lrfd seismic load calculation

huangyhg

lrfd seismic load calculation
i am designing a 961 long flyover. it consists of 31 spans. the girders are considered to be simply supported.elastomeric bearings are used between girders and pier cap. the foundation consists of 9 piles.for seismic load calculation i am using single mode spectral analysis method for seismic load calculation.
my questions are:
1. for calculating seismic load do i need to model for the whole flyover, or can i just model for girder simply supported between two piers, will it be too conservative?
2. also how do i model the substructure, can i consider that the piers are fixed at the pile cap level,or do i have to model for the piles embedded in the soil also?
thanks in advance
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 re  
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   
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   
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.