Sesimic Analysis for Single Span Bridges

[n39]

Hi I was doing a structure analysis on a castelated beam pipe bridge. I was wondering can I use a simple pile cap analysis for the abutment and a simple pile analysis for the strauss pile reinforcement? By being simple, what I mean is to calculate it based on the dead and live load and not considering the seismic load. Since AASHTO LRFD and Standard Specifications call out to neglect sesimic design for single span bridges regardless of any sesimic zone.
Thank you

 

[BridgeSmith]

AASHTO excludes simple span bridges from most of the seismic design requirements due to the inherent stability and lateral strength of single span vehicle bridges. Specifically, their length to width ratios and construction preclude overstress and failure of bridges for which the spec is applicable. The abutments move with the ground, so the only substantial seismic forces on the superstructure are inertial.

Your bridge is a completely different beast. The length to width ratio is very large, so the lateral inertial forces due to an earthquake will produce significant, perhaps critical, stress levels in the beam.

 

[bones206]

What is the relative stiffness of the gas pipe relative to the bridge beams in the lateral direction? In other words, I hope someone is checking the seismic stresses in the pipe!

 

[BridgeNerdGuy]

I would not solely use the AASHTO LRFD Bridge Design Specifications for the design of this structure. The code is written for vehicular traffic bridges which have structural properties and behavior much different than this castellated beam.

Section [1.1] SCOPE OF SPECIFICATIONS states "The provisions of these Specification are intended for the design, evaluation, and rehabilitation of both fixed and movable highway bridges...Provisions are not included for bridges used solely for railway, rail-transit, or public utilities. For bridges not fully covered herein, the provisions of these Specifications may be applied, and augmented with additional design criteria where required."

My interpretation is you are working on a utility bridge which is a structure type not explicitly covered in the AASHTO LRFDBDS so the waiver of seismic analysis in Zone 1 should be used with caution. I would look to ASCE 7 for seismic demands.