Prestressing of Shoring Braces

[jdonville]

PEInc was good enough to point me to information on how to prestress bracing members in reply to one of my posts in the Structural Engineering fora.

I am now interested in learning WHY these members are prestressed. Also, why are only members longer than say, 50ft (~17m) required to be prestressed - i.e., what is the signifigance of that length?

All help appreciated.

Jeff

 

[PEinc]

PL/AE. The longer the brace, the more it can compress when it picks up the full load. The 50 foot reference is probably based on someone's personal experience.

I rarely have had to pre-stress my braces.

http://www.PeirceEngineering.com

 

[BigH]

Jeff - I would suppose that there must be a limit on the amount of movement of the shoring (Singapore has very strict rules) - and this will ensure that the movement is minimized.

 

[BigHarvey]

The only project where I have seen pre-stressed bracing was on Cairo subway stations. The struts were 23 m long. The main goal was to minimize displacements since the existing buildings on shallow foundations were only 4 m behind the diaphragm walls.

 

[jdonville]

Thanks, folks. Since we do have a deflection restriction for this current job, I doubt that I'll be able to sweet talk the CM/design engineer out of prestressing these braces.

J

 

[PEinc]

Jeff, remember that, unless the brace is horizontal, the shortening of the brace is not exactly the same as the lateral movement of the wall.

If you are worrying about compressive shortening of the braces, don't forget to also worry about temperature related shortening and lengthening of the same braces.

http://www.PeirceEngineering.com

 

[PEinc]

Jeff, you did not say what you are supporting. If you are supporting a building with a sheeting wall, be very careful. Underpinning is usually more appropriate when digging near and below the footings of a building.

http://www.PeirceEngineering.com

 

[jdonville]

PEInc,

We're supporting natural gas line-infested soils, no aboveground structures. Horizontal bracing - 2 levels. Total design cut is 40ft.

The project is restricted against using tiebacks because the owner is a natural gas utility. Can't say I blame them.

Jeff

 

[PEinc]

Sounds like fun, Jeff. Good luck with the bracing. Do you need to use bracing at each level or will they let you install tiebacks, under the pipes, for the lower bracing level(s)?

http://www.PeirceEngineering.com

 

[BigH]

jeff

I may be late but got thinking about what they do in Singapore which, as you would surmise, is full of support problems.

They put in a diaphragm walls on both sides - and then they cast a cross beam between the walls; then they excavate. There is an article that I have and will send you. The cross walls there, due to the very deep excavations are up to 10 m high - but of course, for shallower excavations they wouldn't have to be.

 

[BigH]

Jeff - sent you the paper I was talking about. BigH

 

[jdonville]

PEInc, BigH:

Unfortunately, the owner does not want ANY encroachments onto their 'active' property. Even if they would allow ties, we have already procured and fabbed the materials for the internal bracing.

Due to the contaminants in the soil at this site (remediation project), the diapragm wall might not have worked very well, but will keep it in mind for the next go-round.

The project involves excavation and replacement of the contaminated soils in cells, which keeps the earth retention geometry nicer, but still a pain. The current phase is internally braced at the ends with ties along the 2 "parallel" sides. The next phase is more square (more like a parrallelogram, though) and is just internally braced. We are reusing one side of the Phase 1 piles and lagging for sides of future phases.

Thanks for the tips...
J

 

[BigHarvey]

With circular diaphragm walls you could get rid of the bracing. Not ideal to cover the complete zone but can be a solution !

 

[jdonville]

We got down to 40ft below grade recently - but the Ohio River is making it difficult to backfill presently. The water is currently 13ft above the bottom and still rising.

J