CE 382 L3 - System Laoding

8/9/2019 CE 382 L3 - System Laoding

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System Loading

Tributary Areas

Many floor systems consist of a

reinforced concrete slab sup-ported on a rectangular grid ofbeams. Such a grid of beamsreduces the span of the slab andthus permits the designer toreduce the slab thickness. Thedistribution of floor loads on floor

beams is based on the geometricconfiguration of the beamsforming the grid.

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Tributary area of columns A1,

B2 and C1 shown shaded

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Girders on all four sides

Theoretical Tributary Areas

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Theoretical TributaryBeam Areas

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Theoretical TributaryBeam Areas

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Floor Beam Girder

Typical Floor Framing System

Simplified Floor Beam andGirder Loadings

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Example Load

Distribution Problem

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The floor system of a libraryconsists of a 6-in thick rein-

forced concrete slab resting onfour floor steel beams, which inturn are supported by two steelgirders. Cross-sectional areasof the floor beams and girdersare 14.7 in 2 and 52.3 in 2,respectively as shown on the

next page figure.Determine the floor loads on thefloor beams, girders, and

columns.


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Floor Slab Floor Beam Girder Column Schematic

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Building Live LoadReduction

Recognizing that the probabilityof supporting a large, fully loaded

tributary area is small; buildingcodes permit reductions in thestandard (L0) design live loadswhen the influence area (AI =KLLAT) is larger than 400 ft

2

(37.2 m 2) as given in thefollowing formulas:

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0

LL T

15L L 0.25

K A

= +

US Units

0LL T

4.57

L L 0.25 K A

= + SI Units

L

reduced live load0.50 L 0 L L0

for single floor members

0.40 L 0 L L0for multi-floor members

AT

tributary area ft 2 (m2)10


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KLL- element live load factors(IBC2000 Table 1607.9.1)

Type of Element K LLInterior column 4

Exterior column withoutcantilever slabs

4

Edge columns with cantileverslabs

3

Corner columns withcantilever slabs

2

Edge beams without

cantilever slabs2

Interior beams 2All other beams 1

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Load Combinations forStrength Design

The forces (e.g., axial force,moment, and shear) producedby various combinations of loads

need to combined in a propermanner and increased by a loadfactor in order to provide a levelof safety or safety factor.Combined loads represent theminimum strength for whichmembers need to be designed,also referred to as required factored strength . ASCE 7-98has specified the following load

combinations:


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(1): 1.4 D(2): 1.2 (D + F + T) + 1.6 (L + H)

+ 0.5 (Lr or S or R)(3): 1.2 D + 1.6 (Lr or S or R)

+ (0.5 L or 0.8 W)(4): 1.2 D + 1.6 W + 0.5 L

+ 0.5 (Lr or S or R)(5): 1.2 D + 1.0 E + 0.5 L

+ 0.2 S

(6): 0.9 D + 1.6 W + 1.6 H(7): 0.9 D + 1.0 E + 1.6 H

The load multipliers are based onthe probability of the loadcombination occurring as well asthe accuracy with which the

design load is known.


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D = Dead loadL = Live loadLr = Roof Live loadW = Wind loadE = Earthquake loadS = Snow load

R = Rain loadF = Flood loadT = Temperature or self-

strain loadH = Hydrostatic pressure load

Design of a member or of a

segment of a member must bebased on the load case thatproduces the largest force/stress/displacement value.


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AASHTO LRFD Loading


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CE 382 L3 - System Laoding