Thermal Movement in Portal Frames
Thermal Movement in Portal Frames
(OP)
Hi there,
I've been using eng tips as a resource for a while, but this is the first time i haven't found my question already answered in another thread!
I'm designing a large (9x20m bays) portal frame, and the software that I'm using to model the frame doesn't seem to have the option to input temperature variations. This (and a lack of discussions about the topic on the internet/in my office) suggests to me that this is generally ignored in portal frame design. However, I'm worried about locked in thermal stresses and thermal movements in the plane of the frame. I know that the axial forces in the rafters are fairly critical to portal frame design, and I think that a combination of two things will happen, depending on stiffnesses -
- The stiffness in the frame will lock in additional axial compressive forces in the rafters (during thermal expansion), causing failure of the frame at lower loads than predicted by the portal frame modelling software
- The thermal expansion/contraction will not be sufficiently restrained by the stiffness of the frame and will cause the total deflections to increase beyond acceptable limits.
Does anyone have (preferably!) an argument to put forward to assuage my concerns, or a suggestion of how to proceed with the design? My thoughts at the moment are to do a hand calc on the expected thermal movement, and apply the forces by hand in the model to see what happens...
Thanks!
I've been using eng tips as a resource for a while, but this is the first time i haven't found my question already answered in another thread!
I'm designing a large (9x20m bays) portal frame, and the software that I'm using to model the frame doesn't seem to have the option to input temperature variations. This (and a lack of discussions about the topic on the internet/in my office) suggests to me that this is generally ignored in portal frame design. However, I'm worried about locked in thermal stresses and thermal movements in the plane of the frame. I know that the axial forces in the rafters are fairly critical to portal frame design, and I think that a combination of two things will happen, depending on stiffnesses -
- The stiffness in the frame will lock in additional axial compressive forces in the rafters (during thermal expansion), causing failure of the frame at lower loads than predicted by the portal frame modelling software
- The thermal expansion/contraction will not be sufficiently restrained by the stiffness of the frame and will cause the total deflections to increase beyond acceptable limits.
Does anyone have (preferably!) an argument to put forward to assuage my concerns, or a suggestion of how to proceed with the design? My thoughts at the moment are to do a hand calc on the expected thermal movement, and apply the forces by hand in the model to see what happens...
Thanks!






RE: Thermal Movement in Portal Frames
RE: Thermal Movement in Portal Frames
RE: Thermal Movement in Portal Frames
For your large expanse of roof, you will need expansion joints. There are a number of threads on the site, including some with references, about expansion joint spacing. I would look into dividing your roof into separate structures, each 60 x 55.
RE: Thermal Movement in Portal Frames
It all depends on the expected temperture variations and co-efficent of thermal expansion.
For a structure this long i would typically split into three sections with two movement joints.
Kieran
RE: Thermal Movement in Portal Frames
If my cladding is designed to take the thermal movement from the steelwork, and the portal frames are not stiff enough to cause any locked in stresses, then if i design the columns for the resulting out of verticality would that not be sufficient?
I agree that a movement joint would be preferable, but I am trying to avoid them if possible.
Expected temperature variations are pretty low (UK) - assuming a conservative 40 degree variation then the max movement i would be likely to get would be around 45mm - within horizontal tolerances for my column heights.
RE: Thermal Movement in Portal Frames
http://www.modernsteel.com/Uploads/Issues/April_20...
I think avoiding expansion joints in a portal framed and braced building 180 x 220 metres is destined to result in despair.
RE: Thermal Movement in Portal Frames