1997 UBC Load Combinations

[dozer]

In section 1612.3.1 of UBC '97 one of the load combinations is 0.9D +- E/1.4 and it states specifically that no increase in allowable shall be used except for some mumbo jumbo in section 1809.2 that applies to soil. The next section, 1612.3.2 "Alternate basic load combinations" also includeds 0.9D +- E/1.4. This section says, "When using these alternate basic load combinations, a one-third increase shall be permitted in allowable stresses for all combinations including W or E."

So my reading of this is that if I use the alternate basic load combinations I can use a one-third increase in allowables for the 0.9D +- E/1.4 combo but if I use section 1612.3.1 I cannot use it for this same combo.

Is anybody confused besides me? Can anybody explain this?

 

[kvram]

Dear Mr. Dozer,

These two clauses are creating confusing. 1612.3.1 , it is a basic load case of 0.9D +/- E/1.4 where in dead load only contributes. There is no increase of allowable stresses.
1612.3.2( Alternate load comb), in all combinations either live or snow loads are added in combinations . Hence allowable streses can be increased or permitted. In this , 0.9D+/- e/1.4 not to be included .
Definitely it is a confusing statement.

 

[boo1]

Prior to the '97 UBC and the 1612.3.1 load combinations the UBC permitted an allowable stress increase of 1/3 if the corresponding load case involves either wind or seismic forces.
example: fb </= 1.33 Fb'
Known as the allowable stress increase factor. Recognize that it is unlikely that all of the loads in the respective combinations will be at full value. Factor is only applicable when design is based upon allowable stress methodologies. This factor, currently can only be used with the alternative load combinations of '97 UBC 1612.3.2.

Load combinations for allowable stress design are specified in '97 UBC 1612.3.1. They are:

D
D + L + (Lr or S)
D + (W or E/1.4)
.9D ± E / 1.4
D + .75 [L + (Lr or S) + (W or E/1.4)]

 

[dozer]

kvram,

So are you saying that they just should not have included 0.9D +- E/1.4 in the alternate basic load combinations or are you saying that you just have to know that they really didn't mean it was okay to use a 4/3 increase for this particular combination regardless if your using section 1612.3.1 or 16121.3.2?

Either way, I think we agree that the 4/3 increase should not be used with 0.9D +- E/1.4. Right?

 

[JAE]

I do know that the 0.9D +/- E/1.4 combination was actually not included in 1612.3.2 in the first printing, but was added in an errata addenda soon after.

My take is that actually have to follow the precise wording of the code and therefore you do utilize the 1/3 stress increase on it IF you are designing under 1612.3.2 but not if you are using the combinations of 1612.3.1.

While this seems to be inconsistent, keep in mind that this particular load combination is PRIMARILY used for overturning checks and many times does not control the strength calculations of the members or primary connections.

 

[kvram]

Dear Mr. Dozer,

Yes. 0.9D+/- E/1.4 should not have included in alterante load combinations.
At this combinations , seismic force to be arrived for dead load contribution which is there for 100%. Hence sresses need not be increased.