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Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

(OP)
Hi All,

I have an interior braced bay condition where I need to design the foundations for the bracing reactions. In particular I am focused on the horizontal reaction from the bracing. In the past I would have designed the footings for the moment; however, I am trying to take a fresh look at this situation. The piers are going to be surrounded by a concrete slab. Is it reasonable to argue that the slab is going to restrain the top of the pier and only design the footings for the vertical components? Assuming the slab checks out for bearing, etc. What about the isolation joint between the pier and the slab? Look forward to hearing how you guys handle this situation.

Along that same line I also examining how I am handling the braced bay forces in the sidewalls. Normally I use poured concrete foundation wall tied into the piers and the footings and that can easily handle the strut load between the columns. In this case the owner insists on block foundation walls. I would imagine that I would need to fully grout the cores between the two columns and add vertical and horizontal reinforcing. What I am unsure of is the best way to tie into the poured piers. Again I would appreciate any thoughts or suggestions.

Thank you

RE: Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

you will need to dowel in the floor slab to piers.

google foundations for metal buildings, they have different approach how to resist lateral loads (tie rods, etc..)

RE: Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

What you've proposed is extremely common in my area. I hate it frankly. Nobody ever seems to pay enough attention to the load path once the shear hits the slab on grade.

At the the masonry infill wall, my first choice would be to not use it at all. If you must use it, I imagine that you'll need to have dowels drilled and epoxied into the vertical faces of the piers that lap with your CMU horizontal reinforcing.

I think that Delagina was referring to this book: Link. If so, I second the recommendation as I have it as well. Even though I don't agree with everything, it's great to see what assumptions are common in the industry.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

(OP)
Thank you for taking the time to reply. Yes I am quite familiar with the hairpin approach when trying to tie back the outward lateral thrust of a PEMB frame. In this case I am interested in the longitudinal direction both for interior bracing and sidewall bracing. I actually have this book and it has a lot of information on resisting the lateral thrust but I didn't see anything addressing the longitudinal direction. I apologize if I didn't make it very clear. I suppose one could use hairpins on the interior columns but it does seem redundant if the pier is completely surrounded by slab.

KootK, am I to take it that you design all your braced bay footings for overturning and don't rely on the strip footing or the grade wall?

RE: Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

Not at all. At exterior conditions, I'm fine with using the strip footing and grade beam / wall when it's monolithic concrete. I'm just less excited about that scheme when the wall is masonry infill.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

I guess for major buildings with an interior brace linking in to two columns/piers, I would be inclined to tie the two piers together with an underground grade beam and perhaps even extend the grade beam to engage the far exterior, perpendicular grade beams to create some type of load path via passive pressure into the earth.

Linking in a "non-structural" floor slab which usually is separated by an expansion joint doesn't sound right to me - especially if you are dealing with a larger building.
Yes - OK for PEMB but not for more significant structures.

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RE: Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

(OP)
Yes, this is a smaller PEMB, only 70' x 80' and the braced bays are portal frames so the horizontal reactions are halved at each column location. The maximum horizontal brace force at the interior columns is 2.3 kips and 1.3 kips at the sidewall columns. There are no plans for an expansion joint between the portal columns.

RE: Resisting Longitudinal Braced Bay Reactions with Slab and Grade Beams

PEMB's are like the Nevada of building types. Somehow the good judgment that pervades the world beyond just ceases to apply.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

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