Concrete Piers Supporting Pre-Engineered Building 2

[marinaman]

I am designing a foundation for a pre-engineered building. The design includes shallow spread footings supporting cast-in-place concrete piers that then support the pre-engineered metal building.

I am thinking about the plan geometry of the piers.

The piers are isolated from the slab-on-grade. I have designed the building foundation as isolated from the slab-on-grade. The slab is an 8" thick industrial slab.

The attached sketch shows (3) plan pier configurations. I'd like to use the square one (one on the right) because it simplifies the formwork for the piers and simplifies the closed ties within the piers....but....I know that squaring off the face of the pier could result in cracking of the slab beginning at the corners of the square piers.

I'm hoping not to use the 1:1 pier because the 1:1 angle of the interior end of the pier pushes the end of the pier point out beyond the face of the masonry column wraps (columns are wrapped in masonry to protect them from being hit).

Do you guys ever use square piers like the one on the right and simply allow the slab control joint to be sawed up to the face of the pier?....Or do you always use a pier geometry with a "point" on the interior face?

 

[jayrod12]

All the time.

But with the pier being isolated from the slab make sure the PEMB manufacturer has designed for a pinned base. otherwise the pier and footing design becomes a beast really quickly.

Or provide strap beams or tension ties between the piers below the slab.

 

[hokie66]

I see no advantage to the 5 sided pedestals. To control re-entrant corner cracking in the slab, you just need to provide some reinforcing across the corners.

 

[jike]

Use a square or rectangular pier with chamfered corners. Place 3/4" exp. joint against pier and (2) #4 x 4'-0" long diagonal bars at re-entrant corners (corner of piers) in slab before you pour it.

How high does the pier extend above the slab? If it is significant, the horizontal deflection at the top of the pier may affect the strength of the PEMB steel frame.

 

[marinaman]

Thanks guys for your opinions.

jike, to answer your question, my piers are flush with the top of slab.

Thanks again.

 

[DaveAtkins]

jayrod12,

I think you meant to say roller base, not pinned base. The pre-engineered metal building suppliers always design pinned bases--never fixed bases (in my experience).

There will always be a horizontal thrust that must be resisted at the foundation level. It can be resisted by hairpins or continuous bars across the building (on this project that would make sense, since the slab is 8" thick), or, as you implied, the pier and footing can be designed for the overturning due to the thrust.

DaveAtkins

 

[jayrod12]

Tell that to the PEMB guys around here. If we don't specify pinned base, they assume fixed as it allows them to lighten up their structure, so now it's part of our typical PEMB notes. It's frustrating.

 

[DaveAtkins]

Interesting. I have never seen fixed base reactions from a pre-engineered metal building supplier. I am typically given a vertical reaction and a horizontal reaction (thrust), but not a moment.

DaveAtkins

 

[jike]

Horizontal thrusts must be designed for by designing hairpins into the slab, continuous ties, embedded in slab, across the width of the building from one column to the opposite column, or by some other method like buttresses.

 

[hokie66]

As this is an industrial building, I think it very unwise to use the slab or ties to resist the thrust. Industrial buildings are frequently subject to future removal/trenching of slabs.

 

[marinaman]

I'm not going to use the slab to resist the lateral thrust from the PEMB. As Hokie66 says, in industry, I'm certain that a conveyor system or some other pit or tench will be requested in the years to come. This floor is already full of pits and depressions, so I am going to have to go the brute strength route...using only the mass of the footing to resist thrust and overturning.

Another reason I do not use hairpins is because they restrain the slab. Slab restraint leads to cracking where I do not want cracks. I've investigated many slabs where others have used hairpins, restrained the slab, and ended up with radial cracks that are located right at the ends of the hairpins.

 

[hokie66]

Spoken like a true industrial designer, marinaman. My experience exactly. Another approach to the big footing is to use a pair of bored piles under a footing/pile cap.