UBC - Basis for Load Combinations 1

[KSRAO]

Could anyone explain how the load combination specifications of UBC 1612 are arrived at and basis for the associated load factors?

Also, do the load combinations/factors depend on the construction material?


-thanks,
Rao.

 

[boo1]

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.

These load cases are independent of the material.

 

[uleaum]

The load combinations are derived from practical experience, the probalility of exceeding the load of each type, as well as two general considerations. The first considerations is the probality that the maximum loads for every load type is less than 1. The second consideration is that the duration of the load affects the the required value.

There are three load case alternatives in the UBC. LRFD (or strength design), ASD, and ASD alternate load cases. Under the ASD alternative load cases a material strength increase of 1.33 is allowed for wind case loading. This is an older provision that has been repealled by concrete, and steel Standards, but still exists for Wood(in a certain light) and Masonry. Under the IBC the alternate stress increase is still referenced(as a load factor, not a strength increase), even though the material standards have repealled it and the increase cannot be used with ASCE 7-98(according to a contributing author, the factors that lead to this reduction are already including in the force calculation for wind.) I am not sure what the status of masonry is or will be with the new codes but wood still allows a 1.33 increase.

The wood increase is actually 1.6 for some wind case. Unlike other homogenous materials, wood actually does perform differently under different durations of load. The NDS has the different Load Duration Factors for different load conditions. These are material strength increases, not factors for the loads. (you end up with different values of Fb, Fv, Ft,..etc for differnt load cases.) One note, though. Table 2.3.2 on page 2-291 of Volume II of the UBC modifies the load duration factors of the NDS. 1.33 is listed as the increase value. There is a footnote saying the 1.6 may be used for members and connections, but I have had arguements with building departments about this.

Here is the footnote concerning wind:

1.60 may be used for members and nailed and bolted connections exhibiting Mode III or IV behavior, except that the increases for wind are not combined with the increase allowed in section 1612.3. The 60 percent increase shall not apply to the allowable shear values in Tables 23-II-H, 23-II-I-1, 23-II-I-2, 23-II-J, or in Section 2315.3.

Since Mode III and IV behavoir is part of connection failure in wood, I interpret the 1.6 to be usable for members, but one building department feels differently. Under the IBC the 97 NDS ASD is reference directly without modifications, but the 97 UBC references the 91 NDS ASD. The 97 NDS is more conservative than the 91 NDS so be sure of what you are doing. Also note that to use the 1.6 factor you must use the basic load combinations, not the alternate load cases.

Under the IBC the the first consideration is dealt with by allowing reductions when more that one transient load(earthquake or wind) are comibined.

In short you may use the 1.33 stress increase with the alternate load combinations(my office still does, out of habit) but this is by Code and not respresentative of Steel and Concrete current practices.

 

[lsmfse]

To expand on uleaum's point with regard to ASD, the code requires wood elements supporting discontinuous systems to be designed per 1612.4(special seismic load combinations)using omega overstrength factor and an allowable overstress factor of 1.7 It is my understanding that this is an atttempt to simulate strength design parameters for wood.