Seismic design for Zone 1, Elastomeric bearings, AASHTO Sec 3.10.9.2

[maneesha0702]

I am designing a bridge which is of 4 spans and the seismic zone is 1. The total weight of superstructure is around 2000 kips( 200' length , 4 spans)and all the beams are on elastomeric bearings. When I am checking for a pier what is the lateral load I should use. As per sec 3.10.9.2 my As value is >0.05. and if I use 0.25 times vertical reaction it is 500kips. If i use the Csm factor it works out to 0.08 and the friction factor for elastomeric bearing is 0.2 and the response modification factor for multi column pier is 5. Hence it works out to .08*0.2*(1/5)=.0032 *2000 =6.4 kips. I am really confused as to what to do. Any help is really appreciated

 

[BridgeEI]

I apologize that I don't have my AASHTO in front of me, but the 7th edition clarified some language that stated you could use the 0.25 times the vertical reaction for zone 1 or if you decided to do a refined analysis then you could use those calculated values even if they were lower.

 

[EQguys]

As per Section 4.7.4.1, bridges in SZ 1 need not be analyzed for EQ effects. Minimum bearing seat support dimension requirements of 4.7.4.4 and loads from 3.10.9 are to be evaluated. Please keep in mind that the vertical load to a Pier shall be tributary permanent loads. 0.25 times the vertical loads shall be distributed to the N number of bearings. So horizontal load at each bearing is (0.25 times Total tributary permanent loads)/N. You dont have to go into refined analysis.

 

[OSUCivlEng]

maneesha0702,

What Edition of AASHTO are you using? Just for kicks I went back and looked at the 5th, 6th and 7th Editions and they all say for Seismic Zone 1 use 0.15 x vertical reaction when As < 0.05 and use 0.25 x vertical reaction for all other sites in Zone 1.