Safety of Elastic vs. Inelastic Design 1

[Robbiee]

Hi all,
The requirement for providing a minimum level of ductility for structures in certain seismic areas or for certain types of structural systems is to justify the reduction of design earthquake loads for economic reasons and to prevent certain types of materials such as unreinforced concrete block walls to be used. However, for steel and reinforced structures, what I don’t understand why the inelastic design, i.e. design for reduced load with the reliance on inelastic energy dissipating detailing, is safer than elastic design for the full calculated loads. One might say that the design loads could be exceeded in a server event. But, we know that steel and reinforced structures have inherent over strength of a minimum of 30% and if the design load is exceeded, wouldn’t mean that higher than designed ductility is required to dissipate the additional load.
The reason for the question is two conversations I had recently; one with a professor researching the use of carbon fibre as slab reinforcing and concrete reinforcing in general, who agreed that elastically designed structures are not less safe, and the other conversation with a very experience engineer who has won almost every engineering award, who laughed when I said that elastic design is not less safe that in-elastic. Please comment. I wish to know what I am missing here.

 

[connect2]

Are not some structures like nuclear reactors designed for elastic behaviour?

 

[Robbiee]

EngineeringAdam,
I wish I had posted the question of the evaluation of the existing building from the beginning because that was the whole point of my post. In the discussion with the experienced engineer, I said almost exactly what you wrote in your last post except that here in Canada we don't have standards to deal with the evaluation and upgrading of existing buildings, but guidelines based on the NEHRP handbook for seismic evaluation of existing buildings. The experienced engineer did not agree and insisted the building needs new SFRS to comply with the current code.

 

[EngineeringAdam]

Same here Alimar, wish I realized earlier that the existing building application fits perfectly with this discussion.

I should note that many engineers don't practice in the retrofit of existing buildings. Even I have limited knowledge on the specific application of 31, 41, Fema, and NEHRP documents on the matter.

However, since not many engineers practice or even possibly unaware, I am not surprised your engineering friend doesn't agree with your argument.

 

[PicoStruc]

Ailmar,

In Quebec, we have an addition to the NBCC, labeled as Part 10, they state condition that require seismic rehabilitation such as, change in mass or in SFRS capacity (include strength, displacement, ductility, etc...)

This can be a help for your province

 

[Robbiee]

Thanks PicoStruc. In Ontario, the Ontario Building Code also has a similar part for renovations. It is Part 11. It has a section for when a building needs to be upgraded to the current code.