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Chapter 2 – Loads on Structures
�Our objective is to design a structure that will be able to withstand all the loads to which it is subjected while serving its intended purpose through its intended life span. To do so, �we must consider all loads that can realistically be expected to act on the structure during its planned life span. Three classes of Loading
• Dead Loads • Live Loads • Environmental Loads
2.1 Dead Loads Dead Loads are gravity loads of constant magnitudes and fixed positions that act permanently on the structure
Example 2.1: The floor beam is used to support the 6 ft width of a lightweight plain concrete slab having a thickness of 4 in. The slab serves as a portion of the ceiling for the floor below and its bottom is coated with plaster. Furthermore, an 8 ft high, 12 in thick lightweight solid concrete block wall is directly over the top flange of the beam. Determine the loading on the beam measured per foot of length of the beam
2.2 Live Loads Live loads are loads of varying magnitudes and/or positions caused by the use of the structure. Since live loads can move, each member of a structure must be designed for the position of the live load that creates the maximum stress in that member. Building Live Loads (ASCE7): Live loads for Buildings are usually specified as uniformly distributed surface loads in pounds per square foot (psf) or kilopascals (kPa)
Bridge Live Loads (AASHTO Specification): Since trucks make up the heaviest loading on bridges, AASHTO defines two systems of standard trucks H – Truck Loadings represent a two-‐axle truck HS – Truck Loadings represent a two-‐axle tractor truck with a single-‐axle semitrailer
Railroad Live Loads (AREMA Manual for Railway Engineering):
2.3 Impact When live loads are applied rapidly to a structure they cause larger stresses than those that would be produced if the same loads had been applied gradually. To account for the increase in stress due to impact, the live loads expected to cause a dynamic effect on structures are increased by certain impact percentages or impact factors. 2.4 Wind Loads
Wind loads are produced by the flow of wind around the structure. The Magnitude of wind loads depend on
Geographical Location Nearby Obstructions Geometry and Vibrational Characteristics of the Structure
ASCE 7 – 05 Determination of wind loads:
qz = 0.00256KzKztKdV2I
where • qz = velocity pressure at height z (psf) • V = the velocity in mph of a 3 second gust of wind measured 33 ft above the ground. Specific
values depend upon the category of the structure obtained from a wind map. For example, the interior portion of the continental US reports a wind speed of 105 mph if the structure is an agricultural or storage building, since it is of low risk to human life in the event of a failure. The wind speed is 120 mph for cases where the structure is a hospital, since its failure would cause substantial loss of human life.
• Kz = the velocity pressure exposure coefficient, which is a function of height and depends upon the ground terrain. Table 1-‐5 lists values for a structure which is located in open terrain with scattered low-‐lying obstructions
• Kzt = a factor that accounts for wind speed increases due to hills and escarpments. For flat ground Kzt = 1.0
• Kd = a factor that accounts for the direction of the wind. It is used only when the structure is subjected to a combination of loads. For wind acting along, Kd = 1.0
qz = Velocity pressure at height z (psf)
Kz = velocity pressure exposure coefficient Kzt = Topographic factor
Kd = Wind directionality factor V = wind speed (open terrain 33 ft above ground level)
I = Importance factor
Design Wind Pressure for Enclosed Buildings of any height:
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p = qGCp − qh GCpi( )
Where • q = qz for the windward wall at height z above the ground, and q = qh for the leeward walls, side
walls, and roof, where z = h, the mean height of the roof • G = a wind-‐gust effect factor, which depends upon the exposure. For example, for a rigid structure,
G = 0.85 • Cp = a wall or roof pressure coefficient determined from a table • (GCpi) = the internal pressure coefficient, which depends upon the type of openings in the building.
For fully enclosed buildings (GCpi) = ±0.18. Here signs indicate that either positive or negative (suction) pressure can occur within the building
Example 2.2: The enclosed building shown is used for storage purposes and is located outside of Chicago Illinois on open flat terrain. When the wind is directed as shown determine the design wind pressure acting on the roof and sides of the building using the ASCE 7-‐ 10 Specifications.
2.5 Snow Loads Design Loadings depend upon
• Building shape • Roof geometry • Wind exposure • Location • Importance • Whether or not its heated
In the case of a flat roof (slope < 5%) the pressure loading can be obtained by the following equation
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pf = 0.7CeCtIspg
• pg = ground snow loading • Ce = an exposure factor which depends upon the terrain. A fully exposed roof in an unobstructed
area, Ce = 0.8, whereas a sheltered roof located in the center of a large city, Ce = 1.2 • Ct = a thermal factor which refers to the average temperature within the building. For unheated
structures kept below freezing Ct = 1.2, whereas if the roof is supporting a normally heated structure, Ct = 1.0
• Is = the importance factor as it relates to occupancy. For example Is = 0.80 for agriculture and storage facilities and Is = 1.20 for schools and hospitals
If pg ≤ 20psf then use the larger value for pf, either computed from the above equation or from pf = Ispg. If pg > 20psf , then use pf = Is (20psf)
Example 2.3: The unheated storage facility shown is located on flat open terrain in southern Illinois, where the specified ground snow load is 15 psf. Determine the design snow load on the roof which has a slope of 4%
2.6 Earthquake Loads During an earthquake the foundation of the structure moves with the ground, the above ground portion of the structure resists the motion causing the structure to vibrate in the horizontal direction. These vibrations create horizontal shear forces in the structure. For an accurate prediction of the stresses a dynamic analysis must be performed; however, for low to medium height buildings, a static analysis can be performed. The dynamic loads are approximated by a set of static forces that are applied laterally to the structure.
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V = CsW • V = total lateral force or base shear • Cs = seismic response coefficient • W = dead load of the structure
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Cs =SDSR /IE
• Cs = the spectral response acceleration for short periods of vibration • R = a response modification factor that depends upon the ductilityof the structure. Steel frame
members which are highly ductile can have a value as high as 8, whereas reinforced concrete frames can have a value as low as 3.
• Ie = the importance factor that depends upon the use of the building. For example, I3 = 1 for agriculture and storage facilities and Ie = 1.5 for hospitals and other essential facilities
2.7 Hydrostatic and Soil Pressures Structures used to retain water must be designed to resist hydrostatic pressure. Underground structures must be designed to resist soil pressure. Soil pressure consists of both a vertical (weight of the soil) and horizontal component. The horizontal component is much less then the vertical component. When portions of the structure sit below the water table, they must be designed to counter act the combined effect of hydrostatic pressure and soil pressure. 2.8 Thermal and Other Effects Other effects such as temperature changes, blasts, shrinkage of material, fabrication errors, and differential settlements of supports may also induce stresses in the structure. Although they are not addressed in most building codes in cases where they involve significant stresses they should be considered in design. 2.9 Load Combinations Probable combinations of loads must be considered in design. To do so ASCE 7 recommends certain load combinations depending upon the type of structural design being used. ASD (Allowable-‐ stress design) Allowable stress design combines material and load uncertainties into a single factor of safety. Typical load combinations include
• dead load • 0.6(dead load) + 0.6(wind load) • 0.6(dead load) + 0.7(earthquake load)
LRFD (Load and Resistance Factor Design) Strength design separates material uncertainty from load uncertainty. Load uncertainty is accounted for by using load factors applied to the loads or combinations of loads. Typical load combinations include
• 1.4(dead load) • 1.2(dead load) + 1.6(live load) + 0.5(snow load) • 0.9(dead load) + 1.0(wind load) • 0.9(dead load) + 1.0(earthquake load)
Tributary Loadings When flat surfaces such as walls, floors, or roofs are supported by a structural frame, it is necessary t determine how the load on these surfaces is transmitted to the various structural elements used for their support. One-Way Slabs
Other types of systems may also be considered one-‐way. For the following floor system:
According to the ACI 318 code, if L2 > L1 and if the span ratio (L2/L1) > 2, the slab will behave as a one-‐way slab. Two-Way Slabs
When working with a two-‐way slab that is not square our beams and girders will have trapezoidal and triangular distributed loading.
Example 2.4: The floor of a classroom is to be supported by the bar joists shown. Each joist is 15 ft long and they are spaced 2.5 ft on centers. The floor itself is to be made from lightweight concrete that is 4 in. thick. Neglect the weight of the joists and the corrugated metal deck and determine the load that acts along each joist.