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Re[2]: Prestress Losses in a Concrete Dome with a Post-Tensi

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The prestress losses will be due to (1) elastic shortening of the concrete
during tensioning of the tendons (cables), (2) stress relaxation of the
tendons (cables), (3) long term creep of the concrete, (4) shrinkage of the
concrete, and (5) friction loss along the length of a post-tensioned tendon
.
Off the top of my head, I'm not aware of any reference that goes through
these calcs specifically for a ring. But standard texts on prestressed
concrete or the PCI Handbook coupled with some basic stress analysis of a
ring under external pressure load to obtain the concrete compressive stress
(related to tendon force by p=t/r) is one hand calc approach. Will the
tendons (cables) be pre-tensioned or post-tensioned ? For prestressed
circular cables, seems like some hefty hardware in the formwork is needed
to
hold the cables in position and resist the radial forces generated by
pre-tensioned cables, so I wonder if post-tensioning is better for a
structure such as this. One caution: In the finished concrete structure,
the
tensioned cables produce radial forces and associated tension stress in the
concrete exist whether the cables are pre- or post tensioned. Design radial
rebar to resist this tension. A final question: is prestressed concrete
really required here, or is reinforced concrete adequate ?


______________________________ Reply Separator
_________________________________
Subject: RE: Prestress Losses in a Concrete Dome with a Post-Tensi...
Author:  <seaint(--nospam--at)seaint.org> ("Adair, Joel" <JAdair(--nospam--at)Halff.com>) at ROH
Date:    12/03/2001 10:57 PM


Subject:
RE: Prestress Losses in a Concrete Dome with a Post-Tensioned Ten      sion
Ring
---------------------------------




"Thin  Shell Concrete Structures", by David P. Billington, published by
McGraw-Hill  doesn't specifically address prestress loss, but it does
contain
useful  information about design of domes with a prestressed ring  beam.

--  Joel

-------------------------
Joel Adair, EIT
Halff Associates, Inc.
E-mail:  jadair(--nospam--at)halff.com
-------------------------
-----Original Message-----
From: Nels Roselund, SE  [mailto:njineer(--nospam--at)att.net]
Sent: Monday, December 03, 2001 6:12  PM
To: seaint(--nospam--at)seaint.org
Subject: Prestress Losses in a  Concrete Dome with a Post-Tensioned Tension
Ring


I'm designing a concrete dome for permanent support of an existing
unreinforced stone masonry roof and ceiling for stabilization of a historic
structure.  The existing roof/ceiling structure is to be  suspended from
the
new structure.  I'm looking into using a concrete dome  with a post
tensioned
tension ring.  Long-term deflection needs to be  minimized in order to
protect historic finishes on the ceiling.

Can you recommend references that will help me determine long-term
deformations of the concrete dome?

A reference published by ASCE has been suggested: Structural Analysis and
Design of Nuclear Plant Facilities, which has recommendations for
containment
structures.  I don't find that title in the current ASCE publication
catalog.  The copy that is available for me to borrow was published  in
1980.
Is there a more recent title or other publication that may  help with
determining deflections?

My dome analysis is being done using SAP 2000.  Means of  directly
determining the effects of prestress losses do not seem to be  included in
the program.  I assume that, once  the parameters of the prestress losses
are
determined, deflections  can be calculated by SAP 2000 by appropriate
modifications to the properties  of the dome elements.  Does that seem
true?

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net




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