Elastic Seismic Design loads on (older) Bridges 1

[KSRAO]

I understand from literature that a very small % (about only 6%) of gravity weight was used in the design of many of the older bridges (even in California) against earthquakes.

I was wondering if there's any reason/basis for choosing such a small lateral design forces knowing the importance of seismic design?

Thanks in advance.

-Rao.

 

[Qshake]

believe it or not, earthquake engineering wasn't very well understood until recent times. As such, engineers mostly neglected the effects of lateral seismic loads until the codes began to descibe/mandate them.

For example, significant static equivalent lateral force for schools weren't required until after the 1933 Long Beach Earthquake in the US. It wasn't until the State of California effected the Fielding Act.

As a comparison, the equivalent static lateral force really was initiated in Europe following the Messina, Italy earthquake of 1908.

While its true that the equivalent static force method was in use prior to the 1933 Long Beach earthquake, the magnitude remained small. And until the Fielding act, I'm not sure it was mandatory. Hence, you can see the disparity on the two continents.

The response spectrum method wasn't developed until the 1940s and only after significant computational power was available. The latter was necessary to analyze all of the specific data available to arrive at the code definition of the RSA. As one may figure, that technology takes awhile to be absorbed by the engineering community. And to exacerbate matters, highway technology lags behind due to funding.

Thus you can see why only minimal lateral force values were in use until following the 1971 San Fernando event, which really showed the community just how bridges are vulnerable to strong ground motion.

 

[KSRAO]

Thanks QSHAKE. I've two more related questions.

(1) Was it the case with the design of building structures too? I mean, older bldg. structures were also designed for smaller % of their gravity loads?

(2) Theoretically speaking, can we take advantage of the presence of high degree of redundancy in buildings to to design them for lower seismic forces as compared to bridges(say in terms of ARS values)?

Can someone point me to the literature on the comparative study of various design procedures (elastic, capacity and dutility/displacement based approaches) as applied to bridge/bldg. design - preferably quantifying their differences ?

-thanks,
Rao.

 

[CanyonRat]

Do you mean Force Based vs Displacement-Ductility Based? They both pretty much use elastic models for the RSA.

AASHTO and FHWA examples are a good source for the force-based method (R factors).

Seismic Design and Retrofit of Bridges by Priestley, Seible, and Calvi is a good source for the displacement based (Caltrans, etc).

There is a lot of stuff out there now on the net also.

MikeD

 

[KSRAO]

Hi Mike, thanks for our response.

In the references you mentioned, I could only find different approaches discussed separately. There is no talk of QUANTITATIVE DIFFERENCES among different approaches.

Let me try on the net as you suggested (is there any specific site you have to suggest?).

-thanks,
Rao.

 

[CanyonRat]

You are right about comparing the two. Several years ago I was involved in some bridge designs where we performed the seismic analysis / design using both methods. Everything is hardcopy and packed in boxes somewhere.

I remember two main things after finished: 1)Don't try and mix and match the methods and 2)The dispacement/ductility method makes more sense but involves more analysis.

I can't spend too much more time on this but check the Caltrans sites and search the web on earthquake research. The ASCE Journal of Bridge Engineering had some stuff awhile back.

A lot of this stuff is in publications.

MikeD