Pile cap design for single H-pile refer( thread507-158100) and others 3

[kapoA]

I need to design a pile cap for a single h-pile supporting a steel bridge column.
Seems like all the load from the column will be transferred to the pile below (pile located directly below column).

Since the cap will not take any loads, can I just design a small (say 3ft by 3ft and 24 in thickness) cap with minimum reinforcement top and bottom.
H-pile is HP 14x73. I was planning to embed the pile 6 inches into pile cap.

Since I am not very familiar with such design, your feedback would be very helpful.

Thanks.

 

[TLHS]

What kind of loads are you talking about? Pure vertical compression? Theoretically, if you can take pure vertical loading in bearing and carry it through compression there's not a huge amount to design for once those quick checks are done. This is rarely the case though. Even with pure vertical compression loads you'll often have some sort of moment due to allowable construction eccentricities. It can get pretty significant sometimes. Presumably the pile contractor is allowed to be out a couple of inches?

 

[kapoA]

The pile cap has Moments and Shear in addition to compression.
Contractor can be out by 2-3 inches. Any references on how to design this?

 

[kapoA]

This foundation is for a short steel pipe bridge.

 

[SlideRuleEra]

Doubt if you will find references for designing a cap for one pile since, as TLHS discussed, a one-pile cap is potentially unstable when loaded. Not a good idea, IMHO.

Perhaps you could consider the following as an alternative:

1. Drive the H-Pile and cut it off near ground level.
2. Weld the bottom of the column to the top of the H-Pile.
3. Form and place a reinforced concrete "collar" around the welded joint to protect the weld from weather and to LOOK like a pile cap.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[jayrod12]

Do what sliderule said. as usual

 

[msquared48]

Or use a two pile cap.

Mike McCann
MMC Engineering

 

[ztengguy]

What code? Isnt it a requirement to have stability in piles, thus at least 2? Cheap insurance for the setup will be there, just another pile and driving it.

 

[msquared48]

If you have a grade beam system, then a single pile can work, particularly in residential or light commercial/retail.

I would never use a single pile cap that was not tied to a grade beam.

Mike McCann
MMC Engineering

 

[TLHS]

I've used single pile caps for industrial stuff and utility stuff on some occasions. It's generally either lightly loaded or large diameter cast piles. Even then, though, I'd normally frame the superstructure so there's a tie beam somewhere near the bottom. This hasn't *always* been the case though.

I'd probably feel uncomfortable using a single pile (except possibly for monopole construction, but that's a special case) if normal operation/service loads put the resultant force outside the cross section of the pile even if the numbers worked. I'd be concerned about long term settling releasing some of that lateral restraint on the piles. Short term loads I'd be less concerned about. Also, you may not be allowed to use them without grade beams depending on your code and seismics in the area.

For actual design, just draw your free body diagrams at your critical sections and locations.

- Check anchorage of your column into the cap
- Make sure your cap can transfer the forces between the column and the pile. Either treat it as a column, a beam, a combination of the two or strut and tie depending on the situation. I've never really seen much that addresses this specific issue, but you can use a lot of the same concepts you'd use for a column pedastal so looking up design guides for that would likely be a good plan.
- Check anchorage of your pile into the cap

But first make sure your assumptions actually make sense. If you're transferring moment into the pile and it's a single pile cap you really have to make sure that your structure is relatively flexible in comparison to the pile including the geotechnical side or that you've actually modeled the whole system including foundation springs and things. Depending on the framing on top, you could easily have a structure that's an order of magnitude stiffer than the foundation. In that case, regardless of whether you design the piles to take that moment capacity, the connection will only be partially fixed. If that's true, you may be underdesigning your superstructure based on the assumption of a fixed cap.

While a direct welded connection is preferred from the engineering side of things, there are reasons to avoid it depending on the conditions you've got. It can be crappy for making fitup work, especially when you're talking about a single column on a single steel pile. You tend to have to screw around a lot on the welds and they're generally in terrible weld positions. Compression primary loads aren't terrible, because you can shim but if it's a reasonably significant moment connection or there's uplift, it can get ugly.

 

[kapoA]

Big thanks to all, especially, TLHS and SlideRuleEra.
Decided to go with 2 pile. Didn't feel like trying something different