Load Combinations
- As noted earlier in Snow Loads, various loads may act on a structure or
elements of a structure at the same time.
- Because of this, the code specifies various combinations of loads
(also known as load cases), that should be evaluated to determine
which cases are critical to the structure. The '97 UBC combinations
of loads are a significant change from previous editions with an
emphasis given to strength design and a move toward ASCE 7 provisions.
Load combinations for allowable stress design are specified in '97
UBC 1612.3.1. They are:
D (Eqn 12-7) D + L + (Lr or S) (Eqn 12-8) D + (W or E/1.4) (Eqn 12-9) .9D ± E / 1.4 (Eqn 12-10)
D + .75 [L + (Lr or S) + (W or E/1.4)] (Eqn 12-11)
Where D = dead, L = live, Lr = roof live, S = snow, W = wind, and E = earthquake.
- Because of this, the code specifies various combinations of loads
(also known as load cases), that should be evaluated to determine
which cases are critical to the structure. The '97 UBC combinations
of loads are a significant change from previous editions with an
emphasis given to strength design and a move toward ASCE 7 provisions.
Load combinations for allowable stress design are specified in '97
UBC 1612.3.1. They are:
- As specialized for our 1 story building, consider a wood roof system with heavy snow, wind uplift, zone
3 seismic, £ 80 mph wind. How would you use these load cases?
- Roof beam element with 4 different load cases.
- If you were to evaluate the effects, (e.g. V, M, D), of each load case on the typical
beam, you would find:
- Eqn 12-8 will generally control the design, (e.g. member size and material), of the beam.
- But, you will also need to carefully consider the effects of Eqn 12-9 because W > D. In this situation, a net upward acting load occurs causing a reversal of bending stresses and uplift forces at the beam supports. If neglected, these two items could present serious structural problems.
- A typical simple, single-story, load bearing wall section that simultaneously resists lateral and vertical loads
could realize all 5 load cases.
- If you were to evaluate the effects, (e.g. N, V, M, D), of each separate load
case on the typical wall section, you may find:
- Eqn 12-8 will control the pure compression design of the wall.
- Depending upon tributary area to wall, either Eqn 12-9 or 12-11 could control the combined bending and axial behavior of the wall.
- Roof beam element with 4 different load cases.
