seismic for multi-span bridges
seismic for multi-span bridges
(OP)
I am designing a multi-span steel composite beam with concrete deck. I have few questions on seismic design of piers/abutments in multi-span bridges.
Below is the bridge restraint information
Single-span
*End abutment 1 Free in longitudinal direction
Restrained in transverse direction
*End abutment 2 Restrained in longitudinal direction
Restrained in transverse direction
Per AASHTO 4.7.4.3 seismic analysis is not required for the single-span bridges as long as the CONNECTIONS are designed for the seismic forces. I could not find a mention of providing battered piles to resist this seismic load.
Multi-span bridges (2-span/3-span)
*End abutment 1 Free in longitudinal direction
Restrained in transverse direction
*Interior Pier/Piers Free in longitudinal direction
Restrained in transverse direction
*End abutment 2 Restrained in longitudinal direction
Restrained in transverse direction
For multispan bridges, using above restraint conditions, for seismic loads in longitudinal direction, we are detailing the connections such that bridge "rides" over the interior piers in longitudinal direction and transfers all the load to the End Abutment 2.
For the multi-span bridge since the condition in the longitudinal direction is similar to a single-span bridge (free in longitudinal direction) do I need to just make sure that connection at the End Abutment 2 is designed for seismic force and not worry about providing battered piles at the fixed abutment to resist it?
I am providing battered piles for bracking force only. I am also designing the piers and its foundation for lateral load in transverse direction.
Below is the bridge restraint information
Single-span
*End abutment 1 Free in longitudinal direction
Restrained in transverse direction
*End abutment 2 Restrained in longitudinal direction
Restrained in transverse direction
Per AASHTO 4.7.4.3 seismic analysis is not required for the single-span bridges as long as the CONNECTIONS are designed for the seismic forces. I could not find a mention of providing battered piles to resist this seismic load.
Multi-span bridges (2-span/3-span)
*End abutment 1 Free in longitudinal direction
Restrained in transverse direction
*Interior Pier/Piers Free in longitudinal direction
Restrained in transverse direction
*End abutment 2 Restrained in longitudinal direction
Restrained in transverse direction
For multispan bridges, using above restraint conditions, for seismic loads in longitudinal direction, we are detailing the connections such that bridge "rides" over the interior piers in longitudinal direction and transfers all the load to the End Abutment 2.
For the multi-span bridge since the condition in the longitudinal direction is similar to a single-span bridge (free in longitudinal direction) do I need to just make sure that connection at the End Abutment 2 is designed for seismic force and not worry about providing battered piles at the fixed abutment to resist it?
I am providing battered piles for bracking force only. I am also designing the piers and its foundation for lateral load in transverse direction.






RE: seismic for multi-span bridges
RE: seismic for multi-span bridges
If the seismic loads would control the design if you designed for them, then you should design for them.
This includes the piles, and as bridgebuster says, you will get a more robust structure if you use appropriately detailed vertical piles to resist the lateral loads, rather than raking piles.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: seismic for multi-span bridges
Are you saying that we should design the abutment and piles for the lateral load even for single span bridge? If yes, how do I find the lateral load on the abutment when it clearly says no seismic analysis required? Please bare in mind that i am not asking this question with respect to transverse direction. My question is along longitudinal direction.
RE: seismic for multi-span bridges
A multi-span bridge, fixed longitudinally at one abutment only, is a different situation, and clearly the provisons for single span bridges do not apply.
A seismic analysis of the structure will give you loads in both the longitudinal and lateral directions, and if these loads are resisted at one abutment, then that abutment needs to be designed for them.
I should probably leave any more specific advice to those familiar with current US codes.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: seismic for multi-span bridges
On a multi-span bridge you have to analyze for the transverse & longitudinal forces.
I don't have an AASHTO handy but for Zone 3, aren't you supposed to do a multi-mode analysis?
RE: seismic for multi-span bridges
RE: seismic for multi-span bridges
Good luck...
RE: seismic for multi-span bridges
Where do you expect that force to go once it passes through the connection?
Simple span bridges do not have a "structure response" because there is nothing else to give the bridge a different period (like a column). The abutments move with the soil, and the superstructure moves with the abutments. I suspect this, as well as observed performance of single span bridges, is why the analysis requirements have been relaxed.
You will still have dynamic (e.g. Mononobe-Okabe) and inertial forces on the abutments. This is where I include that connection force, because it is still an "Extreme Event."
RE: seismic for multi-span bridges
thank you for the explaination.
"Simple span bridges do not have a "structure response" because there is nothing else to give the bridge a different period (like a column). "
Keeping this in mind, if in multispan bridges we releasae the movement in longitudinal direction for the internal piers and one abutment end, is it not similar to a single span bridge? Will the period in the longitudinal direction be similar to a simple span bridge as it will ride over the interior column?
RE: seismic for multi-span bridges
There is nothing wrong with having your Abutment 2 as the "fixed" support of a multi-span - you just need to design/analyze the support conditions and keep in mind what will happen as the horizontal forces become larger and larger.
You do not have a single span bridge and it is not located in Seismic Zone 1. Follow the code and move on. Seismic is just load case - no more, no less.