CIVL473 Fundamentals of Steel Design British …civil.emu.edu.tr/courses/civl473/Chapter2-Loading.pdfPart 1: Dead and Imposed Loads (1984) Part 2: Wind Loads (1995) Part 3: Imposed

CIVL473-Chapter 2 9/26/2011

Dr.Murude Celikag 1

CIVL473 Fundamentals of Steel Design

Prepared ByAsst.Prof.Dr. Murude Celikag

CHAPTER 2Loading and Load

Combinations

a) The relevant British Standards for LoadingBS 6399 Design Loading for BuildingsPart 1: Dead and Imposed Loads (1984)Part 2: Wind Loads (1995)Part 3: Imposed Roof Loads (1988)

b) The old British Standard for Wind Loading

British standards Institute CP3: Chapter V: Part 2

Loading for most of the structures are obtained from the relevant British Standards, the manufacturers data and similar sources.

1) DEAD LOADS• Own weight of the steel member

(kg/m of the steel section)• Other permanent parts of the building, not normally

movable. (e.g. concrete floor slabs, brick/block walls,finishes, cladding).

Weight is calculated either from density of material (kg/m3) or specific weight (kN/m3).

Material Density (kg/m3) Specific Weight (kN/m3)

SteelReinforced ConcreteBrickworkTimber

785024202000-2300500-900

7723.720-235-9

Typical values of common structural materials


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2) IMPOSED LOADS

• Snow on roofs• People• Furniture• Equipment, such as cranes and other machinery• Semi-permanent partitions that are movable

All imposed loads are based on experience within theconstruction industry and the statistical analyses ofobserved cases. Includes the following temporary loads:

Building Usage Imposed Loading (kN/m2)

ResidentialOfficesEducationalTheatresWarehousingIndustrial Workshops

1.52.5-5.03.04.02.4 per meter height5.0

Typical values of imposed loads

3) WIND LOADS

Wind loads to be used are based on the British StandardsInstitute CP3: Ch V: Part 2.

Basic wind speed, appropriate to the location of the building,is selected and reduced to a design wind speed using factorswhich take into consideration topography, surroundingbuildings, height above ground level, component size andperiod of exposure.

Figure 1: (a) Variation of wind velocity with distance aboveground surface; (b) variation of wind pressure specified bytypical building codes tor windward side of building.


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Figure 2: Influence of shape on drag factor:(a) curved profile permits air to pass around body

easily (drag factor is small);(b) wind trapped by flanges increases pressure

on web of girder (drag factor is large).

(a) Uplift pressure on sloping roof;- The wind speed along path 2 > that along path 1 because of

the longer path- Increased velocity reduces pressure on top of roof, creating

a pressure differential between inside and outside ofbuilding

- The uplift is a function of the roof angle, θ.(b) Increased velocity creates a negative pressure (suction) on

side and leeward face, direct pressure on windward face AA

Figure 3


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Vortex Shedding. As wind moving at constant velocitypasses over objects in its path, the air particles areretarded by surface friction. This process is called vortexshedding. As the air mass moves away, its velocitycauses a change in pressure on the discharge surface.

If the period (time interval) of the vortices leaving thesurface is close to that of the natural period of thestructure, oscillations in the structure will be induced bythe pressure variations. With time these oscillations willincrease and shake a structure vigorously.

Figure 4: Vortices discharging from a steel girder. As vortex speeds off, a reduction in pressure occurs, causing girder to move vertically.

Photo 1: Failure of the Tacoma Narrows Bridge showing the first section of the roadway as it crashes into Puget Sound. The breakup of the narrow, flexible bridge was produced by large oscillation induced by the wind.

http://www.pbs.org/wgbh/nova/bridge/tacoma3.htmlhttp://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacnarr.mpghttp://www.youtube.com/watch?v=j-zczJXSxnwhttp://video.nationalgeographic.com/video/player/environment/energy-environment/wind-power.html


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Figure 5: Structural systems to resist lateral loads from wind or earthquake,

(a) Reinforced concrete shear wall carries all lateral wind loads.(b) Shear and moment diagrams for shear wall produced by the sum of

wind loads on the windward and leeward sides of the building in (a), (c) Plan of building showing position of shear walls and

columns, (d) Cross-bracing between steel columns forms a truss to carry lateral

wind loads into the foundations.

Figure 6: Typical wind load distribution on a multi-story building.

Figure 7: Variation of wind pressure on sides of buildings.


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Figure 8: (a) Wind acts on windward side of building; (b) wind forces applied by windward wall to edge of roof

and floor slabs; (c) side view showing resultant wind forces on each

shear wall.

The design wind speed is equated to a dynamic pressureq (kN/m2).

Owing to building and roof shape, opening is walls, etc.,pressures and suctions, both external and internal, willarise.

Pressure Coefficients external (Cpe) and internal (Cpi) maybe used as shown in the example.

Force on any element = (Cpe- Cpi)q x area of element

Pressure Coefficients external (Cpe) and internal (Cpi) maybe used as shown in the example.

Loads in any structure must be arranged during the designso that the maximum forces or moments are achieved atspecific points of the structure.

Therefore, all realistic load combinations must beconsidered to ensure that all peak values have beencalculated at every point.

One load arrangement would be sufficient to producemaximum moments and forces for simple cases.

4) LOAD COMBINATIONS


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Load Factors and Combinations

Partial Safety Factor for LoadsLoading Load Factor, f

Dead load, D 1.4Dead load restraining uplift 1.0Imposed load, L 1.6Wind load, W 1.4Combined loading (D+L+W) 1.2

1.4DL+1.6LL

1.0DL+1.4WLE

1.0DL+1.4WLW

1.0DL+1.4WLN

1.0DL+1.4WLS

1.2DL+1.2LL+1.2WLE

1.2DL+1.2LL+1.2WLW

1.2DL+1.2LL+1.2WLN

1.2DL+1.2LL+1.2WLS

Example – Wind Loading

2.65m

7.35m

A

B

C

D

E30m

Frames at 6m centres

Basic wind speed 50 m/sLength of building 48m, Open country with scattered wind brakes

S1 and S3 are both taken as 1.0, S2 is 0.88 for a height of 10m.

Design wind speed is Vs= S1S2S3V= 1.0x1.0x0.88x50 = 44 m/s

Dynamic pressure, q=kVs = 0.613x442 = 1.2 kN/m2.

Internal Pressure Coefficients (Cpi) is taken as

+0.2 (maximum) and-0.3 (minimum).

External pressure is obtained from Table 8 (CP 3).

2

Pitch 10o


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Example (continued)

AB D

E

C0.5

0.6 0.6

0.5AB D

E

C0.7

1.2 0.4

0.25

1.0

0.9 0.1

0.050.5

1.4 0.6

0.45 0.7

0.8 0.8

0.7 0.2

0.3 0.3

0.2

0.2 0.3

a) Wind on side b) Wind on end c) Internal pressure d) Internal suction

Case 1 (a+c) Case 2 (a+d) Case 3 (b+c) Case 4 (b+d)

Cpe-Cpi for frame member

Case AB BC CD DE

1. Wind on side + internal pressure2. Wind on side + internal suction 3. Wind on end + internal pressure4. Wind on end + internal suction

0.51.0-0.7-0.2

-1.4-0.9-0.8-0.3

-0.6-0.1-0.8-0.3

-0.450.05

-0.7-0.2

-0.25-0.5

-0.4-0.6

-1.2-0.6

0.7-0.5

Wind on sideWind on end

DECDBCAB

Cpe for frame member

-0.25-0.5

-0.4-0.6

-1.2-0.6

0.7-0.5

Wind on sideWind on end

DECDBCAB



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Cpe-Cpi for frame member

Case AB BC CD DE

1. Wind on side + internal pressure2. Wind on side + internal suction 3. Wind on end + internal pressure4. Wind on end + internal suction


Case AB BC CD DE

1. Wind on side 2. Wind on end

0.2

c) Internal pressure

AB D

E

C

AB D

E

C

a) Wind on side b) Wind on end c) Internal pressure d) Internal suction

Case 1 Case 2 Case 3 Case 4

Documents

CIVL473 Fundamentals of Steel Design British …civil.emu.edu.tr/courses/civl473/Chapter2-Loading.pdfPart 1: Dead and Imposed Loads (1984) Part 2: Wind Loads (1995) Part 3: Imposed