Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Reinforcing Existing Beam

[Subject Prev][Subject Next][Thread Prev][Thread Next]
First a comment - it is described that the current beam has a 6 ksi margin.
Is that based on
checking the beam with dead load only?  If not, can you remove the live
loads prior to retrofit
to increase this margin?

One solution they have used on bridge girders is the installation of bottom
flange post-tensioning.
In your case you might be able to temporarily post-tension the bottom flange
to reduce the tension
stress, add cover plates, and remove the post-tensioning.  

> -----Original Message-----
> From:	P.Rajendran [SMTP:rajendra(--nospam--at)]
> Sent:	Sunday, March 14, 1999 9:15 AM
> To:	seaint(--nospam--at)
> Subject:	Re: Reinforcing Existing Beam
> Nawab:
> The best method will be to convince the client that the existing beam
> needs to be replaced with a significantly stronger beam, which means that
> some equipment shut-down will be required for a short duration.  If the
> client is the average bullhead, then forget  the existing beam.  Select a
> new beam to carry all the loads, i.e existing+new. Note that the lateral
> bracing that exists on existing beam may not be available on new beam, so
> a lower allowable stress is possible.  Install new beam such that the top
> flange of new beam is practically in contact with the bottom flange of
> existing beam.  Install new equipment, which will deflect existing beam to
> become snug with new beam.  Connect the flanges in contact with some bolts
> so as to avoid any accidental lateral movement of existing beam relative
> to new beam.
> Alternatively, is there any way you can transfer load away from the
> existing beam by adding some beams in some other directions?
> Rajendran, P.E.
> Rehan.Nawab(--nospam--at) wrote:
> >           Folks,
> >
> >           I am in a situation, where I have to reinforce an existing
> >           beam (18B40) while it is still carrying load.  The client
> >           requirement does not allow for welding cover plates.
> >           Therefore, I am opting to add a new WF beam to the existing
> >           beam using 3/4" A325 bolts.  This existing beam has full
> >           lateral support, i.e. Fb = 24 ksi.  This existing beam under
> >           existing loads is stressed to 18 ksi.  This only allows for
> >           approximately 6 ksi (18+6=24 ksi)  after adding the new
> >           beam.  Since, the new loads are heavy, this requires a very
> >           large composite section modulus to keep the stress around 6
> >           ksi.  In, this case W33x201 attached to 18B40.
> >
> >           Is this approach overly conservative?  Is there any other
> >           way you would analyze the problem?  Existing beam is up-high
> >           and I don't think, I can have the beam jacked-up to unload
> >           the existing stress in the beam.
> >
> >           Existing Moment = 100 ft-kip
> >           Combined Existing and New Moment = 385 ft-kip
> >           Composite Section Design Moment = 285 ft-kip
> >           Composite Section Modulus = 611 in3
> >
> >           Any ideas?  Thanks in advance.
> >
> >           Rehan Nawab, P.E.
> >
> >           P.S. I posted this question yesterday, but it appears it got
> >           missed some how.
> >