Pier/Footing Shear Transfer

[SteelPE]

I am in the preliminary phase of designing a foundation system for a large hangar. I have the reactions from the metal building company and am trying to conceptually wrap my head around the magnitude of the loads. I have calculated the worst shear case for the column in questions to be Vu=104 kips (this occurs under gravity loads. At this point I have absolutely no desire to use tie rods across the building as there are issues that preclude their use (and quite frankly, I don't like to use them).

So now I have to design the shear transfer mechanism between the pier and the footing. I have two options (see attached sketch).

Option #1 rely on shear friction (I conservatively used only the interior bars in this calculation but I could easily add in the "compression" bars if needed)

Option #2 is to recess the pier into the footing.

What method would you use to resist the loads shown?

 

[WARose]

Definitely Option #1. If the amount of bars it takes doesn't crowd the pier.....I can't think of a good reason not to.

 

[KootK]

Option#1 for me too. The mechanical keying feels viscerally awesome of course. That said, I sometimes wonder if it would actually do what we think it does if that mechanism were truly left to do the job on its own. As shown below, there must be a failure mode somewhat akin to shear pryout in anchorage design. That, particularly given that you'd be delivering the load into what is, effectively, cover concrete.

 

[HTURKAK]

Option one or provide a local shear key .. One way or the other.. the horizontal load will be transfered to the ftg..

Apparently, you do not like or say it is not an option to use tie rods..

Just curious , what is your plan to transfer the horizontal load to the surrounding soil or say, how the ftg will resist Vu=104 kips while the vertical load Pu is 142 kips ?..

 

[SteelPE]

The ultimate load is 104 kips horizontal and 142 kips vertical, the allowable load is 69 kips horizontal and 98 kips vertical. The coefficient of friction is 0.35 if we use nothing below the footing and 0.60 if we use crushed stone below the footing (these numbers are given by Mr Geotech).

The footing I am using is sized for overturning and uplift so it isn't small. The footing and soil weight is 94 kips. Allowable net passive pressure (from geotech) is an equivalent fluid pressure of 250 psf. The passive pressure contributes roughly 25 kips to the resistance. Soil friction = (95 kips footing weight +98 kips vertical)x0.6 + 25 =140 kips............ so I would like to think I have this covered (even if the 0.6 seems a little high).

Reasons why I don't like tie rods:

1) usually by the time we are done sizing the footing for uplift requirements the footing only needs a few more yards to concrete to make it big enough to overcome the gravity load combination that tie rods are used for
2) there is added expense to installing these rods..... if we spend this money on more concrete and reinforcing in the footings we can easily get the footing to work without them
3) we don't have to worry about someone cutting them in the future
4) they tend to stretch quite a bit..... in this structure, if we use the minimum A36 steel for tie rods we would need 3.2 square inches of steel. These would stretch 1.5 inches.... I am not sure if the frames would like that very much (this point is a nit little picky as the footings will rotate under load effectively moving the bases away from each other).

I have 3 of these buildings next to each other.... the footings in the middle of the building are supporting two columns. In these areas the gravity loads will cancel each other out eliminating the need for the rods in the middle of the building (I haven't run the numbers yet in the middle but this is what I am expecting).

I could go on with the nuisances of this particular structure, but this is my reasoning.

 

[HTURKAK]

Apparently these effects are for gravity loading.. If we assume wind loading 60 kips , the horizontal design load could be in the range of 150 kips.. so, you will need approx. 50 kips passive resistance..

I think it is not the best idea to write a warning sign ( trench excavation along the bldg . is forbidden )..


I appreciate your concern for the elongation of tie rods .. But , the rotation of ftg and sliding due to passive soil displacement will be more critical..

Just saying my opinions based on real experience..

Good LUCK..

 

[SteelPE]

I don't know if I explained this properly.

The 104 kip load is an ultimate load (hence Vu = ultimate shear)
The allowable shear load is 69 kips.

So I don't know where you are getting 150 kips of lateral load from.

This is also for an airplane hanger..... so once construction is completed they just cant grab an excavator and start digging around... there is a huge process they have to go through. So putting a note on the drawing should suffice.

 

[HTURKAK]

You explained properly but i suspect i could not.. I just guessed the horizontal load including the wind case ...

If you provide the vertical and horizontal design loads for the combinations C1= DL+LL+S+ TEMP. and C2 = DL+LL+W and provide some more detail for the proposed ftg, you may get more useful responds ..

 

[SteelPE]

HTURKAK,

I wasn't looking for a detailed response on the design of the footing, but rather a response on the design of the connection between the pier and the footing for a shear load of Vu=104 kips. I would like to think I gave all of the necessary information to answer that question (pier size, loads required, and footing thickness) but I guess I didn't.