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Re: Beams Supports

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Ren
án,

If I understand your question correctly, you want to know when neoprene
(elastomeric) pads are used as bearings for bridge beams (regardless of whether
they are steel or concrete beams).

Elastomeric bearings (EB) are used extensively for your average bridge
structure. EB are useful when you need to reduce the superstructure inertial
forces applied to the substructure during a seismic event since they allow the
substructure and superstructure to move independently of one another.  They are
large in plan (due to their low compressive strength) but extremely short.  They
require very little maintenance (the pads have an external rubber covering which
provides some protection).  AASHTO has provisions for specifying a low
temperature grade elastomer so they work in extremely cold environments.  EB are
effective when you have relatively small amount of thermal movement and vertical
and horizontal loads.  Steel laminated neoprene pads will provide greater
resistance than plain pads.  If the thermal movements/loads are extremely large
you generally would want to go with multi-rotational bearings (e.g. pot
bearings, spherical bearings, etc.).  I hesitate giving you maximum movements
and loads since AASHTO allows you to design them essentially for any load, but
as the Shape Factor deviates from a certain value (I believe it's 7;sorry but we
rarely design them anymore, we just take sizes from tables), it is my
understanding that the bearings start to become less cost-effective than certain
types of other bearings.  For NYSDOT, the  published maximum vertical load is
1250 kN (280 kip +/-) with a maximum ONE WAY movement of 48 mm (1.9 inch).

There are a whole slew of high tech elastomeric bearings which fall under the
category "seismic isolation bearing".  There was an interesting story about the
performance of the major seismic bearings in the July, 1999 issue of Civil
Engineering.  I can fax a copy to you if you like.

There is an old publication out by FHWA which covers the design of elastomeric
bearings.  I think I have a copy at home.  AASHTO also has published
specifications for the "Design of Seismically Isolated Bridges".  As to a
publication which discusses when to use them, I'm not sure of one.  Might I
suggest you check AASHTO's web site (http://www.aashto.org) for a complete
listing of their publications or search a bunch of DOT web sites to see if they
publish any guidelines.  The guidelines published by NYSDOT and NJDOT just list
limitations without quantifying them.  You can also check the web for major
bearing manfucaturers (Merriman, Conserv, Cosmec).

I hope this helps.  Any questions, feel free to ask.

Hector






"Renán" <mirosi(--nospam--at)caoba.entelnet.bo> on 09/10/99 01:52:42 AM

Please respond to seaint(--nospam--at)seaint.org

To:   seaint(--nospam--at)seaint.org
cc:    (bcc: Hector Morera/NYC/AmmannWhitney)
Fax to:
Subject:  Beams Supports


Someone knows when it should be to use neopren compound or neopreno simple,
 in
supports of bridge beams? (article of any specification)
Thanks and mispell my english
                                                            RENAN