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I've been working on the design of a reinforced concrete wall pier for a three-span bridge. I roughed out the design based on a number of example plan sets and now that I'm taking a crack at the actual design calculations, I'm questioning the logic of the example plans. (Plus, I just dislike "designing" on the basis of "Well, we've been doing it this way and nothing's fallen down yet!")

The design is thus: Wall Pier Layout

My questions are:

  • Why would the piles be stopped short of the cap?

  • With the piles stopped short of the cap, what is the load path from the girders to the piles for vertical and lateral loads?

I'm also confounded by the fact that I'm typically seeing US #6 bars for the (a) bars (which seems large given that the cap will not see much flexure) and US #5 bars for the (c) bars (which seems small given that it looks like these bars would be necessary to transfer lateral loads from the cap into the wall).

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  • $\begingroup$ These are concrete piles, right? And just looking at it those piles look funky. Never seen piles topping at different levels before. $\endgroup$ – Wasabi Dec 14 '15 at 19:34
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    $\begingroup$ @Wasabi - In the design I'm using H-piles (got lazy with the sketch) but I don't think it would make a difference steel vs. concrete. As for the staggering, my thought is that it might be due to constructability and leaving room for the pile hammer since the piles end up being about 4 feet apart. $\endgroup$ – CableStay Dec 14 '15 at 19:37
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    $\begingroup$ My initial thought is that the piles are at different heights just because it sometimes happens like that when you drive them...and no one wants to scarf on/trim every pile to be exactly the same height. Also, the #6s might just be a rule of thumb, or to make the pile cap stiff enough to effectively transfer the load evenly to all the piles. (Just to qualify my statements - I've never done this kind of design before, just making a guess). $\endgroup$ – grfrazee Dec 15 '15 at 14:23
  • $\begingroup$ This question is an open tab in my browser since yesterday and I'm having some difficulty with it. One piece of information that would help is the sizes being discussed. Is the given reinforcement much larger than the minimum reinforcement? What are the sizes of the elements seen here? $\endgroup$ – Wasabi Dec 15 '15 at 14:31
  • $\begingroup$ From the way the end of the piles are drawn I would agree with @grfrazee it appears that the actual height of the piles is not important as long as they terminate within the Wall (I would expect to see a tolerance measurement for minimum and max embedment though). I would also believe that they don't extend them all the way into the cap to prevent load from going directly toward a single pile if there was some movement, the wall allows for load shedding to the other piers. $\endgroup$ – Dopeybob435 Dec 28 '15 at 14:34
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As @Frank mentions in his answer, the piles aren't stopping short of the cap since the piles are more than sufficiently embedded in the wall. This therefore means that the forces go through the topping beam, down the wall and then into the piles.

That being said, if you're going to make the wall axial-load-bearing, I don't see the purpose for raising the piles so far into the wall. Steel piles usually only need around 1-2 ft of embedment length into the cap for effective load transfer. Here's a diagram provided by a Brazilian pile supplier (Gerdau-AçoMinas):

enter image description here

Very few codes actually define minimal embedment lengths, but most seem to hover around 1-2 ft:

  • This thesis suggests (for cylindrical steel piles) 1 ft with reinforcement or 2 ft without.
  • This thesis has experimental results pointing towards 1.5 pile diameters.
  • This American state survey lists multiple different state's suggestions and most are up to 2 ft (only NE, NJ, ND suggest 3 ft).
  • This report concludes that embedment should be equal to two pile diameters or heights.

Now, all of these assume that the pile is embedding into an actual cap, which must be sufficiently wide in both horizontal directions. For an example, here's another diagram from the same supplier (basically, there must be 25 cm of concrete around the pile):

enter image description here

So, if your wall isn't thick enough to count as a standard cap, I'd guess the extended embedment length allows for the forces to be transmitted to the pile at lower stresses along the wall. No idea how to quantify that, though. Personally I'd rather just thicken the foot of the wall and leave the piles at the minimal embedment length.

The load paths are simple enough. The vertical loads obviously just go right through to the piles (I'm assuming the girders lie directly above the piles). The horizontal loads go from the girders to the bearing pads to the topping beam and then, via shear friction, go to the wall (as you mention, the (c) bars would be responsible for this) and then are transferred to the piles.

Regarding the reinforcements, it's hard to tell without dimensions and loading: are they close to the minimal reinforcement? Is the anchorage length of the (c) bars sufficient?

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The piles are not stopping short of the cap. Of course this depends on your definition. The wall and top beam form the pile cap.

The load path is whatever you want it to be. That's the fun of engineering. Think about strut and tie models, or truss models. Follow the loads and make sure that the materials are capable of resisting the stresses at each level. If you can establish a load path then you will have a stable structure. Now reinforced concrete is much smarter than all of us, and it will find the most efficient path, but it is up to you to give it at least one.

The top transfer beam requires at least minimum steel requirements. Looking at the model in a simple way, one could treat the top beam as a two-span continuous beam between the 3 taller piles.

The vertical bars in the wall also need at least minimum steel reinforcement. Close spacing of smaller bars offer better crack control than large bars spaced further apart.

The two outer piles are battered to offer lateral support to the pile cap. I am assuming that for the other direction, the bending capacity of the piles are sufficient for lateral support, this would need to be determined and checked and additional means provided if required.

The top of the piles may have bearing plates welded to them as well to establish load transfer to the piles.

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This is a little late to help you with your 2015 project, but hopefully it can shed some light on what's going on. I'm a PE in FL/GA, that specializes in bridge design.

a) Why are the piles stopped short of the cap? Most codes do not have a requirement to locate pile heads close to the bearing points. Embedment is generally detailed based on the required fixity of the design, but constructability or other project-specific design constraints may also be necessary. For driven piles in non-seismic areas, an embedment of 1-ft into a concrete element is considered adequate for a pinned connection, and 4-ft is considered adequate for a fully-fixed connection.

For this specific example, it appears the designer is likely using the longer piles to act as flexural resisting elements in the wall pier's weak axis to reduce the need for flexural reinforcement. This is why the (c) bars appear close to the minimum reinforcing required for temp/shrinkage. The two shorter piles are most likely to avoid pile-driving conflicts with the battered piles during construction, but could also simply be a cost-saving measure since the piles were not needed to resist flexural loads. The embedment of the short piles appears to be ~4-ft, which would be a fixed connection, allowing these piles to resist flexure at the base of the wall and lower.

b) What is the load path to the piles? This would be up to the Engineer to determine based on their own judgement and comfort level, but would be based on standard beam or deep-beam theory. Personally, I would design this as follows: 1) The cap as a continuous beam between the top three piles, ignoring contribution of the wall portion, as this is conservative. Typical reinforcing sizes for cap bars are #6 thru #10 for (a) bars, and #4 or #5 for (b) bars. I nearly always use #8 or #10 for (a) and #5 for (b) because steel is relatively cheap and it's very common sizes for highway bridges. 2) Distribute vertical loads between all 5 piles based on lever rule for bearing loads, and tributary width for self-weight of the pier. Remember that battered piles will have additional axial forces induced from lateral loads, and all load combinations should be checked to determine what controls the design. All piles should be the same size, and driven to the same minimum embedment and nominal bearing capacity, regardless of which pile controls the design. 3) Flexural design of the wall-pier (weak axis) would assume no contribution from the concrete/rebar, and all flexure resisted by the 3 long steel h-piles. Wall rebar would be minimum for temp/shrinkage/crack control. Lateral rebar in wall may need to be slightly larger to resist horizontal forces by "spanning" between the 3 piles, but it is still probably controlled by min rebar reqs. Bar sizes for (c) and (d) bars are commonly #4 or #5 to achieve min reqs in wall piers, but sometimes #6 or #7 may be necessary for larger concrete elements. I almost always specify #5 bars @ 12" max as a minimum in foundations, regardless of whether #4s can be used. 4) Flexure at bottom of wall pier and lower resisted by all 5 piles.

Hope this helps!

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