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RE: Pedestrian bridge vibration and strengthening

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Title: RE: Pedestrian bridge vibration and strengthening

Chanon,

The AISC guide D811 ("Floor Vibrations due to Human Activity") gives some guidelines for vertical vibrations of footbridges which you might find helpful. Our research suggests that the approximated dynamic coefficient (alpha) is unconservative for some frequencies, but the overall guidance is useful.

If you find that you are likely to have a problem with vertical excitation and you would need to change the frequency a lot to solve it, then, as Thomas Murphy mentions, moving it out of the critical range by stiffening can be expensive unless you can radically change the modes of the bridge - such as by adding another support. Remember that stiffening the structure will involve adding mass, which in itself lowers the natural frequency! Damping with viscous dampers and/or tuned mass dampers is one approach, but you may find it difficult to assess the effect without software that can analyze complex modes (such as NASTRAN).

As for lateral vibrations, provided your lateral modes are not less than 2Hz as stated, you should not have a problem. If you have lateral modes at lower frequencies then you may have problems irrespective of your structural system.

Obviously, these are general comments and the solution will depend upon the exact form and dynamic properties of the bridge.

Regards,

Andrew Mole, MIStructE

See www.arup.com/millenniumbridge/ for details of the problem and the retrofit.

NB These are my private comments and not those of the company.

_________________________________________________________

From: "Murphy, Thomas P" <TPMurphy(--nospam--at)Modjeski.com>
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: seaint Digest for 11 Jun 2001

Chanon:
        I have found that dynamic problems are best dealt with by using
dynamic solutions.  The "trouble" range of natural frequencies for
pedestrian bridges is generally 1.5 to 3 Hz, and I think you'll find that
the additional stiffness you need to move your bridge out of this range will
be expensive.  I think your best bet is to look into tuned mass dampers.
That is what is being used to fix the millennium bridge in London.  Hope that
helps,

       
Thomas P. Murphy, Ph.D., P.E.
Harrisburg, PA

-----Original Message-----
From: "Benz non" <chanonb(--nospam--at)hotmail.com>
To: seaint(--nospam--at)seaint.org
Subject: Pedestrian bridge vibration and strengthening

Dear all,
I would like to ask for your comments on how to solve a man-induced
vibration on a pedestrian bridge.
The bridge is 30-m long, steel-truss pedestrian bridge with nearly 2 Hz
natural frequency and very light damping.
We are considering strengthening method but we are lacking of experience.
Installation of dampers is one of the solution but we want to know whether
strengthening works or not.
Other solutions are appreciate.

Thanks for sharing

Chanon M.
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