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The time effect factor λ is analogous to the load duration factor CD for ASD design. CD is not applied to LRFD design.
The main difference between CD and λ is that CD is based on the presence of load types in a combination, so that a different CD factor is applied to load combinations generated from a subset of loads than would be applied if all loads existed in the combination. λ is applied based on the combination and is the same for subsets of combinations containing fewer of the ASCE-defined types than in the full combination.
The time-effect factors are given in NDS Table N3 as
ASCE / IBC No. |
Load Combination |
λ |
1 |
1.4D |
0.6 |
2 |
1.2D + 1.6L + 0.5(Lr or S) |
0.7 (when L is from storage) |
2 |
1.2D + 1.6L+ 0.5(Lr or S) |
0.8 (when L is from occupancy) |
2 |
1.2D + 1.6L + 0.5(Lror S) |
1.25 (when L is from impact) |
3 |
1.2D + 1.6(Lr or S) + (L or 0.5W) |
0.8 |
4 |
1D + 1.0W + L + 0.5(Lr or S) |
1.0 |
5 |
1.2D + 1.0E + L + 0.2S |
1.0 |
6 |
0.9D + 1.0W |
1.0 |
6 |
0.9D + 1.0E |
1.0 |
Combinations with Live Occupancy, Impact and/or Storage Loads
It is possible that more than one of live occupancy, storage live, and live impact loads can be on the member. In that case, for load combination 2, the program generates the full combination and subsets of the combination not containing I, L, and/or Ls. The time effect factor is the largest for any of the loads in the combination.
For example, of live and live storage are on the member, then the combination with both has a time effect factor λ = 0.8. A combination is also generated only with live storage loads, without occupancy live loads, with a λ = 0.7.
This is analogous to the procedure used to determine the CD factor for combinations containing more than one load duration category.
The time effect factor for all the subsets of ASCE/NDS load combinations generated by Sizer, designating impact and live storage loads as separate load types, is shown below.
ASCE No. |
No. Sizer |
Sizer Load Combinations. |
λ |
1: Dead only - 1.4D |
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1 |
1 |
1.4D |
0.6 |
2: 2D + 1.6 L + 0.5 (Lr or S) |
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2 |
2 |
1.2D + 1.6(L + Ls) |
0.8 |
2 |
3 |
1.2D + 1.6 Ls |
0.7 |
2 |
4 |
1.2D + 1.6(L + Ls) + 0.5Lr |
0.8 |
2 |
5 |
1.2D + 1.6 Ls + 0.5Lr |
0.7 |
2 |
6 |
1.2D + 1.6(L + Ls) + 0.5S |
0.8 |
2 |
7 |
1.2D + 1.6 Ls + 0.5S |
0.7 |
2 |
8 |
1.2D + 1.6(L + Ls + I) + 0.5S + |
1.25 |
2 |
9 |
1.2D + 1.6 (L + Ls + I) + 0.5Lr |
1.25 |
3: 1.2D + 1.6 (Lr or S) + (L or 0.5W) |
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3 |
10 |
1.2D + 1.6Lr + 1.0(L + Ls) |
0.8 |
3 |
11 |
1.2D + 1.6Lr |
0.8 |
3 |
12 |
1.2D + 1.6 S + 1.0(L + Ls) |
0.8 |
3 |
13 |
1.2D + 1.6S |
0.8 |
3 |
14 |
1.2D + 1.6Lr + 0.5W |
0.8 |
3 |
15 |
1.2D + 1.6S + 0.5W |
0.8 |
4: 1.2D + 1.0W + L + 0.5 (Lr or S) |
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4 |
16 |
1.2D + 1.0W + L + Ls + 0.5Lr |
1.0 |
4 |
17 |
1.2D + 1.0W + L + Ls |
1.0 |
4 |
18 |
1.2D + 1.0W + L + Ls + 0.5S |
1.0 |
4 |
19 |
1.2D + 1.0W + 0.5Lr |
1.0 |
4 |
20 |
1.2D + 1.0W |
1.0 |
4 |
21 |
1.2D + 1.0W + 0.5S |
1.0 |
5: 1.2D + 1.0E + L + 0.5 (Lr or S) |
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5 |
22 |
1.2D + 1.0E + L + Ls + 0.2S |
1.0 |
5 |
23 |
1.2D + 1.0E + 0.2S |
1.0 |
5 |
24 |
1.2D + 1.0E + L + Ls |
1.0 |
5 |
25 |
1.2D + 1.0E |
1.0 |
6: 0.9D + 1.0W |
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6 |
26 |
0.9D + 1.0W |
1.0 |
7: 0.9D + 1.0E |
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7 |
27 |
0.9D + 1.0E |
1.0 |
Applicable Design Criteria
The design criteria that the time effect factor λ applies to for each material is indicated by a λ in the table below:
Criterion |
Sawn Glulam SCL |
CLT |
I-joist |
Sawn, Glulam, SCL |
CLT |
I-joist |
Bending |
Fb |
FbS |
Mr |
λ |
λ |
λ |
Tension |
Ft |
FtA |
n/a |
λ |
λ |
n/a |
Shear |
Fv |
Fvtv |
Vr |
λ |
n/a |
λ |
Rolling shear |
n/a |
FsIb/Q |
n/a |
n/a |
λ |
n/a |
Axial compression |
Fc |
FcA |
n/a |
λ |
λ |
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Compression perp. to grain |
Fc^ |
Fc^A |
R |
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λ* |
Stiffness |
E |
EIeff |
EI |
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Buckling stiffness |
Emin |
(EI)app-min |
n/a |
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Shear stiffness |
n/a |
n/a |
K |
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λ* - Indicates that the criterion is not implemented in generic Sizer