LRFD seismic load calculation 3

Hi benjaman,

I am suprised that for so many spans you are able to justify single mode spectral analysis.

VOD

VOD,

I am a new user of AASHTO LRFD Bridge spec. However, from table 4.7.4.3.1-1 I find for seismic zone 2, irregular type 'other bridge' type, the minimum analysis requirement is SM. Since STAAD is the only analysis tool I have access to I had to resort to Single mode elastic method.

Please Comment.
Also would u please me how would u have modelled the bridge for this situation?

A flyover to most bridge engineers is a curved ramp structure. If this is true for your structure, the curve alone would giveway to irregular structure.

Also, with at total length of 961 ft, the number of spans 31 and only 31' per span....is that correct? If it were meters then the 31 meters would be around 90-100' or good for a prestressed girder structure.

Over 31 spans the stiffness from bent to bent must be very similar to use single mode analysis. And remember that AASHTO is only giving you a minimum.

Typically for large, long, and curved structures with variety of stiffness or bent height changes a multi-mode analysis is preferred such as the Response Spectra Analysis.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

Sorry Qshake for not mentioning the unit. It is indeed 961 m and the girders are post-tensioned girder.

Can u please tell me a bit more about the general practice of modelling the supports for such girders??

First I need to say your analysis should be reviewed by a competent bridge engineer. Should you need to do multi-mode analysis, then have your office upgrade from STAAD. I have found STAAD cumbersome for bridge design, STAAD (REI) mergered with LARSA, so perhaps your office will get some discount for LARSA. This is not a pitch for LARSA though.

Second I do not have a copy of the latest AASHTO Code, but this is my two cents.

Multi-spans can have varying pier stiffness conditions such as bearing stiffnesses, heights, spread footings/piled foundations and founding conditions. After all these are considered, then the decision matrix in AASHTO will come into play.

Since I do not know all about your situation, I can only give you a limited comment. Generally if I am sure of full fixity at the base of the footing, I would model the entire bridge (abutments, superstructure, piers) as a series of members, and then design the foundations accordingly, this will suffice.

HTH

VOD

Thanks,Need to talk to my supervisor about LARSA.

Actually I resorted to this site as he advised me to model the substructure as spring support for simplicity. Again from some LRFD example references, I found, they modelled for the piers as well but fixed the piers at the other end which sounds too conservative to me.

I think I will go for modelling the whole bridge (piers, footings,all). Again this will be too cubersome to be done by STAAD. Because I need to come to a solution quickly and even if my supervisor agrees to upgrade to LARSA, it won't be so quick.

Any further comment will be appreciated.

To this day, bridge design for me has not been quick.

And the quicker you make it, the worse the assumptions and results!

Use care in doing any type of analysis but especially a dynamic analysis.

For a full model that you're discussing it is typical to use spring constants for the foundation to soil connection. In most cases, you need an experienced geotechnical engineer to develop those for you.

You can't simply just look at the fixed foundation model as it will not give the maximum displacement of the structure.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

Benjamin,
Maybe you are already doing this, but I strongly recommend that you keep things as simple as possible. Most importantly, try to model each span as one horizontal line of beam elements. This is not only simple, but most accurate since you can fully account for multiple girders and the deck in the beam element properties. I have seen disasters when people try to model each girder separately. The only way to do that accurately is to model the deck with plate elements, which creates all sorts of complications.

Also try to model each pier as one vertical line of beam elements. You can simulate the bearings using member release commands, and you can specify beta angles to simulate the orientation of the piers and bearings. The tough part will be establishing the boundary conditions for your foundations, but try to keep it simple.

The output of the analysis will be your bearing and substructure forces and moments due to seismic. You can then make a spreadsheet procedure to distribute the forces to individual bearings and pier columns. Good luck.

How does everyone feel about MDX bridge design software? It is very easy to set up and analyze bridges with this software. We modeled a dual span curved girder bridge with it and had the beam sizes determined very quickly. STAAD flat out stinks to model a bridge in, very time consuming. MDX will even float the lanes for you.

Graybeach,

Thanks for ur valuable post. I had indeed started with beam elements to model each span. I will use beam elements for pier and piles as weel but planning to use plate elements for the pier and pile caps. How does this sound to u?

Benjamin,
I try to stay away from plate elements, especially with seismic analysis, if at all possible because I find it much more difficult to interpret the output and translate it into a design. I do not know the details of your case, but I would say to try to use beam elements for the pier and pile caps. This might mean combining the piles in the model so they are in single plane.

I agree with graybeach to stay away from plate elements. One reason that comes to mind is the limitations for plate elements which is typically based on a very thin plate compared to the width and length dimensions. Civil structures of concrete may not always fit the plate element assumptions.

As graybeach suggested you may have to use rigid body transformation to obtain a single set of DOFs to represent a pile matrix or even a set of bearings. Once the bearings or piles are reduced to a single node they can be easily placed at the ends of beam elements.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

buening - you should start your own thread for more responses to your comments.

MDX is a good program which does a lot for you and of course the output should be checked thoroughly. There are many updates to MDX as it's developers update it continually based on feedback from the engineering profession.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.