MBS Foundation - Large thrusts without tie-beams or hairpins 1

[dbnerds]

I'm currently designing a proposal for a client. Its a MBS with a (3)100'span (300' wide building) and frames are at 40'oc. As you can imagine the loads are considerable. The client GA drawings require us to avoiding tie-beams or hairpins. In addition no slab will be installed until the client finds a tenant. Additionally, the only geotech info I have is a allowable bearing pressure of 3ksf.

Question: I see the only option (at this proposal phase) is to employ a ballasted footing design that depends on soil friction. I can likely get the column baseplate low to avoid any additional moment onto the footing. IBC provides some friction coefficients. However, this just doesn't to be "enough" in my opinion. These are rather large loads and I'm trying to design with some half-hearted info on soil condition, YET stay competitive.

In normal circumstances, our company stays clear of ballasted footings. We also avoid hairpins when spans get larger than 70'. However, this is above and beyond all of this.

Thanks,

 

[AELLC]

What about casting the footings against soil (no footing forms) and including passive pressure against face of footing?

 

[hokie66]

In that situation, I think coupled bored piles are appropriate. I think you have a wise client, in avoiding hairpins and tension ties, as they restrict the flexibility of the facility.

 

[dbnerds]

@AELLC - At this proposal stage, I don't have any info on passive pressure, only bearing (which I can quasi-translate to coefficient of friction, using reco's from IBC). I've read that you shouldn't try to utilize friction and passive at the same time.

@hokie66 - Bored piles sounds interesting, but that would require way more geotech info than I have at my current disposal. I'm also curious to the financial trade off (huge mass of concrete with ballasted footing vs. the installation of bored piles). As for the client being "wise" maybe - but the client is creating a building, but has no tenant yet. The tenant may not end up needing the flexibility being engineered here, yet the client will have paid for it. Risky, IMO.

 

[hokie66]

Maybe risky, but he may have some idea or experience of the type tenant who would want the facility. Having to rip out a floor slab or cut trenches through it, then having to do something with the footings, is not something you want to risk either.

 

[H57]

Another option to consider would be drag struts. Given that your client doesn't want tie-beams, I don't know if he would sign off on using these. Even though they would only extend into a portion of the building.

In the projects I've seen drag struts they tend to be trenched soil cast grade beams (8" to 12" wide) extending from the pad footing into the building. The top bars are anchored to the footing to transfer lateral thrust into the drag strut. It is resisted by side friction on the trenched footings. Where I'm at these can be about 3' deep or deeper.

 

[XR250]

What about using helical piers as tie-backs for the footing?

 

[MJB315]

Going back to an earlier comment-- why is not appropriate to use both foundation-soil friction and passive pressure on the foundation? True, the foundation needs to move to engage the passive pressure block-- but the friction force does not necessarily disappear when that happens. Whereas I understand that kinetic friction values (i.e. the footing is sliding) are generally less than static friction (i.e. the footing has yet to move)... I have not seen literature on how that applies to this situation.

"We shape our buildings, thereafter they shape us." -WSC

 

[JedClampett]

If you use passive pressure, you need to get the PEMB designer on board. The movements required to develop passive pressure are very likely to exceed their assumed values (probably zero) causing large moments in the columns.
And note that those movements are based on a lot of empirical data, not rigorous analysis. So if one engineer says 1 inch and another says 4 inches, you're going to have a hard time deciding. I'd stay away from it.
I would not feel bad at all about assuming a conservative value for friction (.3?)and using a massive foundation. They've put a lot of constraints on your design. Concrete is heavy and cheap.

 

[WARose]

How about batter piles (or batter micro piles)? You’d need more geotech info……but such a system would be stiff and take out the lateral forces without causing a great deal of moment to be transferred to a base. I’ve always been apprehensive about foundations seeing overturning moment on a day to day basis: it leads to uneven settlement.