increase in allowable stress due to duration 2

[ichibansan]

Is it permissable to take a 15% increase in allowable stress of wale beams used to brace/support sheet piling around a 30'x32'x26'deep excavation? Obviously, the shoring is temporary, hence the increase. Any comments?

 

[JAE]

aub,

We have done similar adjustments for temporary shoring situations as you describe. What we always do, however, is adjust the applied soil load, not the allowable stresses in the steel or wood.

We normally check with a geotechnical engineer who is familiar with the types of soil that exist behind the shoring and get a somewhat smaller equivalent fluid pressure to use. The concept is that the build-up of lateral earth pressure would not occur immediately but require a long-term condition. Be careful, however, to make sure that the contractor keeps water runoff from spilling or draining into the material behind your wall.

 

[austim]

For many years I have argued strongly against using ANY overstress for all temporary work.

I have no problem with JAE's approach that uses a realistic short term load (similarly for wave, wind, earthquake risk), but...

Fatal accidents resulting from failure of temporary work are still all too common (with cofferdams being well represented in the statistics).

Remember that the hydrostatic component of the earth pressure doesn't know that you are treating it as 'short-term'.

There may be a tendency to treat the entire matter of temorary work design and construction as being less 'important' than permanent work, hence less supervision in the design office and on site.

Overloading of temporary work is not as rare as it should be.

There is a risk of unauthorised site modifications to all temporary work (due to unforeseen obstructions to sheet-piling, etc).

There is a significant risk of excessive loads due to crane operation outside any initially planned needs (on one project with which I was concerned, the there was an accident involving crane planned for a critical lift on the day before the lift, and the replacment crane loaded a temporary wharf in entirely different loactions from those expected in our design office). A similar state of affairs could increase surcharge locally on the material around you cofferdam.

All in all, talk of any general overstress allowance horrifies me - whenever I lecture on the topic at our local Uni I tell my students the very reverse - allow for the totally unexpected (eg vertical/horizontal loads at midspan of cofferdam struts to allow for impact from concrete skips), and ensure that temporary designs have a good reserve of robustness.

 

[JAE]

Good points, austim,
Your point is very well taken in that the majority of structural collapses occur during construction. One visit to any jobsite does a lot for an engineers education in what really gets built.

 

[Trussdoc]

One point to keep in mind is that a 15 prcent duration factor represents a period of two months during which the wood is fully loaded. If the project lasts, or can be reasonably expected to last, any longer, this factor should not be included.