Thermal Restraint Forces

[snowmachine88]

I have a slab supported on very short concrete members. The members are 3.5' from bottom of slab to top of the wall they are supported by and are 12" square. The slab is supported by a wall on one face. The slab is 56' x 33'. If I assume the slab expands due to a 45 degree temperature change and the load factor for force effects is 0.5 based on AASHTO, I get an expansion of 0.053" and 0.045" in the two perpendicular directions. Then if I assume the columns are fixed at the top and bottom I calculate end moments of 95 k-ft and 81 k-ft for orthogonal directions. I am not going to be able to design for this moment because as the column size increases so do the forces.

With a total displacement of approximately 0.07", can I assume the column will crack and relieve all of the thermal forces, or do I need to do something different to design for the thermal restrain loads?

I attached my calculations of the loads.

 

[hokie66]

I must admit that my comments were generic. I couldn't see any 3.5' tall columns on the attached drawing.

 

[snowmachine88]

Suppose the columns were to rotate as rigid bodies and develop monster cracks at the tops and bottoms of the columns.

I like your reasoning. My original numbers were mearly for force calculations according to AASHTO. They allow you to use the gross section and then reduce the forces from temperature (shrinkage also) by half because of stress relief from cracking. Therefore, if I use the 1.2 load factor for temperature and include shrinkage I get a total deflection of 0.448 in. .448/42*12= 0.128in assuming rigid rotation about the "toe" of the column. AASHTO lists the assumed crack width for Class I exposure as 0.017. I designed the rest of the structure for an assumed crack width of 0.011in based on the same criteria. However, AASHTO also says there is no correlation between crack width and corrosion. I am using epoxied steel throughout the structure to minimize corrosion. A crack on the order of 1/8" would be an issue, but it shouldn't open up that much in actual use.

The worst distress in the columns will be due to shortening of the slab parallel to the wall

I realized this after I said it. The best news is for the corner columns; the restraint is the greatest, the load will be maximum, and the visibility of the cracks will be easiest.

I agree that ultimately the loads will be relieved so the columns will just remain designed for the vertical loads.

I'll just have to slip out there to put some gray caulk on the columns after the slab has had a chance to cure a little and it's a cold day so the owner doesn't ask too many questions, lol.

 

[snowmachine88]

I must admit that my comments were generic. I couldn't see any 3.5' tall columns on the attached drawing.

Here's a drawing showing how the bottoms of the columns frame in. The columns in the middle of the slab are even shorter because beams frame into them in one direction.

 

[hokie66]

May be just me, but I still think it is a poor detail. I would have required the columns to be cast first, then the wall, with a soft joint between.

 

[snowmachine88]

Hookie,

I have no doubt the whole project is rife with poor details since I am a bridge designer by trade. The reason the wall is cast as a solid piece then the columns on top is due to the fact that the wall is actually a weir so I didn't want to introduce any joints below the top of the wall. I do agree that would be a good solution to this problem though. It's that kind of thinking that solves these issues. You wouldn't happen to have any more ideas, would you?

 

[hokie66]

Ideas, I am full of them. And I don't mind a good argument...as some of these other folks can testify.

 

[KootK]

When I look at unrestrained temperature movement I get a total if about 0.25 in which translates to 0.125 per end column and 0.035" cracks using the rigid body rotation business. That's about twice what you're shooting for. But then, the rigid body rotation concept is an upper bound way to look at crack width.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.