Loads - Types

48 Pages·2012·0.35 MB·English

Loads

Moayyad Al Nasra, PhD, PE
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Loads - Types
• Dead Load, D: Load of constant magnitude,
specified direction, and well defined point of
action. Ex. Self weight of beams, slabs, …etc.
• Typical unit weight of structural materials
– Reinforced concrete 150 lb/cu ft
– Structural steel 490 lb/ cu ft
– Plain concrete 145 lb/cu ft
– Hardwood flooring(7/8) in 4 psf
– Suspended ceiling 2 psf
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Loads – types cont’d
• Live load, L: Load that may change in
magnitude, direction, or position such as:
– Floor loads, specified by the design codes
• Apartments 40 psf
• Public rooms 100 psf
• Office building 100 psf
• Storage, light 125 psf
• Storage, heavy 250 psf
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Loads, types, live loads, cont’d
• Traffic Loads: Loads of varying magnitude
caused by groups of truck or train wheels
• Impact Loads: Loads due to vibration of
moving or moving loads.
– Live load impact factor
• Elevator machinery 100%
• Motor driven machinery 20%
• Reciprocating machinery 50%
• Hangers for floors or balconies 33%
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Loads, types, live loads, cont’d
• Other live loads
– Blast loads
– Thermal loads
– Longitudinal loads, stopping loads of trains, trucks,
…etc.
– Centrifugal loads, curved bridges
– Hydrostatic pressure, dams, reservoirs, water
tanks,…etc.
– Earth pressure, retaining walls, basements, … etc.
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Loads, types, cont’d
• Environmental loads
– Snow loads: 1 inch of snow is equivalent of 5
psf. Common values ranges between 10 psf to
40 psf.
– Rain loads: The best method of preventing
ponding is to have an appreciable slope of the
roof at least ¼ in per foot.
– Wind load, Hurricanes and tornados, … etc.
– Earthquake loads, seismic forces
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LRFD load combinations
• U= 1.4 D
• U= 1.2 D + 1.6 L + 0.5 (Lr or s or R)
• U= 1.2 D + 1.6 (Lr or s or R) + (0.5L or
0.8W)
• U= 1.2 D + 1.6 W + 0.5 L +0.5( Lr or s or
R)
• U= 1.2 D +/- 1.0 E + 0.5 L + 0.2 S
• U= 0.9 D +/- (1.6 W or 1.0 E)
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LRFD load combinations, cont’d
• U= the design or ultimate load
• D = dead load
• L = live load due to occupancy
• Lr = roof live load
• S= snow load
• R = nominal load due to initial rainwater or ice,
exclusive of the ponding contribution
• W= wind load
• E= earthquake load

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Example, Load combination
» W12X120, 10’ O.C

– Floor system has W12X120 beams spaced 10 ft on
center and is supporting a floor dead load of 60 psf, and
a live load of 100 psf. Determine the governing load in
lb/ft that each beam must support.

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Solution
Self dead load = 120 lb /ft
Floor dead load = 10X 60 = 600 lb/ft
Total dead load = 720 lb/ft
Live load = 10 X 100 = 1000 lb/ft
U= 1.4 D = 1.4 X 720 = 1008 lb/ft
U= 1.2 D + 1.6 L + 0= 1.2 (720)+1.6(100)=
2464 lb/ft
The governing factored load = 2464 lb/ft
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