×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Period of a simple span Bridge

Period of a simple span Bridge

Period of a simple span Bridge

(OP)

 How can I calculate the period of a simple span bridge ?How the period is affected if the Bridge is jointless i.e.there is no joint between the deck slab and the approach slab.

RE: Period of a simple span Bridge

Well, you will find the period of this type of structure much like you would any other system.  You start with the basic equation involving the mass and stiffness of the system - the system here is really very basic though.

The system can be treated for simplicity sake as a lumped mass at the center of the bridge.  The stiffness of the system will be the typical deflection formulas for simple beams.  

If your interested in the transverse, lateral response, then for the case you describe, it will be necessary to use the fixed-fixed case for deflection.  The difficulty will be in determining the properties of the bridge cross-section.  For slab on girder bridges, the slab will control the properties so an esitmate can be made from there otherwise you'll need to brush up on the parallel-axis theorem and dissmilar properties especially if your dealing with steel and concrete.  Anyway, using a unit load (1 kip, or 10 kips or something similar you can find the value of p/delta.  The period is easily determined from here.

One word of caution, if your looking for the vertical period of the single span bridge - you will have to look closely at the boundary conditions.  Jointless bridges aren't really fixed per se in that direction as the abutment is usually founded on piling.  It is the properties of the piling that allow a rotation to take place beneath the abutment beam thereby creating a pin connection.  Thus for the vertical case, you need to use the equation for deflection of the pin-pin case.

I hope this helps.

RE: Period of a simple span Bridge

(OP)

 You mention nothing about the stiffness of the abutments.In your opinion abutment stiffness shall not be included in the calculation of the period of a simple span bridge ?

RE: Period of a simple span Bridge

Hi KOKONIS36,

If you are looking for the first flexural frequency for a simply supported bridge, isolated from the abutment by its bearings:

f1=(PI / (2*(L^2))) * (EI/mass per unit length)^0.5

where E in Pa(N/metre^2)
      I in metre^4
      mpul in kg/metre
      L in metres
      f1 in Hz

This formula is found in the Commentary of the Canadian Bridge Code CSA-S6-88

With regards to the effect of a jointless bridge I would not count on the contribution of the approach slab over the life of the bridge as the integrity of the approach slab is a function of the soil beneath.

HTH

Regards

VOD

RE: Period of a simple span Bridge

Yes, there is no account for the abutment stiffness...simply a rule of thumb treatment for determining the fundamental period of the bridge and not higher modes where it is likely that the soil structure interaction will be prominent.

If you'd like to condsider the abutment stiffness then the problem changes to that of a multiple degree of freedom problem that may necessitate a computer solution.

However, the FHWA - Seismic Retrofitting Guidelines for Highway Bridges gives a good example for soil stiffness and pile stiffnesses at abutments.  In summary the pile are afforded a stiffness of 40 kips/in/per pile while the abutment is given a value of 200 kips/in/foot of abutment width.  Note that while the pile will act at each abutment, the soil stiffness due to passive pressure will only be active for the compression case.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources