H-Pile Design Question

[kxa]

I'm designing shoring for excavation of a 15' deep cellar which will later have 14" concrete foundation walls and concrete slabs.this will a 5 story concrete bldg. This will also be a cantilever H-Pile wall with no tie backs. I have two questions:

1. Since the H-Piles will be encased in the concrete wall, can I design them as temporary piles and increase the allowable stress by 1.33

2. In sizing the H-Pile, I use the moment and calculate the section modulus. Should I also be concerned with deflection.

Any advice is appreciated.

 

[MotorCity]

If the piles are encased in the wall and the wall is part of the 5-story building, how can the piles be temporary?

Not sure what your detail looks like, but I would be careful about trying to encase H-piles within a 14" thick wall. What size piles do you plan encasing in the wall?

Many of the recent codes do not allow a 1/3 stress increase. Even if it was allowed, I would tend to use it as a last resort and not rely on it in my design

I would check the deflection and make sure its not too excessive, use your judgment for what would be acceptable (i.e. you don't want people working in the area to become alarmed if they see the wall leaning)

Just a thought....15' is not a very deep excavation. Have you considered an open cut or benched excavation instead of shoring?

 

[kxa]

Attached is a wall cross section. the soldier pile wall will be inside the concrete wall with minimum 2" cover. The concrete wall will be restrained at the top and bottom according to the section. The piles are H 12x63's at 4 to 6 foot spacing. The excavation depth is about 16' and top of first floor to top of cellar slab is about 12'. Can't do open excavation because of close proximity to adjacent properties.

 

[MotorCity]

So during excavation the wall is spanning horizontally between H-piles. Since the piles are spaced every 4' to 6', the main horizontal reinforcing is interrupted by a pile every 4' to 6'. The horizontal bars will probably be very tedious and time consuming to install. How do you develop the horizontal bars?

Since your piles are 12" and the wall is 14", it looks like there is no tolerance on the outside of the wall since the piles are flush with the face of the wall. There is only a 2" tolerance on the inside face of the wall. Pile driving is not an extremely precise operation. If the piles are slightly off, you will end up with a wavy wall.

You may want to consider constructing shoring separate from the structure and either removing or abandon in place.

 

[kxa]

Piles will be installed in 2' diameter pre drilled filled with 2500 psi concrete.

With top and bottom slabs shown in the section view, the span becomes much smaller and it is no longer a cantilever hence using the temporary scenario.

 

[dcarr82775]

I would never used the 1.33 increase for soldier beams, as the load isn't that temporary. I know folks that use 0.75Fy for their allowable bending stress in the soldier beams, but I don't subscribe to it. One good rain event is all it takes to ruin a skimpy design. You indicated you have adjacent structures, if those are in the zone of influence you need to check deflections as a cantilever wall of that height will move a fair bit and anything it the zone of influence will be effected. Quite a few softwares out there can those calcs for you, or the old NAVFAC manuals have a decent hand method to estimate deflections.

With the 2ft pier and 4ft spacing your need to reduce your resistance strengths for overlapping pressure which will hurt your deflections as well. The H12 feels a little small for a 16ft cut, at least for the 6ft spacing.

Are the H piles your permanent structure as well?

 

[jayrod12]

The tallest I've ever got H12s to work cantilevered is around 11-12 feet. Albeit that's in mucky clay. It seemed to keep the deflection in check as well.

 

[oldestguy]

kxa: By not providing any soil data, I don't know how anyone can comment positively on your design, nor comment on deflection.

 

[oldestguy]

A few more items. Drilling a 2 ft. diameter hole will extend into adjacent property.
Will the hole be filled to the starting ground surface with concrete? If not, what will fill that space above b'smet floor grade? In order to install timber lagging some added excavation and stuffing of voids will also be beyond the PL. Any nearby shallow footings that impinge load on this wall? Is the lagging suitable for the loads imposed? What do local building codes say about this type of job? Who is responsible for holding up nearby property?

The more I look at this I think it needs the input of a geotech engineer and advice from an experienced contractor versed in this type of work.

 

[kxa]

Here is some more information: The soil is dense silty sand to about 26' and then dense sand. Bearing capacity 2-3 tsf. Angle of internal friction is 31 degrees. There is really no bldg. in the immediate area (15'+ away)and those have basements with foundations at least 12' below grade. Also have car ramps for under ground garage in between the buildings and my foundation walls. So, there is really not much load coming from those areas from the ground level down. Only two sides of the property that sidewalk exists will have soil all the way up to the first floor level.

I used Retain Pro and included 400 pcf (from soils report) for passive earth pressure, multiplier of to for passive wedge and 300 psf of surcharge to design the piles. The program gives the required embedment length and the max. moment but not the deflection because it does not recommend sizes. Used Fb=0.6*(Fy=50 ksi) and 1.33 factor to calculate section modulus and also increased the embedment depth by 1.2 factor.

 

[dcarr82775]

If nothing else I learned to never buy RetainPro, the deflection is the only part that is tricky to calc. So thank you for that. Why are you multiplying the moment by an embedment depth factor? Does RetainPro only give you the moment at the base of the cut and not the actual maximum moment in the beam down in the embedded portion? I do not know how RetainPro calculates the embedment (presumably with some multiplier on the pier width), but generally if your 2ft piers are closer than 6ft o.c. (3 diameters although some folks use other multipliers) the soil resistance needs to be reduced.

Another reason you need to calc the deflection is that the HP12 will deflect a lot at that depth of cut with those soils. I wouldn't be surprised is it deflected more than 2", enough to stick out of the finished wall.

 

[oldestguy]

Maybe it is time to get out the calculator and the calculating pad. Also, with all that nearby space, an agreement for tie backs looks needed.