Thermal Restraint Forces
Thermal Restraint Forces
(OP)
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.
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.






RE: Thermal Restraint Forces
1) Once the columns crack, their moments of inertia will drop substantially. Depending on how much axial load you've got on your columns, you may be able to take advantage of that.
2) You might be able to capacity design your columns. Assume that you get flexural hinges at the top and bottom and make sure your shear works comfortably at that level of load.
Are the columns part of your lateral system?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Thermal Restraint Forces
The axial load on the columns is very small, on the order of 5 kips.
If I use the same assumptions to calculate shear I get 54K and 46k in orthogonal directions, whereas a 12" square column can only support up to 60k with maximum steel.
The columns are not part of the lateral system. The wall should support all of the lateral force and provide restraint I am worried about.
I attached the drawing of the situation. I'm looking specifically at the lower larger slab. The supporting wall is at El. 842 (about 3.5' below the bottom of the slab). Also, it is hard to tell from this detail, but there will not be a beam on the north and south column lines.
RE: Thermal Restraint Forces
RE: Thermal Restraint Forces
I'm not sure but you may have misinterpreted my suggestion regarding shear. It wouldn't be the thermal shear that you'd design for. Rather, it would be phi_V = (2 x M_pr x overstrength)/3'. Same kind of capacity design that we do for seismic.
I actually got the idea from an article I read a few years back. An engineer used the same idea to design the steel columns at the far ends of some giant stadium trusses. I wish I could find the article...
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Thermal Restraint Forces
I admit to being biased, however. I learnt my seismic skills in NZ.
RE: Thermal Restraint Forces
Then is my shear capacity only dependent on my rebar, since I am assuming the concrete has sheared? So my shear capacity would be Phi*As*fy*0.6?
Would it make any sense to stop the steel at the base of the slab and put a bit. felt joint between the tops of the columns and the bottom of the slab as a bond breaker? Would I need to introduce something more like an elastomeric bearing pad that is used in bridges?
What would the detailing look like to make the capacity design work?
RE: Thermal Restraint Forces
You'd want to use the yield moment capacity of the columns. 1.25 Fy for the rebar.
Yeah, that's about the gist of it. Good instincts.
I don't see a need for this.
Mostly lots of stirrups and no laps in the plastic hinging regions.
I'm kind of torn with this. I've actually seen exactly the kind of issues that you're worried about with short columns. However, my gut instinct is that you'll be fine here. Any chance the structure at the bottom of the columns will be subject to the same thermal strains as the structure at the top of the columns? Shrinkage may actually be a bigger issue.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Thermal Restraint Forces
RE: Thermal Restraint Forces
CEL can you give me a link to NZS 3101?
RE: Thermal Restraint Forces
Perhaps. But only if the columns don't crack first. If it's an issue, you could throw in some delay strips.
While the NZS document will contain some interesting goodies, everything that you need to capacity design your columns should be available in your local code. ACI 318 and CSA A23 both have seismic chapters where this information will be found.
For your problem, you can probably just simplify the special detailing it to this:
1) Don't splice your column verts.
2) Use four column verts as small as you can make 'em.
3) Provide 10M, one piece ties at 4" o/c with 135 hooks.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Thermal Restraint Forces
Mind you, for full disclosure, I haven't worked with the ACI very much.
Sorry, I don't know of a free link for the NZS. Actually buying yourself a copy of "Reinforced Concrete Structures" by Park and Paulay is probably a better investment if you aren't going to be using NZS to design.
RE: Thermal Restraint Forces
What is a delay strip? Based on your comment I assume it will make the slab crack before the columns?
I assume this also means no hooks into the slab? Just continue straight bars to w/in 2" of the top of the slab?
Is spacing key here, or can I use 13M (#4) bars @ 6-8" for the same area of steel?
Finally, when I listed my shear strength as phi*As*fy*0.6, I am assuming I should use my main steel As since the shear would be through the vertical bars and not really engage the ties. However, then I still need ties since the shear will be transferred down into the column at that point and then I can use the concrete+tie strength. Is that the correct assumption?
RE: Thermal Restraint Forces
Delay strips are strips of slab that are not poured until some time after the adjacent slabs. The idea is to allow as much slab shrinkage to take place prior to locking the slabs on either side of the delay strip together. It doesn't cure all shrinkage woes, but it helps. It's particularly helpful when you've got stiff lateral elements on opposite ends of long slabs. In those situations, slab shrinkage tends to draw the lateral elements together and, theoretically at least, generate very large forces.
Hooks are fine. In general, get yourself as much development length as possible at either end of your column verticals.
Spacing is key here. Keep it tight.
It will be your column ties that deal with shear within your columns. The vertical bars should be used only for shear friction at the interfaces between the columns and your slabs.
I recommend that you consult the seismic chapter of your local concrete code and work out a solution that is consistent with the detailing requirements expressed there. You don't want to be basing your design based solely on our discussion. For what it's worth, my jurisdiction is Canada.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Thermal Restraint Forces
RE: Thermal Restraint Forces
Does it matter that the short columns are supported on a wall of the same thickness? The wall will be stiffer than the columns, but less so than a typical footing. It may allow for some additional movement. Would additional steel in the slab help reduce the shrinkage enough to matter? Thanks for the input!
RE: Thermal Restraint Forces
RE: Thermal Restraint Forces
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Thermal Restraint Forces
I sit ready to be corrected... One does not idly disagree with Hokie66!
RE: Thermal Restraint Forces
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Thermal Restraint Forces
RE: Thermal Restraint Forces
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.
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.
RE: Thermal Restraint Forces
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.
RE: Thermal Restraint Forces
RE: Thermal Restraint Forces
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?
RE: Thermal Restraint Forces
RE: Thermal Restraint Forces
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.