Thermal Analysis in Risa 3D

[karthik_m]

Hey guys, I'm doing a thermal analysis in Risa 3D. I have a couple of questions :

1. What direction is the thermal load applied in RISA? I know it has "T" direction symbol to apply a thermal load. But when applied in the model it shows that the load is applied in the downward direction. Can someone clarify this for me?

2. Do I need to include P-big delta effects in my analysis for thermal loading? Per ASCE 7-10 2.5.2.1, I'm using a load combination of 1.2 D + Ak( in my case temperature) + 0.5 L. My idea for including P- big delta effects is that earthquake and temperature occuring together is a rare combination hence when I'm doing the load combination as mentioned above I feel it makes sense to include P-big delta effects. Please let me know if that makes sense. Since thermal loading would cause expansion and contraction.

3. The building I'm analyzing has buckling restrained braced frames as the lateral system. All other connections are pinned. So I'm applying thermal loading only on the beam and columns which has the braces since it would restrain any expansion and thus cause increase in stresses. Does that make sense? If so when I'm including P-big delta effects it is diverging at some nodes and thus I'm getting an error. I checked the structure without P big delta effects and it has no instability. Also, the structure under normal gravity loading 1.2 DL + 1.6 LL performs fine including P-big delta effects. Please let me know what could be the possible cause for this?

Thank you!!

 

[Ron247]

I absolutely am not a thermal expert in RISA, but I use RISA. I always ass-u-med thermal loads only expanded or shrunk the members length only. I could be wrong. Thankfully, we have some RISA experts to clarify this. Now I am curious too.

 

[Celt83]

From the RISA-3D Manual: "..For members, these loads cause the axial expansion or contraction of the member along its length, i.e. axial stress only. The temperature is assumed constant across the member's depth. .."

My understanding is it just uses the temperature values to contract or expand a member assuming the temperature change applies uniformly throughout the whole cross section. So depending on the coef. of thermal expansion a -T would typically shorten the member and a +T would typically elongate the member.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering