boundary conditions in a 3D HVAC model due to thermal loading

[henerythe8th]

I am currently working on an analysis for a HVAC duct system in a nuclear facility. The analysis is necessary to determine what type/strength of supports are necessary in the system as well as determining the deflections at various connections to this system. The system has a temperature change of 200degF that causes "growth" in the system. It is being debated whether the boundary conditions should restrict motion in the three orthographic directions or whether the moments about these axes should be restrained as well. Can you please lend your expertise on this matter?

Also, what is the preferred method to define an expansion/isolation joint?

 

[DRDearth]

There are several ways to model the Expansion joints. One is to use contact/surface gap elements. Another way is to model nonlinear deflection material models, i.e. for a defined amount of strain (deflection) no load resistance, then after the defined deflection is obtained, kick in the stiffness of the elements.

How you define your boundary conditions can only be determined by looking over you drawings. You will have to make that decision.

David R. Dearth, P.E.
Applied Analysis & Technology
E-mail "AppliedAT@aol.com"

 

[henerythe8th]

I am currently modeling the expansion/isolation joints with "beam releases" to allow the axial growth in the system.

As far as the boundary conditions are concerned, I guess what I really meant to ask was, "In a "normal" HVAC system do the duct supports typically restrain motion in the three principle/orthographic directions only OR do they also restrain the moments about the principle/orthographic axes?"

From what I've seen it would be a pretty particular type of support that would restrain the moments as well. This is somewhat of a "bum deal" because in the fairly complex geometry of a multiple HEPA filter bank area, I am having trouble adequately restraining the system without the use of moment restraints
OR
if I model without them the software locks them...

Thanks for your input...