AASHTO - Temporary Structures 1

[bhargav]

Hello,

I remember reading that the allowable stresses can be increased by 25% for temporary structures. I don't have the relevant AASHTO with me.

Can any one site the reference / clause No. to me.

Thanks,

Bhargav

 

[Qshake]

There are a number of places that this could surface, however, a quick review of Section 3 Loads doesn't come up with this explicitly. Yet there is the table which includes all applicable loads and acceptable overstresses. Also I looked at "Construction Handbook for Bridge Temporary Works" and "Guide Design Specifications for Bridge Temporary Works" wherein different load cases are noted for different structures. With each load case is a different overstress ranging from 100 to 133%. So you'll have to be more specific.

 

[breaks]

Like QShake says, tough to pinpoint what you're trying to find. The only allowable overstresses that I know of are from 100% to 150% for different load cases listed in AASHTO Table 3.22.1A. This is allowed because of the extreme nature of these load cases and the improbability they all the listed loads would occur at once for an extended period of time.

 

[ishvaaag]

The only reason I see to keep the reductions on the safety factor is economy, through easir constructability with less weight. This may be enough, but I am becoming less and less sympatizing with the reductions of safety factors for the work periods, for it reinforces the idea of some conditions of life being more valuable than others, as happens with the goods.

In all, the reduced safety factors whilst in the works can be tantamount to the statement of that accidents during the works are less important than elsewhere, or economical loss should be more bearable there than elsewhere, an implicit justification and maybe real cause of accidents happening in the works.

Uniformity in the safety of humans is a condition of just government. I support the same safety factors be used during the works for anything that needs such verification.

 

[austim]

I have put my view on record elsewhere, but here it comes again.

In my opinion, ishvaag is absolutely correct in his anti-overstress stance.

If anything there are very good arguments for REDUCING the allowable stresses, on the clear evidence that accidents resulting from faulty design/construction of temporary works are all too common.

I put the increased risks down to several factors - design by site staff who may not have the necessary design background to be doing it; a general view that temporary work should be designed by the more junior staff (such structures being seen as 'second class citizens', not deserving the serious attention of 'real' structural engineers); design-in-a-hurry or design-by-guess; insufficient consideration of the possible accidental loads (striking a cofferdam strut with a full concrete skip can be a great way to kill your colleagues); careless assembly; lack of proper quality control, or quality control by personnel with insufficient experience of temporary work and the realities of site operations; overloading of temporary work by personnel who do not understand these matters. I could go on...

The major cause of the high accident rates could well be that some codes have encouraged the belief that a short design life IN ITSELF should justify higher design stresses. A moment's consideration should demontrate the flaw in that.

'Standard' design stresses are set to allow for the maximum likely Live Load that will occur in a long design life. THus the full design traffic load on a long bridge will be very rare event (I wonder if the Golden Gate ever saw full Live Load until the nature of road traffic changed radically?)

Although temporary work (eg bridge falsework) may only be loaded for a single day, it will almost certainly be loaded up to its full design load (or higher), since that will be primarily the dead load that it will support. Thus the added real factor of safety that we get when we design 'permanent' structures is not available to the temporary work designer.

 

[breaks]

ishvaaag and austim,

Do you know of references that allow for an increase in allowable loads for temporary structures? As I said above, I only know of this for Load Combinations in bridge design (which is definitely not a temporary structure!!).

What you both say makes sense but I'd like to try and pinpoint a reference that allows this increase.

Thanks, Matt

 

[ishvaaag]

Maybe NJDOT, first link in search...

http://www.google.com/search?q=temp...reduction&hl=es&btnG=B%FAsqueda+en+Google&lr=

 

[ishvaaag]

By NBE AE 88 the presently mandatory code for loads in Buildings in Spain, section VIII.8.6 for foundations and for eccentrical loads allows a 25% increase of allowable pressure against the soil. It needs not even come from earthquake or wind, being eccentrical is enough.

 

[austim]

hi again, bhargav

I suspect that one of the 'culprit codes' was BS 153 Steel Girder bridges Part B: 1958, which has often been read out of context to permit a general allowance of 30% overstress for temporary conditions.

In fact what BS 153 says is this:

"Combination 'c'. The worst possible combination of forces during erection...

c. For the forces of Combination 'c' above, the allowable working stresses shall be those stresses given in Clauses 25 to 36 inclusive increased by 30 per cent. Additional material shall be added or other provision shall be made to keep stresses within that limit..."

What BS153 does NOT say is that you can accept any overstress at all for the basic combination of dead load plus construction live load.

I have little problem with BS 153 as written, but what has happened is that many engineers have (entirely incorrectly) taken it to mean that you can accept 30% overstress under static gravity loads alone, and ignoring the possible effects of impact, wind, temperature etc.

The general acceptance of overstress has, I suspect, developed from that type of provision into a culture of "everyone uses 25% overstress for temporary work"