Pier Caps 2

[frv]

In my area, straight shaft piers are very common.

For several years now I have been "designing" pier caps based on nothing other than company standards.

What I mean by "pier cap" is a rectangular cap poured after the pier is poured, on which a column (typically an X-Brace or moment frame column) is located. The purpose of the cap is to allow the geometry of the required base plate and embed to be properly supported.

When designing pier caps, I've followed what I believe to be good detailing practices, based on both papers I've read and suggestions from both fellow engineers and professors.

My issue, however, is that quite honestly, I have no fundamental understanding of the forces or stresses in a pier cap. To my knowledge, a pier cap is nothing more than an extension of the pier itself. I extend the pier reinforcing to the top of the pier cap, and terminate it with an ACI standard hook. I extend the anchor bolts into the pier cap sufficiently to develop the forces in the column. I additionally add closed ties to encompass both the anchor bolts and pier reinforcing (within the pier cap) in order to provide confinement.

My question is, what is the proper way to analyze this? Is there any standard way to determine proper geometry and thickness?

The reason I haven't been more concerned with this before is that I have mostly been designing buildings similar to those my company has been designing for decades, so I didn't really question our standard details. Additionally, the buildings have not been more than a few stories and the loads have been relatively small. I will now likely be in a position to design more substantial buildings and I do not want to design by "feel". I am really looking to the more senior engineers on this forum to guide me a bit. Any suggestion is appreciated.

 

[rowingengineer]

I assume to date you have been designing pier caps for single piers. When you have more than one pier or pile the cap it's self becomes a transfer member. For this type of situation there is many different methods to analysis I prefer the strut tie method. There are also preferred arrangements to suit detailing method. The Istructe “Manual for the design of concrete building structures” has a good run down on pile cap design and spacing.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[frv]

rowing..

exactly.. a single pier.

There is sufficient literature on pile cap design; I'm looking for guidance on a single pier cap.. I honestly have no clue why we reinforce them the way we do.. It seems reasonable to me, but I have no idea why..

 

[rowingengineer]

I would treat a single pier cap as a short column or plinth and reinforce accordingly.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[JAE]

frv - done hundreds (thousands?) of these and basically follow what you outline.

1. Use at least 1% steel - tend to match the pier or column rebar somewhat.

2. Use additional ties near the top (per ACI requirements).

3. Ensure that the cap is tall enough to allow pier rebar development into it - and anchor rod or column reinforcement above to be developed down into the cap.

4. If there is moment at the base of the column above - do a column design for the cap to ensure adequate reinforcement for the axial and moment forces (simply a very short column design).

 

[Lion06]

How tall are the piers (total)?

Do you assume the slab on grade braces the top of the pier or do you design the pier and fdotjng for the axial force and moment (from the shear times the pier height). This doesn't seem unreasonable for a short building, but seems like it could be difficult for anything over a couple stories (especially in a braced frame building where the lateral loads are
more concentrated than in a moment frame building (mostly because you get more frames when they're moment frames).

 

[BAretired]

I regard single pier caps (pile caps) as an extension of the pile. Their sole purpose is to allow the contractor to accurately set the anchor bolts after the piling contractor has left the site.

They may be formed with cylindrical cardboard forms (Sonotube or similar) of a diameter large enough to accommodate anchor bolts with a little room left over in case of slight misalignment of pile.

The depth of pile cap must be adequate to contain the length of anchor bolts with a little clearance. I usually make them two feet deep.

There is not much design to a single pile cap. Simply extend the pile steel.

BA

 

[frv]

StrEIT..

The length of the piers in this area is typically 10-30 ft, depending on final bearing stratum.

I'm not sure why you say that the moment would be the shear times the pier height. I can't remember the specific wording in the IBC, but you are typically allowed to assume soil braces your pier (even if it's soil from a different stratum than your final bearing stratum). So I think it would be somewhere along the lines of shear times the distance to the first stratum of soil plus whatever distance is required to transfer the lateral loads into that stratum of soil.

 

[Lion06]

My mistake. In re-reading, it's clear that you are talking about drilled piers (caissons). Disregard my post above.

 

[JAE]

frv - one other thing - most pier caps I've designed have really been just part of a cast-in-place concrete beam/slab/joist system of some kind so there is usually some level of lateral constraint in the pier cap supplied by the floor system. So then there wouldn't be really much moment involved as the cap-beam interface negates moment in the cap.

A lot of old plans in south Texas that I used to review had the term "column under first" used to refer to these caps, identifying them as a column section, located directly below the first level.

BAretired - I don't really disagree with your statement that they are simply extensions of the pier - for interior columns surrounded by a slab on grade they are just that. But technically a pier is under ground, surrounded by soil, and many times pier caps are more a part of the floor system, or located in a crawlspace and would then fall under ACI 318 column provisions vs. being treated as a drilled pier/caisson.