Thermal Effects on Steel Beam in Storefront

[KHoff]

I have a project currently under construction that includes a segment of storefront approximately 30 feet in length with a 5 foot cantilevered canopy attached. A steel tube was designed to span through the storefront for support and attachment of the canopy. This tube is rigidly attached to a precast wall on each end. I feel that this is a fairly common condition and I have not had issues with this on other projects, but in this case the contractor is seeing some cracking occurring at the tube steel beam connections and this is being attributed to thermal effects as the temperature is dropping. I can see how temperature could be a factor, but I'm wondering if there is more going on here. One thing that might be different in this case is that the interior space is not yet conditioned. If it were I imagine that would help alleviate thermal issues to some extent. If that is a factor, is it the responsibility of the engineer to design for such a condition that would only occur during construction? Also, it seems that getting a connection to work for the required loads (vertical, lateral, torsion) that also allows for longitudinal movement would be difficult.

I appreciate any suggestions on how to approach this problem, both as a new design and how to potentially fix the issue.

 

[Agent666]

What type of cracking? A photo would help a lot towards diagnosing what may have occurred?

 

[KHoff]

Here is a picture. Hopefully this helps.

 

[KHoff]

And the other end.

 

[EZBuilding]

It is hard to see the connection through these pictures. Can you expand on the connection type? Is it possible that excessive heat was applied during welding which caused expansion/contraction of the steel elements and created these cracks? Are the pictures of the same tube, or different along a span of members?

 

[3DDave]

What is the attachment? It looks like there is no accommodation for differential thermal expansion/contraction.

 

[KHoff]

The steel tube is welded directly to an embed provided in the precast. As I said, I understand that temperature change is probably a factor, although I've never had an issue before on this type of condition. Any thoughts regarding the interior space being conditioned vs. not? Are you suggesting you would always have a connection that allows longitudinal movement? What might that look like?

 

[3DDave]

Ok. Typical steel has an elastic modulus of 30 psi/ppm and a thermal coefficient of expansion of 12 ppm/degree F. From that, it is roughly 360 psi/degree F. A 10F delta is therefore 3600 psi.

This assumes as a first order approximation, that the concrete does not move. Multiply the difference in temp (deg F) from when the steel was installed by 360 psi/ degree F to the current temp and then by the section area of steel to find the total retention force required.

Since the cracking looks really similar in extent on both ends I expect the load is elastically deflecting the embedment and fracturing the concrete over it. Though the cracks widen away from the beam, it looks like the amount of deflection is not very large; you can divide the total crack width by the overall beam length, divide by 1 Million and multiply by the elastic modulus of 30 psi/ppm to understand the current load.

 

[KHoff]

New piece of information. On our drawings we indicated the reaction of the tube steel beam at the precast, including vertical load, lateral load and torsion. Apparently the precast engineer made the assumption that we gave a total load and divided by 2 to get the reaction at each end of the beam. So the embed is designed for half of the required load. The canopy is not installed at this point, so I don't believe this caused the cracking, but it is another issue that will have to be addressed.

So in summary, we have concrete cracking potentially caused by thermal contraction in combination with an embed connection designed for half of the required load.

 

[IRstuff]

If the temperature is dropping, the beam is shrinking, causing it to pull on its anchors, so sounds plausible that the concrete couldn't take the tension.

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[Agent666]

Do you have a drawing of the cast in item so we can relate it to the cracking. Certainly crack pattern seems to indicate some tensile loading may have popped off the cover concrete.

 

[KHoff]

Unfortunately this specific connection was omitted from the precast submittals we received.

At this point I am convinced that thermal contraction has caused the cracking. I admit this was not considered in the design, in most cases where the tube is attached to structural steel it would not be an issue as the steel would simply deflect (we are talking about a change in length in the order of 1/10 of an inch). Here it should have been considered as the tube is attached to a rigid precast wall on both ends.

Now, the bigger issue becomes how is the canopy to be supported when the embed connection is not adequately designed...