wall as well as the studs in the wall. The studs must be continuous from the top of the bottom plate to the underside of the top plate, except where openings occur. This provides a continuous tie from the top of the wall to the bottom of the wall maintaining a continuous load path. Walls using plywood or OSB panels provide an unblocked diaphragm similar to the floor systems with the same nailing requirements.

Wall plates must be lapped (laced) at the corners. This ties intersecting walls together at a critical point in the load path as this is where the majority of the compressive and tensile forces are concentrated during and earthquake.

Wall plate joints must be staggered from one another by at least one stud spacing. This minimizes the possibility of a weak spot in the wall system.

aboveCodetechniquesforMitigatingearthquakeLoadsonWallsThe walls most prone to failure in a building are the cripple or pony walls supporting the main floor of a building around the crawlspace or serving as walls for the stepped portions of a foundation and the main floor walls. A reason for these failures is that they are at the bottom of the load path; therefore, they are supporting the accumulated loads of the entire structure above. Another reason is that they may be inadequately secured to the foundation. Also, they typically have larger openings for garages and windows as the bottom floor often encompasses the main living areas of the house which are generally designed with larger windows. This is often referred to as a “soft storey”, meaning that its design, larger openings, makes it inherently weak.


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Cripple WallFigure 13.

A cripple wall supporting a floor system over steps in the foundation walls should be tied together with the sill plate on the wall above acting as the double top plate on the cripple wall below. This will provide a connection that utilizes the foundation anchorage system of the sill plate above. If possible in multiple stepped foundation walls, notch the structural panel sheathing around the step in the wall rather than cutting it flush with the wall and starting a new panel at the step. This will provide a better connection between the stepped walls and increase overall stiffness of these walls.

Stepped pony walls and cripple walls around a crawlspace can have blocking tightly fitted between all of the studs at the sill plate. These blocks increase the lateral strength of the wall by minimizing the stud’s ability to rack as well as providing greater surface area for nailing the sheathing at the bottom. These walls are often found in the unfinished portions of buildings allowing us to add a layer of structural panel sheathing to the inside of the walls that will not interfere with the interior finish.

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Rim joist

Framing clip

Floor joist

Blocking

BlockingStructural panel

Foundation

Still plate bolts

Special framing for cripple/pony wallsFigure 14.

Studs should be continuous, i.e., non-finger joint Douglas fir, larch, or western pine. These species provide greater stiffness than other commercially available species. When nailing the wall studs to the top plate it is important to keep the nails even to the outside of the plate, approximately ¾” from the plate edges. This prevents uneven loading which may cause the plate to pull away from one side of the wall.

Walls sheathed with structural panel products such as Douglas fir plywood or OSB have much greater resistance to lateral loads than other forms of lateral bracing such as let in bracing or 1 × board sheathing. Many builders in residential construction will use the 1×8 boards used as form sheathing for the foundation walls as wall sheathing. This practice is acceptable, but can be best utilized if the boards are applied on the upper floors rather than the lower floors and installed on approximately a 45° angle. Horizontal board sheathing provides little resistance to lateral loads and should be avoided except for short wall sections (less than 3’).


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Board wall sheathing installed to maximize lateral strengthFigure 15.

Wall sheathing creates a diaphragm very similar to a floor system and as such can gain from the same blocking and nailing adjustments. Blocking at all panel edges and decreased nail spacing to 100mm - 75mm (4” - 3”) will create what is generally referred to as a shear wall, the equivalent of a floor diaphragm. If the thickness and the number of plys of the sheathing are increased, a corresponding increase in structural strength is achieved.

The sheathing itself should be applied continuously from one end of the wall to the other whenever possible, including interior walls designed to provide lateral support. Sheathing applied above and below openings should be installed to avoid joints at the sides of openings. Consistent nailing should be maintained around all openings to ensure the continuity of load transfer around what is inherently a weak point in the wall system. An effort should be made to use full-size panels as much as possible and to avoid narrow strips of sheathing.

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Overlapping sheathing at rim joistFigure 16.

A very simple yet effective way to ensure that the walls and floor systems stay together to allow for a continuous load path is to hang the panel wall sheathing over the perimeter joists from the top and the bottom and fasten to the perimeter joists themselves. Although a very common sense solution, it is often overlooked as it requires that an intermediate piece of panel sheathing be installed on each wall to accommodate for the amount that the sheathing is installed over the perimeter joist. This approach can also be employed using board sheathing by extending the boards on to the perimeter joists at a 45° angle.

The compressive and tensile forces concentrated at the ends of the walls can be further supported on exterior walls by extending the wall sheathing from one wall across the end stud of the intersected wall on outside corners. The laced wall plates and the nails fastening the one wall to the other are inadequate at such a critical load transfer point in a building. The sheathing greatly increases this connection. Again, this is often overlooked since it is more work to plan for and extend the sheathing.

Although the BCBC allows the use of staples for attachment of wall sheathing it is a poor choice when lateral strength is required. Another factor that is often overlooked is the importance of not overdriving the fasteners. Sheathing nails that are overdriven by 1/8” can reduce the strength of a wall by as much as 40 to 50 percent. Hand nailing may be the best option but if air driven fasteners are utilized they must be properly adjusted to prevent overdriving the fasteners.


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SeismicHardwareAlthough Simpson Strong-Tie TM does have steel shear wall panels, the majority of their seismic hardware for walls is designed for two primary functions. First, they must withstand the concentration of compressive and tensile forces at the ends of the walls. They must also maintain the overall connection from wall to floor system. These functions must be maintained across all of the building elements from top to bottom and is often accomplished with a continuous tie down system.

There are generally two common forms that continuous tie down systems take. The first uses an anchor such as the Simpson TM HD19 pictured to the right. This hold down is secured to the concrete foundation with an anchor bolt. The hold down is then bolted through a series of solid studs at or very near the ends of the walls. The number of studs and their nailing requirements will be determined by an engineer but generally decrease the higher they are in the building. At the top of each wall an inverted hold down is bolted through the same stud package as the bottom, and a threaded rod is used to connect it to the next stud package in the wall above. This is repeated at each floor level and all the way to the roof providing a continuous tie from the top of the building to the foundation.

Simpson HD 19 hold downFigure 17.

The second system accomplishes the same task as the first, which is to provide a continuous tie from the top of the building to the foundation, but it uses threaded rods and couplers to transfer the loads rather than the stud packages.

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The key for both of these systems is that they must extend from the foundation to the roof and they cannot skip floors.

Connectholddownswithall-threadrod andCNW coupler

Threaded rods and couplersFigure 18.

The wall to floor system connection can be reinforced by the use of straps, typically on the outside portion of the wall, and/or various framing anchors or tie downs used on the inside of the wall. When using a mechanical fastener such as a strap on the outside of a wall, a corresponding fastener should also be utilized on the inside of the wall. The reason for this is that loads transferring through these systems can be restrained at the outside of the wall with the sheathing overlap and straps, but this often causes excessive loading at the inside of the wall resulting in failures.

External strapsFigure 19.


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Another important element for maintaining a continuous load path is the use of drag struts. The main purpose of a drag strut connector is to connect a beam or truss to the top plate of a collinear bearing wall. This completes the load path and allows loads to be transferred out of the roof or floor sheathing into the wall system where the shear walls are.

Drag strutFigure 20.

Soft storeys, as discussed earlier, are critically weak links in a buildings ability to withstand the loads as a result of an earthquake. They are often the result of excessively large openings, such as those created by garages and the use of large windows or short walls with a single opening that encompasses the entire wall. The lack of structural panel sheathing on these walls severely limits its ability to resist lateral loading. The solution to this problem may be to utilize a steel moment frame. A moment frame is incorporated around the opening to strengthen and stiffen the wall beyond the capabilities of the sheathing.

Steel Moment frameFigure 21.

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Roof and Ceiling Construction

earthquakeLoadsandhowtheBCBCrequirementsacttoMitigatethemWoodframe roof-ceiling systems, regardless of the pitch of the roof, form a roof diaphragm that transfers earthquake lateral loads to braced walls in the story level immediately below the roof. This is done in the same manner that floors transfer loads from interior portions of the floor to the braced wall lines of the story below. The lateral loads in the roof-ceiling are based on the mass of the roof-ceiling assembly and a portion of the mass of the walls in the story immediately below the roof.

The BCBC details the minimum ceiling joist and rafter spacing based on lumber species, spans and on centre spacing. Nailing requirements for rafter connections at the ridge, rafter to plate nailing at the outside walls, rafter to ceiling joist connections and ceiling joist to ceiling joist connections over bearing walls, are designed to tie all of the roof and ceiling components together across the width of the building and act as an integrated system. This system is primarily designed to resist the outward forces resulting from gravity loads, but also invariably acts to keep the roof together while subjected to earthquake loads.

A roof system acts very much like a floor diaphragm, with the same sheathing nailing requirements. The fact that the roof is at the top of the structure means that it has the lowest loads applied to it than all of the other elements in the building. But a failure at the top of the building can be just as dangerous as any other failure in the structure.

There are a number of specific considerations that the BCBC makes for the roof-ceiling system. All ridge boards must be at least the full depth of the rafter plumb cuts to prevent the rafters from deflecting under load if they are not fully supported. All hips and valleys must be a minimum 2” deeper than the rafters that they are supporting to provide continuous support for the full depth of the plumb cuts, as well as in recognition of the extra loads being distributed onto them from those connecting rafters. All rafters must be “shaped” to provide even bearing surfaces which facilitates effective fastening of the rafters to the tops of the walls.

aboveCodetechniquesforMitigatingtheeffectsofearthquakeLoadson roofs and CeilingsLike the floor and wall systems discussed previously, the roof-ceiling system is essentially a diaphragm that can be improved upon by decreasing the on centre nail spacing, providing blocking at all of the unsupported edges, and gluing the sheathing. Using spaced wooden boards is not recommended unless absolutely necessary due to the lack of lateral strength resulting from partial coverage of the roof surface and lack of nails.


Notes

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For any building system to work effectively it must have a direct load transfer to the systems immediately below it. The roof–ceiling system transfers its loads to the walls that support it, so these connections are crucial to its ability to withstand the forces of an earthquake. The load transfer point is at the top of the walls where the rafter/ceiling joist or trusses are secured to the top plates of the wall. What this connection typically lacks is continuous contact as each member is spaced with gaps between. An easy solution to this is the addition of solid wood blocking placed between the roofing members and firmly secured to the adjacent roofing members as well as the top of the wall below. This should be done on the outside walls as well as all interior load bearing walls. The panel sheathing can then be nailed to the blocking at the outside walls creating a continuous load transfer free of any gaps. This also has the added benefit of resisting the rotational loads that want to roll the rafters or trusses. Roof ventilation must be maintained.

Blocking at rafters and perimeter nailingFigure 22.

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The hips and valleys of a roof system will be required to support concentrated loads during an earthquake. To increase their ability to withstand the negative effects of these loads intermediate support can be provided with the use of columns or props that bear down onto laterally supported walls such as shear walls. This will minimize the chance that these members will fail under load as well as help dissipate the forces over a larger number of load paths. The common rafter spans can also be supported by purlins or knee walls which will also increase the number of load paths placing less stress on individual members.

Some roof systems are more susceptible to damage from lateral loading than others. Hand framed or trussed Gable ended roofs can suffer from the “hinge” effect. These roofs often pivot under load at the wall to roof line connection. This is the result of the lower wall, which forms the exterior of the living space, and the upper wall, which encloses the attic space, sitting one atop the other. They are often sheathed independently as separate walls and therefore the structural connection is limited to the toe nail fastening of the set down gable truss or ceiling joist to the double top plate of the lower wall.

There are a couple of solutions to this problem – the simplest being to sheath across the two walls so that it extends across the lower wall onto the attic wall. This will take some of the stress off of the nails and help loads transfer from one wall to the other more effectively. Another more effective solution is to frame the whole wall continuously to the underside of the roof rafters. This eliminates the “hinge” altogether – although this method will not be practical for trussed roofs.

A trussed roof system or a particularly steep roof may have to rely on a series of braces, such as those in the adjacent image, to sufficiently support the gable end.

end nails toblocking (typical)

4 - 10d to each block 2× blocking at

48” on center

4 - 10d

2×6 diagonal strutsat 48” on center toprovide lateral supportfor top of wall

not steeper than 2:1

sheet metal angleclip, 2×6 diagonal

to wall top plate

roof truss or framingat gable end wall

elev

atio

n w

all

Gable end bracingFigure 23.


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SeismicHardwareRoof system seismic hardware is generally designed to perform three functions: resist rotational loads resulting from lateral forces, anchor the roof system to the wall system, and prevent uplift. Simpson Strong-Tietm provide a number of hardware solutions that can be utilized depending on the application.

Rafter/truss to wall anchorFigure 24.

Rafter tie down with blockingFigure 25.

Documents

Carpentry - BC Trades Modules Line J1 — 1 Learning Task 1 Notes Describe the Forces Acting on the Building Structure All buildings are subject to loads created by gravity, weather,