How to find the shear force of a concrete pile(Shanghai Bldg Collapse) 1

[jwripple]

Hi all,
I'm trying to model the Lotus Riverside Apartment building collapse in Shanghai, China using Slope/W. It appears that the building used 2' circle open-ended lightly reinforced concrete piles. Is there any quick way to calculate the shear force of these piles so I can best represent them in SLOPE/W. Plus, any additional information pertaining to the collapse would be greatly appreciated.

 

[ishvaaag]

Quick I would say not. Even more that you want to use the value as input for a probe (even if theoretical) into the case. To your purpose, especially if for some real world application, the most accurate soil-structure interaction should be used; but if thinking that thing out of reach, a common structural analysis should not be in any case. Note that since related to some ruin many other aspects may have ignited the failure and (except that you know the contrary is the case) shear atop the piles might not be as relevant as say, solid rotation on lack of bearing, a weakened bad detail at the caps, etc... I mean, of course with your model you may get some interesting appraisals of what could have happened yet one would have to consider I think a number of concurrent or alternative scenarios to come closer and closer to a correct report.

 

[BigHarvey]

the shear resistance of the piles is neglectable compared to the driving forces of the slope failure. And I'm sure you wouldn't design the piles to resist the slope failure !

 

[jwripple]

I see, its just that in Slope/W they require you input the shear force of the pile and I'm not too sure what value to put there. An example analysis uses a shear force of 300kN, but I'm not sure how they calculated that or for what kind of pile. Any thoughts? Thanks.

 

[Zambo]

I think these are spun piles which are common in China. The first problem is that you are not sure of the diameter, you think 2'. But spun piles are available in 500, 600 and 700mm diameter plus the wall thickness can be varied to give the required properties.

Try looking up "spun pile" on the internet.

 

[concretemasonry]

The initial question referred to the "shear force" of the piles, but obviously the force is induced by the soil, conditions and has no bearing on the properties of the pile itself.

After seeing the site, it seems strange since the adjacent structures had no apparent distress and the problem could be related to the conditions/loads applied later that could include soil, drainage and moisture.

With some familiarity with the massive projects going on concurrently, I would think it is a "cookie cutter" design of similar or identical buildings with the same materials from the same plants within a one year or so period (24x7 construction).

Only an observation.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[jwripple]

Thanks for the input. I believe the "shear force" required by Slope/W is the force induced by the piles unto the surrounding soil, as some kind of reinforcement. It seems to me that one of the key factors in the failing is the fact that the face of the excavation was left unsupported. They did not construct any structure when excavating the garage pit. This was mentioned by the assistant for the lead investigator of the collapse. Since details to this collapse are vague, I have just been making assumptions based on the pictures and comments others have made. After the initial analysis' in Slope/W; if there was reinforcement to the face of the slope, the factor of safety increased substantially, perhaps even to prevent the failure. I have tried analyzing the slope without the surcharge of the soil behind the building, but the factor of safety did not change much. I am also unsure how deep the piles are, so that could also determine where exactly the weight of the building would lay; either in the failure zone or beneath it. I'll post of my final results when completed. Thanks.

 

[dgillette]

The shearing resistance may be entirely irrelevant! Compare the bending moments in the piers with the moment capacity, and I believe you will find they fail in bending with very small lateral forces indeed, much smaller than the capacity of the pile in direct shear. (Imagine the pile as a cantilever with the load applied 2-3 m above the fixed base, and compare the ultimate load with the direct-shear resistance. It might be more correct to assume a moment connection at the top, but that just makes for a more complicated calculation with numbers you haven't got.) You can't analyze them as if they are in direct shear; they are long, slender structural elements surrounded by soft soil.

Also, consider the possibility that there was no slope failure; instead, the piles on the side toward the excavation may have buckled and the building simply fell over. The combination of the excavation and the softening of the soil from heavy rain may have drastically reduced the lateral confinement that those lightly reinforced piles were relying on to resist buckling. Sometimes you need to think like a structural engineer.

DRG

 

[FixedEarth]

I think you are asking of the lateral restraint force and its point of application rsisted by the drilled pier foundation. If so, clay soil under an rest earth pressure with level backfill can be calculated as: (0.8 x Ko x Gamma staurated x retained height x pier spacing). This can be assumed as a triangular shape. It's point of application is at 0.33H from the bottom of retained height. Ignore cohesion.

 

[jwripple]

Thanks for the good info. I'll take those things into consideration and see what I come up with.

 

[jwripple]

Based on what you all mentioned and the results in Slope/W, seems like one scenario of how the collapse happened is that with the heavy rains, the soil became increasingly weaker. The garage excavation created a very unstable slope, greatly reducing the confining pressures close to the building foundation. The open-end circle piles seem to have relatively low shear strength and lateral strength (from the pictures there does not seem to be much reinforcement, if not any at all). The mound of soil on the other side of the building does not seem to affect this failure too much(based on Slope/W. As the slope either failed or had greatly reduced confining pressures. The pile began to buckle from the weight of the building and lack of confining pressures, then the building simply toppled over unto it's side.

I strongly believe that if they supported the garage excavation slope, it might have prevented this collapse. The support would of kept the confining pressures around the foundation, allowing the pile to keep the building upright. There is a picture of the adjacent apartment building with the garage excavation in front, however there appears to be some kind of reinforced wall, perhaps a concrete retaining wall. This adjacent building still remains standing. (By the way, the assistant to the lead investigator specifically mentioned that there was no structure created to support the slope.) What do you all think about this?

 

[dgillette]

I had seen that photo with the exc for the adjacent building, but never noticed the exc and the retaining wall until you pointed them out.

The slope for the garage exc wouldn't actually need to slide if they simply got it too close to the piles and the soil was soft enough. I saw some "cartoons" of how it happened (NOT photos or engineering drawings) showing the exc going almost right up to the building and being pretty steep.

Regardless, yes, what you describe is an entirely reasonable mechanism, with the main question being whether there was lateral movement of the building before the piles began to fail, or whether the ones closest to the exc just failed due to axial load alone. Buckling, rather than slope instability, has been my leading hypothesis for the initial failure since soon after it happened. However, without some form of stability analysis and viewing it from pictures only, I didn't want to totally write off lateral movement driven by the stockpile on the side away from the exc. We'll never know for sure, since whatever evidence that survived removal of the structure may have been inconclusive, and it probably got quickly s___-canned anyway. I think I read that people are doing hard time in prison.

Having seen some construction sites in China (from a safe distance!), they seem to be a bit more casual about worker safety than we are here.

 

[jwripple]

Thanks for the feedback. Attached is a pdf of my Slope/W slope stability analysis of what I believe the site conditions were at. Let me know what you think.

 

[pelelo]

It seems there are several approaches for computing what slope/W calls the "shear force" on piles.

jwripple,

On the "soil strength and slope stability" book by Duncan, in page 258, there are in details several authors which have been working on evaluating the shear forces and moments in the piles for slope stabilization. Among them: Poulos (1995), Yamagami (2000) and Reese and Van Impe (2001).

Reese and Van Impe use p-y analisys in order to obtain the driving force acting on the pile. It doesn't seem a quick way nor friendly either.

Also, in your SLOPE/W model, it seems you are missing the shear force (i don't see the arrow/symbol). SLOPE/W models the shear force acting on the pile against the sliding surface.

What about the bending moment?, did you finally take it into account for your slope stability analisys?, I really don;t understand where do I need to included it, other than using in order to determine the number of bars for the final pile section.

FixedEarth, you provided an approach of computing lateral restraint force under clay. Do you know an approach for c-phi soil?, any references?