Wind load stress increase?

[SteelPE]

I am in the process of reviewing some calculations for the design of lt guage girts that support a foam panel siding. The local jurisdiction follows IBC 2009. The person who ran the wind load calculations used 0.75 W as a load combination. From what I can tell, this load combination is not allowed for pure wind cases. Are there any other material specific reasons why a 0.75W case would be allowed?

 

[SteveGregory]

I agree with your assessment of the the current building code.

 

[frv]

Is that the only wind load combination he used?

There is one ASD combination that contains 0.75 WL in the 2006 IBC. I think the 2009 is essentially the same. But it is only one of several combinations.

 

[SteelPE]

frv,

I agree with your assessment however, in this instance, the only load the girt can see is from wind. The girt can not be loaded by live or snow loads. So you are governed by the 1.0D + 1.0W load combination.

 

[msquared48]

Technically I think you can if you are looking at only the girt, not the whole structure.

The thought being that the lateral wind force to the whole strcture is divided between a positive force on the windward wall, and a suction force on the leeward wall. Hence, the lesser of the two would control the girt design, not the sum.

Mike McCann
MMC Engineering

 

[SteelPE]

msquared

You can take a reduction because the force is on the girt and not the whole structure? The loads applied to the girt is calculated based upon the Components and Cladding requirements of ASCE7. In this analysis you calculate windward and leeward pressures and then take the worst scenario (usually leeward) to design your member. I don't see how your argument would apply?

 

[woodman88]

SteelPE,
What wind load W are you talking about?
If the girt can only see either the windward or the leeward load (of the total wind load case) at one time, than only the windward or leeward load case would apply to the girt loading, for the total wind load case. Maybe the engineer felt that upon doing the required calculations that the the windward or leeward load to the girt would be less than 75% the total wind load and that 75% the total wind load case and would control over the components and cladding load cases or maybe they did the all the calculations and are only showing the worst case wind load calculation for the girt.
You would have to talk to the engineer to find out why the 75% was used.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[SteelPE]

woodman88

I tried talking to the engineer, he wouldn't return my calls. The client has a tight time line so I didn't want to waste to much time trying to contact him. I marked up the calculations and sent them back.

As an EOR I have the ultimate say on how I want my building designed. He was using the C&C values straight out of ASCE 7 for simplified structures. In my opinion if he uses these numbers then he must use 1.0W and not 0.75W.

He was real sneaky about it too. He calculated the moment based upon the full wind load and then took the reduction when he calculated the required section modulus (Sx=M*12*.75/Fb).

 

[JAE]

I agree with 1.0W

 

[SteelPE]

So I marked up the calculations and sent them back.

Spoke briefly with the engineer who was a little short with me and explained that he didn't use the IBC for his design,he used the equations from the AISC (which doesn't even govern the design of lt guage girts). He said he would switch everything over to strength design and that would solve his problem. OK good so far.

Get the calculations in today and he back figures the section modulus using the equation Sx=(M*12*.75/.9Fy)*1.6. So basically he is taking a stress increase in strength design when he uses 0.75*1.6=1.2 ......

 

[JAE]

Just ask him where in the world is the 0.75 coming from.

 

[pcronin]

0.75W shows up in IBC 2009 for ASD load combination Eqn. 16-13

You are allowed to use 0.70W when calculating deflection. 1.0 for strength, 0.70W for deflection. Footnote "f" on Table 1604.3 IBC 2009.

 

[JAE]

Yes but he "switched over to strength design" on the last post by SteelPE and still used the 0.75.

 

[SteveGregory]

Your "friend" is confused or on drugs. He needs to use ASD or LRFD. You can't mix and match methods.

 

[bookowski]

It should be 1.0 for pure wind.