Built up section modeling

[tedamiao]

I have a tough question on a project I am working on. We are using concrete pile foundation with shorecrete wall to retaining soils. Retaining height varies from 8ft to 25ft....I am using 24'' to 30'' concrete pile with steel W beam in the middle. Basically it is a soldier pile but it is not temporary.
There are huge moments at the base (assuming it is a cantilever columns embeded into soils). Steel beam is very large if considering steel bending capacity only. I am thinking to use the composite section but do not know how to model this section in SAP2000.

Does any engineer here know how to model concrete pile section with W beam in the middle? Thank you very much.

 

[Taro]

When the depth of excavation exceeds the capacity of the bare steel section as a cantilever, we usually add one or more tiers of tie-backs. This ends up being cheaper than a composite cantilever. Sometimes tie-backs are difficult because they can extend beyond the property line. Even then, it is usually more economical to negotiate a temporary easement to install the tie-backs.

 

[tedamiao]

Tie back system can not work because it is over property line. Outside of property line is CALTRANS right of way. No way to get permission from them.

 

[Taro]

If tie-backs are not an option you can try to design the composite cantilever system. You will need to reinforce the concrete cover and provide transverse confinement. See AISC specification chapter I.

You would be surprised how resourceful contractors can be in securing permission to install tie-backs. I once had a project with a 4-story deep excavation and the adjoining property owners refused to allow tie-backs. We designed the excavation shoring using a cross-lot bracing system. The contractor obviously hated the bracing because it complicated his construction sequences. So he negotiated the rights to install the tie-backs with the neighboring property owners and kept increasing his offer until they said yes. It still ended up being cheaper than the braced option. Public agencies deal with easements all the time, so it might not be as impossible as you would think if you have the right connections and political savvy.

 

[tedamiao]

It will take a lot of time to get permission from Caltrans, probably 1 year. For sure, client can not afford this timeline. Perhaps, there are some telephone, electic or other embeded wires overthere, there will be a great chance for tie back cable to hit them.......

 

[JAE]

Don't forget to check the capacity of the soil as well. I just checked a 30 ft. high secant wall system and the concrete piers, at 60" diameter, would overstress the soil at the base of the excavation. And the deflection at the top of the wall was 4 to 6 inches. Way more than would be acceptable.

 

[msquared48]

If tiebacks are not warranted, then you might consider a raker system.

Mike McCann
McCann Engineering

 

[bkjd]

rackers are not bad, although hard to work around, you might try internally braced assuming your excavation plan dimensions are not to large, either way it is a pain.

you can freeze the ground big bucks....

soil nails may be an option depending on the geotech conditions, unless your right against the property line.

 

[tedamiao]

It is close to property line, only 15' to 20' away.
The retaining wall is part of the building (five stories total). We are planing to use 10'' concrete mat foundation. I think soil capacity shall be ok if we following flat pole design to get embedment (could be 50ft or more embedment) by putting piles at 8'-0'' o.c. W beam is W24x250 if no composite section.

What is raker system??? I do not have idea for that.

Yes, it is a pain.......Feel so bad.

 

[Taro]

Rakers are inclined braces on the interior side of the excavation. Can be efficient, but you face the same constructability issues I mentioned with the cross-lot bracing.