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horizontal thrust - rigid frame to concrete footing
4

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.

RE: horizontal thrust - rigid frame to concrete footing

3
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.

RE: horizontal thrust - rigid frame to concrete footing

(OP)
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?

RE: horizontal thrust - rigid frame to concrete footing

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

RE: horizontal thrust - rigid frame to concrete footing

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

RE: horizontal thrust - rigid frame to concrete footing

(OP)
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?

RE: horizontal thrust - rigid frame to concrete footing

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?

 

RE: horizontal thrust - rigid frame to concrete footing

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

RE: horizontal thrust - rigid frame to concrete footing

(OP)
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?

RE: horizontal thrust - rigid frame to concrete footing

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.

RE: horizontal thrust - rigid frame to concrete footing

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

RE: horizontal thrust - rigid frame to concrete footing

(OP)
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!

RE: horizontal thrust - rigid frame to concrete footing

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.

RE: horizontal thrust - rigid frame to concrete footing

(OP)
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.

RE: horizontal thrust - rigid frame to concrete footing

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.
 

RE: horizontal thrust - rigid frame to concrete footing

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.

RE: horizontal thrust - rigid frame to concrete footing

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.

RE: horizontal thrust - rigid frame to concrete footing

(OP)
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.

RE: horizontal thrust - rigid frame to concrete footing

wire rope stretches, yes?

RE: horizontal thrust - rigid frame to concrete footing

(OP)
Yes, wire roper stretches.  I figured the 3/4" cable would elongate about 1 3/4" under the design load (7/8" deflection at each end), and added a  cage of 8 -#4 bars and a #4 tie every 12" in the circular piers to address shear and help handle this deformation.

Tie rods do not stretch?

RE: horizontal thrust - rigid frame to concrete footing

Your slab is definately going to show distress if it's subjected to 7/8" movement. I think rods deform much less than rope and would have been my choice. The cheaper way that seems to be in trend is the hairpins in the slab, but severely limits slab removal or replacement; never was my 1st choice.

RE: horizontal thrust - rigid frame to concrete footing

(OP)
no slab in this building, just a gravel floor

RE: horizontal thrust - rigid frame to concrete footing

Rigid frame metal buildings are designed by computer programs which assume that the foundation does not move. Perhaps a metal building engineer can confirm this. When a tie is used between foundations, it will stretch as it is put under load. For this reason, you would not want to use a high strength tie (like a cable) since it will stretch more than a low strength tie. You may also use a reduced allowable stress level. The stretch of the tie can be calculated by PL/AE.

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

You did not say what type of soil you are dealing with. As I am sure you know, some soils do not heave.

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

In addition to the bond strength between the hair pin and the concrete that you talk about, don't you also have to figure the friction at the interface of the slab/sub grade to resist the thrust after the load is in the slab?  If columns are spaced at, say 20' centers, we would only be able to use the friction created by the DL of the slab within the 20' trib. width right?  And then I think we would have to really watch where the contractor would want to put construction joints.  Or am I way off here?

RE: horizontal thrust - rigid frame to concrete footing

a 'Hairpin' anchor is only as good as the 'Concrete Slab on Grade' it is cast into. Go figure ????. There's at least 1000 posts (????) about 'concrete slabs on grade' and failures I'm sure.

RE: horizontal thrust - rigid frame to concrete footing

as noted cable stretches mostly by the twine turning if at all able or the fibres pulling inward and everything else corrodes

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

klipper brings up an excellent point that I haven't seen anyone address in any of these forums about hairpins.  I know hairpins are commonly used, but what about control joints and their affect on the load transfer?  Most if not all of the reinforcement in a slab is supposed to be stopped through a control joint.  This would mean that the thrust from the frame would have to be transfered by friction through a 15'x15' max section of floor slab to the sub grade below.  That doesn't sound realistic to me.  Am I missing something?  I have used hairpins in the past, but until I can find something or someone who can address this concern, I think I am going to use spliced bars in a trench below the slab.  I know I will be fighting contractors especially in wider buildings, but the hairpin thing just doesn't seem to resolve itself.

RE: horizontal thrust - rigid frame to concrete footing

Have you considered minimaly post-tensioned concrete struts between opposing bents/columns?  I have seen this used with engineerd steel buildings that have shallow continuous footings and an earthern work floor.

RE: horizontal thrust - rigid frame to concrete footing

What happens if the owner modifys the floor in the future by cutting out a section of the slab?  This occurs frequently in factories when new equipment is installed.

RE: horizontal thrust - rigid frame to concrete footing

The engineer who is responsible for the modifications should review the as-builts, discuss with the owner the ramifications of cutting the slab. It is prudent that the owner consult with the initial EOR to evaluate any alteration and the impacts that they may have on the structure.

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.

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