horizontal thrust - rigid frame to concrete footing
horizontal thrust - rigid frame to concrete footing
(OP)
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.
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.






RE: horizontal thrust - rigid frame to concrete footing
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.
RE: horizontal thrust - rigid frame to concrete footing
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?
RE: horizontal thrust - rigid frame to concrete footing
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
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
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?
RE: horizontal thrust - rigid frame to concrete footing
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?
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
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?
RE: horizontal thrust - rigid frame to concrete footing
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.
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
I want to thank Lutfi and ishvaaag for your valuable assitance with my problem you guys are great!
RE: horizontal thrust - rigid frame to concrete footing
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.
RE: horizontal thrust - rigid frame to concrete footing
http://www.beaverplastics.com/pdf/frostcushion_ds_01052...
RE: horizontal thrust - rigid frame to concrete footing
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.
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
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.
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
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.
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
Tie rods do not stretch?
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
As a result, the load capacity of the steel frame will be reduced. I would suggest running an analysis to find out what the corresponding load reduction is and then increase the specified load for the steel frame by that amount.
RE: horizontal thrust - rigid frame to concrete footing
There are a number of ways to deal with frost heave. I would agree with JAE that a concrete encasement down to frost level is the most practical.
Other possible ways to limit frost heave include:
1) Utilizing a frost point on the bottom of concrete grade beam. I do not recommend this method unless there is inspection to make sure the contractor does not excavate flat bottom beams. You also must consider if the soil would grab onto the sides of the grade beam and try to lift it. Insulation or visqueen along the sides of the beam will make it difficult for the soil to grab hold of the sides.
2) Use non frost susceptible fill below and adjacent to the grade beam. If you already have that type of soil, you are all set.
3) Use extruded insulation below and adjacent to the grade beam.
These are the common ways to limit frost heave.
Don't forget to consider that the frost depth is usually deeper for an unheated building.
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
can you not envisage a pile build with a spread base so the piles are held by base expansion core and concrete pumped i.e. a wider base than the bore insert and as far as possible force pump the base once some extraction has taken place, or extract, form pile and refill the top
this might avoid the lateral motion, but more importanly while using high volumes the material would be cheap.
You obviously have some problem with glacial till as whatever happens it is hard then moves under load when wet and compressed, exposed and weathered, so excavation and emplacement are a menace, but in the long run it might be safer and cheaper than sophisticated ties that are going to suffer a lot of stress in the type of force conditions tills can produce
best wishes this is the best geotechnic description I have yet come across at least you are working it out, some of the situations are horrors from the start with little concept of the need
MikeHydroPhys
mdshydroplane
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
RE: horizontal thrust - rigid frame to concrete footing
Serious issue and it is done without the engineers knowledge, I do not see how he can be held responsible. Of course some trial attorney will make some argument to pin the responsibility on some other than the owner.
Regards.
RE: horizontal thrust - rigid frame to concrete footing
The article is part of Butler's foundation manual. I am not sure if they still publish it. If you can get a copy of it is there. It is simply based on bonding of bras t othe concrete.
Regards.