Vukazich CE 160 Live Load Notes [L5] 1

CE 160 Notes - Live Load Live Load – gravity loads that can vary in magnitude and position (e.g. furniture, people, vehicles). Live loads are usually prescribed by codes (e.g. IBC table 1607.1 - floor live loads for buildings) Roof live loads are prescribed along the horizontal plane for sloping roofs.

Floor Live Loads (L) IBC Table 1607.1 gives minimum floor live loads based on use or occupancy of structure Examples

Residential Floor: 40 psf Sidewalk: 250 psf or 8000 lb point load Schools

Classrooms: 40 psf or 1000 lb point load Corridors above first floor: 80 psf or 1000 lb point load First floor corridors: 100 psf or 1000 lb point load

Lr – roof live load (Lab 5)

D – dead load (Lab 4)

12 in

F (rise in inches)

Vukazich CE 160 Live Load Notes [L5] 2

Vukazich CE 160 Live Load Notes [L5] 3

Floor Live Load reduction is permitted in most cases for members with large influence area (AI) Tributary Area (AT) – Area assumed to load a given member (Lab 3) Influence Area (AI) – Total area assumed to influence the loading of a member. In practical situations, loads outside the tributary area may influence the performance of a member.

AI = KLLAT AT = Tributary area of member in ft2 KLL = Live Load Element Factor (IBC Table 1607.9.1)

The consideration of the influence area in determining the unit live load is based on the probability that high unit loads are more likely to occur over small influence areas and less likely to occur over a large influence area.

Vukazich CE 160 Live Load Notes [L5] 4

If AI > 400 ft2

Then the floor live load is

𝐿 = 𝐿! 0.25+15𝐴!

where AI is in units of ft2

where

LO = Basic (unreduced) floor unit live load from IBC Table 1607.1 AI = KLLAT AT = Tributary area of member in ft2 KLL = Live Load Element Factor (IBC Table 1607.9.1)

Note: If AI < 400 ft2 then L = L0 (no reduction) If L0 > 100 psf then L = L0 (no reduction)

⎩⎨⎧

=columns) (e.g. floors moreor 2 supporting membersfor 4.0

beams) (e.g.floor 1 supporting membersfor 5.0

0

0min L

LL

Roof Live Loads (Lr) The IBC Basic Roof Live Load is 20 psf. The basic roof live load is the same for all roofs as it is associated with roof maintenance.

Roof Live Load reduction is permitted for members with large tributary area (AT) and for roof slopes in excess of 4 inches per foot.

𝐿! = 20𝑅!𝑅! where Lr is in units of psf

𝑅! =1 𝑓𝑜𝑟 𝐴! ≤ 200 𝑓𝑡!

1.2− 0.001𝐴! 𝑓𝑜𝑟 200 𝑓𝑡! < 𝐴! < 600 𝑓𝑡!

0.6 𝑓𝑜𝑟 𝐴! ≥ 600 𝑓𝑡!

𝑅! =1 𝑓𝑜𝑟 𝐹 ≤ 41.2− 0.05𝐹 𝑓𝑜𝑟 4 < 𝐹 < 120.6 𝑓𝑜𝑟 𝐹 ≥ 12

where AT = Tributary area of member in ft2

F = roof slope in inches of rise per foot

Note: Minimum allowable 𝐿! = 12 𝑝𝑠𝑓

Vukazich CE 160 Live Load Notes [L5] 5

Lab #5 Problem

1. For our example steel building, calculate the roof live load and draw the

idealization of the following members: A. One of the W14x22 beams between lines A and B loaded with both dead

load and roof live load (recall that the dead load is 24 psf); B. The W18x50 girder on line 2 between lines B and C loaded with both dead

load and roof live load (recall that the dead load is 28 psf); C. The W10x33 column at C-1 loaded with both dead load and roof live load.

Find the compressive load that the column must support, PD+Lr (recall that the dead load is 29 psf);

Vukazich CE 160 Live Load Notes [L5] 6

2. For the idealization of the W14x22 beam found in problem 1.A., loaded with dead

load and roof live load, draw the V and M diagrams using the “usual” civil engineering sign convention and indicate the maximum shear and maximum bending moment.

3. For the idealization of the W18x50 girder found in problem 1.B., loaded with

dead load and roof live load, draw the V and M diagrams using the “usual” civil engineering sign convention and indicate the maximum shear and maximum bending moment.

4 ft

16 ft

PD + Lr