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.

 

[KootK]

0.07" sure doesn't sound like much does it? A couple of thoughts:

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.

 

[snowmachine88]

KootK,

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.

 

[CELinOttawa]

Funny to see another engineer recommend Capacity Design up here in the NA of the world... I agree with Kootk; Remember that your detailling is the trick in making that work!

 

[KootK]

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.

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.

Funny to see another engineer recommend Capacity Design up here in the NA of the world

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.

 

[CELinOttawa]

Capacity Design is the core assumption for 90% of the seismic work done in New Zealand... It is far superior to most other methods, in my honest opinion.

I admit to being biased, however. I learnt my seismic skills in NZ.

 

[snowmachine88]

I have not used capacity design before. Should my calculated shear (2*M*Overstrength)/3' use my moment from my gravity loads then? Also, should my over strength factor be 1.33, I think I saw that at one point.

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?

Remember that your detailling is the trick in making that work!

What would the detailing look like to make the capacity design work?

 

[KootK]

I have not used capacity design before. Should my calculated shear (2*M*Overstrength)/3' use my moment from my gravity loads then

You'd want to use the yield moment capacity of the columns. 1.25 Fy for the rebar.

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?

Yeah, that's about the gist of it. Good instincts.

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?

I don't see a need for this.

What would the detailing look like to make the capacity design work?

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.

 

[CELinOttawa]

You may want to have a look at NZS 3101. It has explicit detailing requirements for this... The Kiwis have done a lot of research on it!

 

[snowmachine88]

Kootk, I would think the structure underneath will see less thermal strain. There could be water on the lower structure so the temperatures will likely be different, as well as shading from the above structure. I agree shrinkage may be an issue also. Will the slab crack and relieve these stresses?

CEL can you give me a link to NZS 3101?

 

[KootK]

Will the slab crack and relieve these stresses?

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.

 

[CELinOttawa]

The real value of the NZS is the best commentary... Their code presumes you know everything (nothing cookbook at all), but the commentary explains everything right down to the smallest fundamental. It is far easier to use than either ACI or the CAN/CSA equivalent.

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.

 

[snowmachine88]

If it's an issue, you could throw in some delay strips.

What is a delay strip? Based on your comment I assume it will make the slab crack before the columns?

Don't splice your column verts.

I assume this also means no hooks into the slab? Just continue straight bars to w/in 2" of the top of the slab?

Provide 10M, one piece ties at 4" o/c with 135 hooks.

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?

 

[KootK]

What is a delay strip?

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.

I assume this also means no hooks into the slab? Just continue straight bars to w/in 2" of the top of the slab?

Hooks are fine. In general, get yourself as much development length as possible at either end of your column verticals.

Is spacing key here, or can I use 13M (#4) bars @ 6-8" for the same area of steel?

Spacing is key here. Keep it tight.

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?

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.

 

[hokie66]

I would disagree with my colleagues here. Very short columns supported on walls are notorious for cracking due to slab shrinkage, and to a lesser extent to temperature. The fact that they crack is not the issue, but the cracking is often very unsightly, and with loose bits that fall out.. I have seen these short cracked columns replaced with steel.

 

[snowmachine88]

Hookie66,

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!

 

[hokie66]

No, it doesn't matter. The worst distress in the columns will be due to shortening of the slab parallel to the wall. Steel in the slab doesn't prevent the shortening, it just helps to control slab crack widths due to direct tension. Perpendicular to the wall, the problem may be less, but all perimeter columns, regardless of height but especially short columns, tend to crack on the exterior due to slab shortening.

 

[KootK]

My gut instinct is still that nothing needs to be done here. The system is just too small in plan. Suppose the columns were to rotate as rigid bodies and develop monster cracks at the tops and bottoms of the columns. Using OP's numbers, those cracks would be 0.053 x 0.5 x 1/42 x 12 = 0.0076 in wide. Shrinkage may well be worse but the same logic should apply.



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

 

[CELinOttawa]

Agreed... The source of the forces doesn't matter, only that we are able to handle the loads. With the limited size of the slab, we should only see minor loads.

I sit ready to be corrected... One does not idly disagree with Hokie66!

 

[KootK]

I started my Xmas holiday this morning so I feel as though I have the emotional resources available to go toe to toe with the Gandolf of Eng-Tips. I might even have another shear friction thread left in me.

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