Thermal Movements of Exposed Materials

[Valstone]

I am working on a project where the project specifications state the following:

Thermal Movements: Allow for thermal movements from ambient and surface temperature changes. Base calculations on surface temperatures of materials due to both solar heat gain and nighttime-sky heat loss.

1. Temperature Change (Range): 120 deg F (67 deg C), ambient; 180 deg F (100 deg C), material surfaces.

My question is do I determine the thermal movements based on a temperature increase of 180 degrees or do I determine the thermal movements based on the differential of the ambient and material surfaces range, which in this case would be 60 degrees.

Any enlightenment would be greatly appreciated!!

 

[Ron]

The surface temperature will determine the thermal movement of the material. To give you some perspective on this, go out to a building with some dark-colored aluminum trim. Check the ambient temperature, say 90F. Put your hand on the aluminum trim at, say, 3:00 pm on this sunny day.....see how long you can hold your hand on that trim! It will be significantly warmer than 90F.

 

[ishvaaag]

I don't agree with Ron, this time.

As I understand the specs (rather vague, more when there are even more complete specs in some codes) the intent of the same is that you consider 2 situations, one where you impart 180 deg F to the surfaces and then examine what happens to the building (or part of your interest) and the other where the ambient temperature has a difference of temperature 120 deg F with your building or part of interest.

Of course if you are interested only in some aluminium cladding the inner part of wich is not air conditioned, the steady state temperature average in the thickness might attain soon an average higher than the 120 deg F (even if the analysis of consideration seems to be the steady state, and not a comparison with the average, for the case).

For other more traditional structures, and massive reinforced concrete structures the change in temperature towards the interior can progress little if conditioned (or buried etc), and hence to take the surface temperature as the one of interest for the structural design of the building would be excessive.

So I think that the code can give light (and may be override) your specs ... for example, for the steady state you will need to make some assumptions not contained in the specs, that can entirely change the thermal movement and stresses resulting from the analysis.

 

[Ron]

Valstone...reading your posting again, I will add that you are given two criteria that might result in different magnitudes of movement. In that case, you check both and consider the worse case. I believe ishvaaag is pointing this out as well.

As ishvaaag notes, the steady state will control in some instances, particularly for those not exposed to direct sunlight or other direct thermal influence.

...and ishvaaag...you are correct. Your answers were more complete than mine and you pointed out the disparity between surface and ambient in some cases; however, whether for structural or trim, the condition that creates the most movement is the one for which that movement must be considered and compensated.

 

[Valstone]

What I have is an acrylic and glass enclosure. I believe the inside will be temperature controlled, but the outside of the wall and roof elements will be exposed to the temperature. I need to provide the thermal movements which will be easy for the roof elements, but the wall elements will be a little more difficult since I need to provide vertical support, but allow the panels to move vertically. I think I will use 180 degrees to be safe, but I haven't seen any industry standard for this specification section.

 

[ishvaaag]

If you use 180 ºF as a general increment of temperature you may end not staying safe, since the actual distribution of temperatures in the outfit may be causing curvatures pushing too much the deformed glass against points that can't yield against the push, so causing the glass to break.