metal building foundation slidng check

[delagina]

Attached is the pdf (2 pages)

Based on metal building foundation design guide, for overturning and sliding check, you only consider the "haunch" portion.
I don't have issues with OT moment check but failing on sliding check.

How do I engage the slab as sort of tie beam so the horizontal force cancel out and I don't fail is sliding check.
I only have minimum rebar for the slab #4 or #5 @ 12"

 

[delagina]

I don't think making the haunch bigger is a good idea just to meet the FS sliding.
I need to engage the slab so the horizontal force will cancel out.

 

[dik]

Will not the passive earth pressure on the outside of the stiffened edge easily resist horizontal movement? I use hairpins to resist the outward thrust, but, in a seismic/hurricane event, the lateral forces should be taken by the soil.

Dik

 

[WARose]

Hairpins are the most common way that the slab gets engaged. Another means to address it (and get the horizontal forces to "cancel out" where they are equal and opposite) is to use a tie rod. Use to see them a lot for metal building foundations.

 

[delagina]

Hairpin was added for the anchor bolt. Although I'm sure it will help with sliding check of the foundation/haunch I'm not sure how to quantify it to increase the area/weight near around the haunch to resist the sliding

 

[delagina]

I don't know if tie rod can be added on a slab with haunch type of foundation

 

[WARose]

I don't know if tie rod can be added on a slab with haunch type of foundation.

Happens all the time. I've seen some people anchor the rod in the anchorage zone of the column anchor bolts, or have some type of attachment to the column itself. If you set the column below the level of the floor (i.e. elevation of the bottom of base plate), it then becomes a simple matter to make the attachment. (I.e. by a clevis & pin perhaps. I've seen that a time or two.)

 

[KootK]

I use hairpins to resist the outward thrust, but, in a seismic/hurricane event, the lateral forces should be taken by the soil.

Why is that dik?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

I've even used a 'levelling plate' with holes to match the anchor rods and rebar welded to the U/S of the plate that extend into the slab.

Dik

 

[delagina]

I don't want this to be too complicated. the slab rebar would be in tension and if that tension capacity is way more than the horizontal force then I should be ok, I think.

 

[MotorCity]

Are we talking about horizontal thrust from the column reactions or sliding of the foundation? As long as the foundation is surrounded by fill, sliding wont be an issue (although passive pressure could be).

 

[SteelPE]

FWIW, I used to use hairpins but rarely use them now. I always find it more reliable to depend on moment resistant footings to resist the required loads. In my area, the frost depth is 4'-0" so sliding resistance is rarely an issue. Overturning is the problem.

When used we I was told to keep the loads in the pins to between 10-12 kips, anything larger I was told to use tie rods. So in this instance, if I was pushed into it, I would be using tie rods and not hair pins. Again, reliability is a bit of an issue, but something you can make clear on your design documents.

On a side note stretching of the rods at 100'-0" in length might raise some concerns. A 20 kips load would require a roughly .5 in^2 rod. If we used 2x that amount (1 in^2) you would expect the bases of the columns to move .82" under the applied 20k load. I might be inclined to call the metal building manufacturer and see what that does to their design.

 

[delagina]

Why would you choose moment resistant footing over slab with haunch? Is that for frost depth only? slab with haunch is a lot cheaper.

As far as overturning, if footing is 4' below grade then it's 20kips x 4 as opposed to just 20kips x 2 ft if it's slab with 2' thick haunch

I used to use moment resistant footing for metal building foundation until a client complained and showed me a slab with haunch design by others and it's half the cost

 

[delagina]

I could not justify anymore when to use moment resistant footing. If it's frost depth, I can use insulation

 

[RHTPE]

When resolving the horizontal thrust at the column-to-footing connection, I would offer these comments:
- Check with the PEMB designer regarding how much horizontal deflection (movement) would invalidate the design of the building frame.
- 100 feet of slab is going to have control joints. Depending on how the (effective) control joints are designed, are they capable of sustaining the tension force? OR:
- Does one rely on subbase-to-slab friction to resolve the horizontal thrust?

Do a quick model of a PEMB frame with the expected loading and allow the bottoms of the (2) columns to move (deflect) horizontally - how does the frame moment differ from the same model with NO outward movement? Sometimes surprisingly little column base movement makes for serious differences in the overall frame moment for the same loading.


Ralph
Structures Consulting
Northeast USA

 

[dik]

KootK... unless your soil liquifies, there is a lot of it to resist lateral movement... even if you have a 1/2" shrinkage gap between the soil and the concrete. If a seismic event is sufficient to break the bond between the SOG underside and the supporting soil, the 1/2" slack is likely to be the least of your worries...

Dik

 

[SteelPE]

delagina,

In the jurisdictions I practice in frost depth is a concern and foundations are placed 4'-0" below grade. Given varying grades this means we would place the footing 4'-8" below the slab. So sliding is never really a concern in the area that I practice as you have 4'-0" of passive load to resist the sliding force.

Now, I have had a long conversation with an old engineer who used to work for a large metal building manufacturer. We both believe that hairpins are unreliable and tie rods are uneconomical. With the extra work associated with tie rods (trenching, concrete, special connection etc) you might as well just make your footing a little bigger and not have to worry about things like construction sequencing etc. So we have used moment resistant foundations for quite some time. Occasionally there will be a use for tie rods, but I don't see any use for hairpins.

 

[delagina]

yes, hairpin is not required for moment resisting foundation because the pedestal/pier has ties/stirrups that will take of the anchor bolt horizontal force.
I have a slab so I need hair pin for the anchor bolt horizontal force.

I'm not considering the hairpin for sliding check at all. it's used only for the anchor bolt blowout.