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.

 

[jimbo2]

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?

 

[Stress02]

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.

 

[jimbo2]

no slab in this building, just a gravel floor

 

[jike]

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.

 

[jike]

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.

 

[klipper]

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?

 

[connect2]

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.

 

[MikeHydroPhys]

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

 

[furter123]

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.

 

[MPENN]

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.

 

[witchdoc]

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.

 

[Lutfi]

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.

 

[Lutfi]

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.