Load Combinations

[MOOK]

In IBC 1605.4 Page 283.

It was mentioned that these two equations are valid for both LRFD and ASD methods.

1.2D+ f1L + Em (eq 16-22)

0.9D + Em (eq 16-23)

I always thought these equations are valid ONLY for LRFD method while the following three equations are valid for ASD method

D+0.70E

D+0.75(0.7)E

0.6D+0.7E

Am I missing something??

 

[Hromis1]

Mook,

I just had a discussion about this the other day. The opinion of the engineer I spoke with was

Thats these equations take into the account the likelyhood of the live load actually being present via the f1 factor.....(varies from 0.5 to 1.0)

They make a difference on odd structures.

Typing this for the 2nd time. Hope the network does not crash again....

Hromis1

 

[bcy]

In IBC 1605.4 Page 283. It was mentioned that these two equations are valid for both LRFD and ASD methods.

1.2D+ f1L + Em (eq 16-22)

0.9D + Em (eq 16-23)

I always thought these equations are valid ONLY for LRFD method while the following three equations are valid for ASD method

D+0.70E

D+0.75(0.7)E

0.6D+0.7E

Am I missing something??

I would follow ASCE 7 Section 12.4.3.2 for load combinations with overstrength factor. It seems to me that this was an overlook in IBC 2006. In IBC 2009, these clauses are re-written in section 1605.1.

 

[MOOK]

Hormis

I really did not get what you mean.

bcy

I used to use ASCE equations with either redundancy factor or with over strength factor depending on the case.

 

[Hromis1]

Mook, that is ok, some days I do not know what I mean.

To make this clearer..our opinion was that this was a way to "back-door" in a seismic live load reduction for lightly loaded structures (ie the f1 factor in equation 16-22). This only applies to live loads less than 100 psf. (this equation is the additive case = gravity and seismic work in the same direction.)

bcy is correct...I think this equation was only intended to be used with the overstrength analysis.

equation 16-23 appears to be the opposite case. Normally this "subtractive" case would be for uplift on columns.

The equation has you use 0.9D+Em (16-23), I compare it to 0.9D+E/1.4 (16-21). It appears there is a penalty in this case. Penalty may not be the correct phrase. Rather that 1.4 reduction value cannot be used. (Note the equation the x/1.4 value is numerically almost the same as the 0.7E value in equations 16-12 and 16-15 etc).

That was a really good question! Maybe bcy has some more to add.

 

[bcy]

I just happened to find this article talking about this. see link (page 16, hope it works):

http://gostructural.epubxpress.com/wps/por...zFvA!!/