Question about redundancy 3

[Lion06]

I have always wondered this about redundancy in a system. My understanding is that the idea behind redundancy is to provide an alternate load path should a support be lost. Is this an accurate assessment?
If it is then my question is this. What exactly is the point of having redundancy in a system. It seems to me that unless you consider every scenario for which a support could be lost, you could spend way too much time figuring out alternate load paths - this seems very subjective in terms of which supports one would assume to be lost.
Also, in the case of these redundancies (say a continuous beam over let's say 3 supports) you have redundancy such that if one of the supports (columns) is lost there is an alternate load path. Here is the crux of my question. Why does this matter if the member is not robust enough to handle the load at this new span?
Every continuous member I have seen designed is around 80%-90% stressed. If you suddenly turn this 2-span (25' each) into a single span (50') or a 25' span with a 25' cant, it is likely this member is going to fail.
Is this common or do you generally consider the potential "new" loading conditions assuming a given suppport is lost? If not, is this really designing a "redundant" structure or just taking advantage of the inherent benefits of continuous beams?

 

[apsix]

I don't think that the term redundancy is particularly helpful or accurate.
I don't know about the US, but elsewhere the need to avoid disproportionate collapse is often the requirement.

"Disproportionate Collapse:
A3. The building shall be constructed so that in the event of an accident the building will not suffer collapse to an extent disproportionate to the cause." (From Approved Document A - 2004 of England & Wales)

Also the load factors used for the 'accident' case are a lot less than for normal load cases. From memory they are something like 1.05 & 0.4 instead of 1.4 & 1.6, for dead & live loads respectively.

 

[SlideRuleEra]

IMHO, the alternate load path does not have to meet the strict requirements of building code specified load combinations and allowable stresses. All it has to do is "not collapse" without regard to deflection, yielding, etc.

For example, you mention that structural members you work with are 80%-90% stressed. I assume, this would be of ALLOWABLE stress (say 24 KSI for A36 steel). For the redundant system situation the stress could go up more than 50% (say, to 36 KSI). Also the "design" load for the redundant situation would likely be lower.

Of course there are also many occasions where the redundant system must be capable of replacing the primary (sort of a belts-and-suspenders scenario). Naturally, the cost goes up for this.

http://www.SlideRuleEra.net

 

[UcfSE]

Ideally you want to have an alternate path for life safety. This isn't typically required but in some cases designing for progressive collapse is a requirement. In this situation you have some options, one of which is providing an alternate load path.

In your example of a continuous beam becomgin simply-supported, ask what you mean by fail. Do you mean fails its design code checks, or do you mean collapse? Are you accounting for catenary action in the beam as it loses its flexural capacity? It becomes important to define what exactly you mean by failure and what it takes to get there.

 

[shin25]

A non redundant system will collapse suddenly without any notice, even if just a single support fails. On the other hand, a redundant system may fail, but will give a warning before it does so. This may give enough time to move out the users from the harms way. Particulary, the example of redundant system that you have given, if the middle support fails, this member will provide catenary action before failure. This probably will buy some extra time.

More over, now a days some of the essential structures are to be designed to resist progressive collapse. Where your statement holds true, if a middle support fails, the member needs to be capable of carrying load effect from the span that is now twice as long.

 

[csd72]

redundancy is not about the building being able to support the full load via an alternate load path.

It is about the building being able to stand up some-how if a support is removed.

You are talking about letting continuous beams act as cables and sway frames act as vierendeel trusses.

This still requirs some thought though.

I would suggest you do some reading:

http://www.aisc.org/Content/Content...Resources/Facts_for_Steel_Buildings_Blast.pdf

http://www.ce.berkeley.edu/~astaneh/1-Publications/Astaneh-9ASEC Blast Paper 2003.pdf

http://dimacs.rutgers.edu/Workshops/NJHomeland/slides/mdobossy.ppt#21

http://www.wbdg.org/ccb/DOD/UFC/ufc_4_023_03.pdf

csd

 

[Lion06]

Thank you all, I appreciate the input