Truss analysis loads to consider concurrent

[AdamJSR]

Writing this from phone. Please excuse mistakes....

When analyzing a truss what loads are considered concurrently? I thought that I had read for analysis of roof loads we don't have to consider them concurrent, but I can't find hard reference in any codes (or I'm just not looking in right place)!

Thanks guys.

 

[paddingtongreen]

You are kidding,right? Or are you able to tell the wind when to blow, the dead load to stop pushing while the live load pushes etc.?

Off hand, the only one I can think of where you would be right is that we don't combine wind and earthquake.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[AdamJSR]

Well. That's why I was asking. I know in design that they are considered with factors, but for analysis I thought I read differently.

 

[AdamJSR]

Let me explain a bit better. If I want to analyze an existing truss for a rooftop installation I find the snow load, wind load, and dead loads. Then I find member loads and stresses, compare them with the lumber type used in each member.

Would I take the governing loads (typically the snow load) plus any dead loads only ( I thought I had read this someplace) or would I have to consider all forces acting concurrently? I am not designing, I am only analyzing if there is additional capacity.

Thanks

 

[JAE]

The building code in your area (wherever that is) should have load combinations dictated.

These would be the various combinations of your dead, snow, wind, etc. that must be checked - each and every combination provided.

 

[AdamJSR]

Ok. That's what I thought. I just seemed to think that it was different for the analysis of existing members, as opposed to design of new members.

I am basically using the load combination in the ASCE-7 and analyzing from there.

Thanks

 

[AdamJSR]

Another quick Question....

When I plug-in my loads in the load combinations provided in ASCE-07 I am a bit unsure of how to apply.

For instance, 2.4 Load combination number 1 = D + F. Since I am trying to analyze a truss that has top chord DL, and a bottom chord DL, would I just sum the DL's and plug it into the load combinations?

Thanks for the help.

 

[SteelPE]

Are you sure you know what you are doing?

Apply the load combinations how they are shown in IBC. If you don't have am F then F=0 and your combination would just use dead only.

 

[AdamJSR]

Quite sure I know what I am doing. I am a bit new to trusses, but I am trying to learn. I am a PE, but work under another PE with a great deal of experience. IT is always more beneficial to learn on my own, as opposed to just being told how to do it. So I go and run some numbers, think of questions and pose them here. This is not my full-time job, but I like doing it and want to learn more about it. I figure this forum is a great reference with a wide and deep knowledge base.


I guess this is my path forward.....Once the DLs and LL's are calculated the member size and grade is chosen to resist said loads after the frame is analyzed to determine member forces and stresses. This I can do easily. I get a bit stumped when it comes to actually plugging in Loads into the formulas to determine the worst case scenario of load combination. Since I have 2 DL's (TC and BC) do I just sum them to get "D", then plug in that summarized "D" into the load combinations and determine a total load (worst case of 1-8). then go back and use that Total factored load case to analyze the truss?

Thanks again for the guidance. I appreciate it.

 

[structSU10]

Anything that is a specific type of load need be applied when it is being combined in a load combination. The truss as a whole has dead load applied on top and bottom chord, so that is you D. Remember with snow that you have multiple conditions if there is a sloped roof.

You need to also consider bracing of the various members, which includes checking the tension chord as a brace for the truss web members, to ensure it is strong and stiff enough for the desired behavior.

 

[AdamJSR]

Thank you for the input.

 

[XR250]

Also, the NDS gives a nice adjustment on the capacity side of the equation for wind- if needed

 

[woodman88]

Way before the IBC/IRC the bottom chord live loads were considered to be non-concurrent with the top chord live loads. But then they found out just how much stuff the home owners were putting up in the attic space of trusses.

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.

 

[AdamJSR]

So, when a truss designer goes out to design a truss he will also take the load combo's, or would that be the job of the engineer/architect specifying the truss?

For instance, if I tell them that we are in a 60psf GSL area, would they reduce that snow load for the pitch of the roof? Would I determine all loads (Wind, and snow), and convey that to the truss manufacturer? OR would I take the worst case scenario of the load Combo's and tell them that is what I want to truss designed to, plus bottom chord DL, and any additional TC DL?

Just looking to get more insight. Thanks guys.

 

[woodman88]

If you want to know what the truss designer designs the their trusses for. Look at the information that the IBC/IRC requires on truss designs.

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.

 

[ztengguy]

dont forget to check service limit states too.

 

[AdamJSR]

Service Limit States.. Will do.