Earthquakes! Earthquakes!

[skavuvie]

With all these earthquakes that are happening recently and great in magnitude (chile, haiti, tiawan, aftershocks in chile)in like a month, how is it affecting your design viewpoint. are you being more conservative in your design now then before? just wondering what the average design engineer is thinking about now? as always all comments are appreciated. (discussion is good)

 

[JAE]

Just following my normal, good, somewhat conservative engineering judgement.

If you look on the USGS or other websites - the frequency of seismic events really isn't statistically changing all that much - just more quakes in areas of higher population.

 

[steellion]

I'm not in a high-seismic area, but I trust the experts that wrote the code. Chile's earthquake was 100X stronger than Haiti's but caused 1/1000th of the damage due to stringent building codes and good seismic detailing. I never go about any project trying to design all members to 100% stress, but I don't have my own arbitrary safety factor that I add either.

 

[bones206]

I'm not really a design engineer, but it strikes me that the conservatisms that structural engineers are used to applying to their designs are counter-intuitive when it comes to seismic designs. Rather than being more conservative for seismic design, I think that the real goal is to be more precise.

For example, A615 rebar can be conservatively specified grade 60 ksi for elastic design because there's a cushion for the yield strength. But for seismic design, you might want the rebar to yield at a specific stress level, in which case having a MINIMUM grade 60 ksi is not appropriate or necessarily conservative.

So I would say that rather than being more conservative, I would concentrate more on the precision of my design and materials and the predictability of my structure's true behavior.

 

[Teguci]

bones206,
The "Quake"ers have already implemented a factor in order to make sure that maximum yielding is obtained for high seismic areas. Its called Omega for "System Overstrength Factor" and it usually runs at 2.5 times allowable. Doesn't really meet my definition of precision but it does serve the purpose of keeping plastic hinges away from brittle connections.

 

[hokie66]

I am in favor of doing our best to produce structures to perform satisfactorily in earthquakes, but "precision" is a word which has little place in structural engineering.

 

[sandman21]

Omega is not a factor to ensure maximum yielding in a structure, it is there to allow items that are critical to the performance of the system that resist seismic forces to stay close to elastic as possible allowing the the inelastic range as a reserve capacity for when the forces exceed that level, and they will. The big one in So Cal. is expected to generate forces in a building 2-3x what is required under current code. Good detailing and the reserve capacity of the system is there to prevent a total rapid collapse of the building. Rebar is a poor example, the detailing that goes into the RBS or the hinge in a SCBF are better examples of areas that we use more "precision".

 

[apsix]

As hokie said; "precision" is a word which has little place in structural engineering, and even less place in defining eathquake loads.

 

[jenofstructures]

you can't fight the angry nature. But then again, we can always be ready on everything that will happen.

I'm actually afraid of these earthquakes. Our country is part of what they called ring of fire. Our government decided (just now?) to check our old structures and found out that some of it have cracks already. Even the bridges have cracks. We are lucky that the earthquakes occurred recently is not that strong.

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[bones206]

Just to illustrate my point...

In ACI 318,

21.1.5.2 — Deformed reinforcement resisting earthquake-
induced flexural and axial forces in frame
members, structural walls, and coupling beams, shall
comply with ASTM A706. ASTM A615 Grades 40 and
60 reinforcement shall be permitted in these members if:
(a) The actual yield strength based on mill tests
does not exceed fy by more than 18,000 psi; and
(b) The ratio of the actual tensile strength to the
actual yield strength is not less than 1.25.

The yield stress of A615 rebar has to be more "precisely" controlled for seismic design, for lack of a better word. I may be misguided here and out of my range of professional competence, but I think there's a difference between being conservative and being rigorous. I dont design buildings but thats my 2 cents worth.