Temporary Loading Increase for Structural Steel 1

[BsB56]

I am designing a temporary soil retaining structure to be used during construction. The structure will only be in place for a day or two till it is removed.

Is there any code or provision I can use to justify increasing my allowable strength to yield? In order words designing the steel with a factor of safety closer to 1.0 then the standard allowable safety factors per AISC 13th ed?

 

[MiketheEngineer]

Not that I am aware of... Seems like every one or two day project ends up being one or two months - or years!!! Plan on it.

 

[PEinc]

Some older soils and foundation books mention the use of higher stresses for temporary earth retaining structures. Others mention of using 80% of the empirical earth pressure envelope to design the sheeting or soldier beams, while using the full envelope for designing the bracing or tieback anchors. It is not uncommon to use a 20% increase in the allowable stress for temporary steel sheets, soldier beams, braces, and wales. Finding a reference for this is hard, especially for cantilevered members.

http://www.PeirceEngineering.com

 

[msquared48]

As an corollary here, a lot of "temporary" shoring becomes, in effect, permanent shoring if the cut and shoring are placed, then the project is delayed or cancelled.

I have seen this happen in larger projects, and the shoring system just sits there for years.

Consequently, I don't trust "temporary".

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[kelowna]

My approach is not to reduce factors for temporary conditions. I can accept a shorter return period for environmental loads (10 year return wind rather than 50 or 100y) and not to use earthquake loading.

Otherwise the factors are there for a reason, and that reason is independent of the time the structure is in place. The factor that reduces steel strength deals with variability of the steel material, not with the time the loads are in place.

 

[csd72]

BsB56,

This is currently what i am doing full time so I have recently put some thought and research into this.

I would not recommend reducing the factor of safety as this allows for both steel variables and for variations in the soil from those that have been estimated.

The stress reduction mentioned by PEinc is a valid reduction which is mentioned in UK excavation literature (particularly TRADA - timber for excavations). In this book they talk of reductions of 25 to 50% in the flexural members due to arching of the soil between the much more rigid prop points. But as this is due to arching then the full load still needs to be allowed for at the props.

 

[PEinc]

There may be several reduction factors for use with temporary earth retention structures - to account for arching, temporary allowable stress, and using about 80% of the empirical earth pressure load for the sheets or soldier beams. If you apply one reduction factor, you will probably be OK. If you try to apply more than one, the odds are that you will soon have a problem job.

Terzaghi & Peck's earth pressure envelopes for braced sheeting walls were intended to calculate the maximum anticipated bracing loads - not the load on every brace. Therefore, the envelope is generally conservative. That's why you can usually reduce the pressure to design the sheet piling or soldier beams.

For a few thousand temporary projects, I usually used the full earth pressure envelope for calculating the tieback and brace loads but then allow a 20% temporary overstress for the temporary sheet piling, soldier beams, and wales. It is almost always approved without question. Sometimes, a railroad will allow only a 10% overstress.

http://www.PeirceEngineering.com

 

[JAE]

Check out ASCE 37.

https://secure.asce.org/ASCEWebSite/BOOKSTORE/BookDescription.aspx?ProdId=5255

 

[ishvaaag]

Current spanish code doen't allow for reductions anymore. I agree this is against all prior practice but they have gone (sometimes irrealistically) conservative.

Safety factor is 1.6 for "transient" or persistent loads (no temporary anywhere). It can go up to 1.8 if the level of control of concrete is low.

 

[ishvaaag]

What above would apply for steel as well, but no the 1.8 for low control of concrete.