horizontal thrust - rigid frame to concrete footing 4

[jimbo2]

I have been asked to specify a foundation for a rigid frame building with a 70 foot clear span. The horizontal thrust at the base plate, as provided by the building manufacturer, is 19 kips. I have determined to use a 3/4" wire rope tie between opposite piers to resolve this lateral load, terminating the cable in a 10" x 10" steel plate and pin. The column base plate is 8" x 12" with four 3/4" anchor bolts.

I understand the base plate/concrete surface will provide "shear friction" and the anchor bolts will take up a portion of the thrust through tension. My question - will the top of the 42" diameter, 3,000 psi concrete pier require any additional reinforcment/bars in the shear transfer area? If so, what is the preferred design here.

Thanks for any advice.

 

[Lutfi]

There is a simpler and more frequently used detail to transfer the thrust from the base of the rigid frames to the slab. They use a one to pair of hairpins. I use one if only 2-anchor bolts are used. If 4-anchor bolts are used, I double the hairpins.

Hairpins are simply reinforced rebars bent at about 30-45 degrees from the frame center line and the legs are projected or embedded into the floor slab. Theoretically, thrust is resisted by the slab and no lateral load is seen by the footing. This will eliminate any overturning moment in the footing. 19 kips is not a reaction. To compute the length of the hairpin projection is to simply compute the bond strength between the rebar and the concrete.

If no slab exists, then your choice is reasonable. You may want to consider housing the cable in a sleeve and covering it with concrete (say 12”X12) to protect it.

I hope this helps.

 

[jimbo2]

Lufti

Thanks for the good info. This building will have no slab, just a gravel floor. I am not clear on how the lateral load will be transferred from the column base plate, thru the concrete to the cable tie. I am running the cable through/between the stem of the 4 anchor bolts, and initially thought this would be enough to transfer the shear load from the column to the cable. Will the base plate friction and anchor bolts be sufficient without any shear reinforcement in the concrete?

I specified the cable be encased in an 8" x 8" grade beam, What about frost heave in the grade beam?

 

[Lutfi]

The slab will resist lateral load via its weight and friction between slab and concrete. Also the slab, if existed, it will be interconnected with reinforcement of some sorts (bars, WWF, Etc.).

It sounds like what you are doing is fine. 8”X8" may be a little small, I would use 12X12. Since the concrete is only placed to protect the cables, I would not worry about the frost line. You made me think of my early days doing designs in VA and WV when I used to worry about the frost line all the time to a depth of 30 to 42 inches. I live in Florida now and all I have to worry about is hurricanes.

I think you are on the right track. Another option would be to reinforce the grade beam and use its rebars to carry the tension. Make sure your bars will develop their lengths from end to end of the grade beam. If frost is a concern, I would wrap the grade beam with some thermal insulation to protect it.

Good luck

 

[ishvaaag]

If you are worried about shear transfer adding a welded shear steel parallepiped at the underside face of the plate won't make harm.

 

[jimbo2]

Lutfi/ishvaaag

Thanks ever so much for your input. I feel better about how I am thinking of this now.

Its hard not to think about frost up here in Central New York State with our -20 to -30 F wind chills lately, and this building will not be heated.

yes I am concerned about shear transfer, and will seek additional information about the base plate. Somehow I still want to reinforce the concrete, how paranoid am I?

 

[haynewp]

I have always had a slab available to use the hairpins. But I haven't ever tied piers together with a cable. So I am only inquiring:


If the 19kips due to lateral load and not gravity load at the base of a rigid frame, there may exist an equal 19 kips at the opposite pier acting in the same direction. How does tying the tops of the piers together with a cable accomplish taking the lateral load at the top of the piers out?

Also, I am wondering why you wouldn't have to place the bottom of the grade beam below the frost line. If the beam heaves in the middle, couldn't this lift each pier at the ends since it is tied to them?

 

[haynewp]

On second thought if the beam heaves it seems it could cause damage to the beam itself, (shouldn't lift the piers).

 

[jimbo2]

haynewp:

According to the load vectors in the building drawings, the horizontal thrust from the rigid frame occurs outward on both sides of the building. The wind loads do result in thrust in one direction on opposing piers, but the DL + LL is a gravity load, and the wire rope tie holds the piers together.

I'm thinking the heave is a real concern here, but how bad can it get? If the grade beam is lowered to below the frost line, this will move it significantly away (about 4 feet) from the thrust line. Would we then have to resolve some bending/moment loads through the concrete?

 

[haynewp]

I see where the tying together works for D+L. So I assume you checked for whatever wind or seismic reactions are given that the piers themselves are adequate in lateral bearing to take it.

I would also think that you would want to stay below the frost line with the bottom of the beam. The bending due to the heave on the concrete beam may overstress your cable you are tying at each end.

If below the frost line is the correct treatment then yes I would check the piers to resolve the 4 feet of moment. Make sure the concrete is strong enough to take the moment, and check the overall stablity of the pier about it's point of rotation.

But again this is the first time I have approached this method of resolving thrust loads. Maybe someone else can comment.

 

[haynewp]

Aren't void forms used to counteract frost heave on grade beams? Maybe that is a better solution than going below frost level.

 

[jimbo2]

haynewp - Great - I love it! your right, I think that will solve the heave situation without getting into a more complicated design, and I appreciate the input.

I want to thank Lutfi and ishvaaag for your valuable assitance with my problem you guys are great!

 

[JAE]

Wait a minute...if you put void forms below the grade beam, what holds the grade beam up? The void forms will eventually deteriorate, as they are intended, and the beam will drop.

All you have to do is trench out your grade beam tie down to below frost. So let's say your frost depth is 4'-6". You would have a concrete grade beam, with its bottom at a depth of 4'-6" and its top perhaps 12" below the gravel floor. The wire rope, or rebar, or whatever tensile element you use, can be placed near the top of the trenched grade beam. That way the eccentricity of the tension in the column piers is kept to a minimum. The grade beam could also have some supplemental rebar placed in the top and bottom as well.

 

[haynewp]

If you can find this synthetic stuff, it will take a million years for it to deteriorate.

http://www.beaverplastics.com/pdf/frostcushion_ds_010529.pdf

 

[jimbo2]

JAE

On haynewp's suggestion, I quickly found an "inert" closed cell expanded polystyrene (E.P.S.) product manufactured by Beaver Plastics, Ltd of Alberta, Canada at

http://www.beaverplastics.com

The product name is Frost Cushion, and I plan on specifying it for this job.

I had a bond breaker at the pier/grade beam connection point, but now I wonder if I should run dowels between the grade beam and pier, then pin this grade beam down (with deformed bars) at some interval over the 70 foot span.

 

[haynewp]

JAE is correct. Even if the plastic stuff holds up against the elements, it is not meant to have but a certain amount of weight applied to it. If this is a farm type shelter for instance, vehicle traffic could collapse the beam and the form since you are not designing the beam to support vertically applied loads.

 

[jheidt2543]

Just a couple of additional thoughts:

1. I have used a solid tie rod to resist the horizontal thrust in a moment framed building (metal building), but never a cable. I would have some concern over corrosion. Having it encased in concrete, as noted above, solves that.

2. Hair pins in the floor slab is certainly the least expensive solution - IF there is a floor slab.

3. What happens to hair pins and/or a tie-rod/cable if the building use changes and the floor or subgrade gets dug up cutting the hair pins and/or tie-rod/cable?

4. The BEST solution, but also the most expensive, is to design the column footing, piers and adjacent frost wall to accomodate the horizontal thrust loading. For one reference see the book "Metal Building Systems Design and Specifications" by Alezander Newman, 3rd Ed., McGraw-Hill, ISBN 0-07-140201-2, Example 12.1, pages 348 - 355. Another reference is put out by the Metal Building Dealers Association (MBDA), sorry I don't have the complete citation.

 

[PMD123]

I agree with JAE comments and also agree that the piers and foundations can also be designed to resist the rigid frame outward thrusts. But make sure if you use hairpins and tension steel/rebar tie backs that the reinforcement is spliced per ACI requirements for tension members. i.e mechanical or welded splices that develop 125% of bar tensile strength. If welded splices are used, then A706 rebar should be used or weldable rebar with the proper carbon equivalent.

 

[jimbo2]

The subgrade pad for this building was carved out of a hillside two months ago, forming a cut/fill situation under the building. The fill soils were not compacted. As a result one side of the building will be constructed above native, hard glacial till soils, and the other side on moist to wet, soft mixed fill up to 8 or 9 feet thick. I thought about a gravity foundation to resist the horizontal thrust, but was concerned about the wet soft soils and the limited passive resistance they will provide. The way I looked at this, the foundation piers would become quite large by themselves should they be required to resolve the lateral loads. As it is the 42" diameter concrete piers on the fill side will be about 11 feet deep so they can bear on the hard till (about 30 kip dead plus live load). Hence, the tension cable/grade beam between opposing piers.

The prepsence of the softer fill is a concern, especially since it exists only on the eastern half or third of the building. Good point about the surface loading. To address this, and the potential for differential settlement, I figure to specify a prepared/compacted 12 foot wide (equipment issue) subgrade for the void form, and a similar width surface course of compacted NYSDOT No. 2 subbase stone across the top of the 3,000 psi concrete grade beam.

 

[Stress02]

wire rope stretches, yes?