Concrete Pile Caps - Shear Design

[bedstead]

In designing the pile caps for a simple span bridge with piles that are centered under each girder, ie/ 5 piles/ abutment and 5 girder arrangement for the superstructure all which are centered over the pile locations, apart from the self weight, wingwalls and soil pressures, there appears to be relatively no load in the cap between the piles. What then would be the basis for the shear deign? Simply minimum reinforcemnt requirements.

Thx in advance for any advice.

 

[wiktor]

The cap beam will carry no load, only if assumption is made that the supports are indefinitely stiff. However, both piles and supporting substrata are elastic, so I would advise to re-analyse the cap beam treating it as a beam with elastic (spring) supports. This approach would definitely provide shear forces in the cap beam. Also, if the connection in between piles and cap beams are rigid or semi-rigid, there will be shears and moments in the cap beam generated by transverse loads (wind, seismic).
If the piles used to support the cap beam are friction type, uneven settlement will generate moments and shears as well.

 

[Qshake]

In many cases, DOT's will strive to place piling under each girder at abutments in order to minimize the amount of reinforcing that otherwise may be required. And so you'll wind up with the results you've noted for vertical dead and live loads. For the DOT, who will do hundreds of bridge designs per year, this makes sense economically. Yet, as consultants working on only a single bridge for this or that agency we might lose sight of that.

At the same time, you don't want to forget to evaluate the impact of some of the loads mentioned by wiktor above. However, in most cases for the situation you've described these will result in negligable shears in the beam. Though their impact might be greater for other parts of the structure.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[wiktor]

I forgot to add one of the most important requirements for the cap beams, which should be capable of supporting the jacks during servicing/replacement of the bearings.
I agree with Qshake, that agencies are pushing for reduced loading in such elements, in order to reduce the costs. However, I have seen a bridge, where outer pile failed due to the scour, and the extra reinforcement in the cap beam (not needed by normal calculations) prevented catastrophic collapse.
I would reinforce cap beam for the 1.10 of the vertical reaction from the girder (outer bay, inner bays less) for shear, and provide enough rebars for bending to avoid collapse, but with damage, with any single pile failing.

 

[CanyonRat]

Maybe consider the wet concrete of the diaphragm if there is one?

Most states have standardized reinforcing for simple pile caps at abutments and the reinforcing need not be checked if piles are under the girders. Make sure you satisfy yourself with this of course.

One easy method is to just check your cap beam as simple span between piles and provide same reinforcing for positive and negative steel.

I have treated the piles as springs and artificially created the shears but it's just an excercise to create some loads.

If the piles settle they should move together (as in downdrag). If the bridge is getting wide (100+) you may consider it if the borings indicate a lot of changes going on. Consult your geotech...

Right now I have a wide bridge where the bedrock slopes 20% so it's something to consider and possibly provide for.