Need a book? Engineering books recommendations...

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

PRECAST/PRESTRESSED BEAMS

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Candi Anderson wrote:

"Can someone give me a lesson on debonding of prestressing strands in
this
type of construction?  What is the purpose?  Is it always necessary?
How
might the length be determined? Anything else?
Thanks-

Candi Anderson, P.E."


Candi,
In a prestressed beam, at midspan you want the strands to be near the
bottom of the section, just like a normally reinforced beam.  However,
if the strands are straight, this leaves you with the same eccentric
prestressing force (i.e. moment) at the ends of the beams, where there
is no DL moment to counteract it.  If you try build a beam like this, it
usually explodes at the ends when you release the jacks holding the
strands, because there is bending due to the eccentric prestressing
force at the ends that results in too much tension in the top of the
beam.

There are several ways to deal with this.  Sometimes, the prestresser
will harp the strands, which means that they are positioned at middepth
of the beam at the ends, but held down at midspan to get the depth you
need.  This works well but adds complexity to their work.  The
hold-downs get cast into the concrete, so you just bought a bunch of
hardware with each beam, and as a bonus you get to look at a torched-off
piece of steel at either the top or bottom surface.  Alternatively, in
theory, you could put rebar in the top to counteract the unbalanced
moment at the ends, but the concrete has to crack to mobilize the rebar,
so this method is not favored.  The other way to solve the problem is to
get rid of the prestressing force at the ends of the beams, and arrange
for it to start far enough into the beam so that there is some dead load
moment to counter the eccentric force.  That's what debonding does--it
eliminates the prestress force in the debonded area at the ends of the
beam, by letting the strand slip within the concrete.

Prestress manufacturers make beams, planks, etc. on long casting
beds--basically long sidewalks with a large anchorage at each end.  They
might be 500 or 600 feet long.  They thread the strands into the
anchorage at one end, jack them tight (really tight) from the anchorage
at the other end, and lock them off.  Then they set forms about the
strands and pour the concrete.  When the concrete has enough strength
not to explode or crack (hopefully in 16 or 18 hours, so they can cycle
their beds daily), they cut the strands at the ends.  Depending on what
they're making, they may build bulkheads to start and end each piece
(more typical of large bridge beams) or they may cast a continuous
ribbon of product and sawcut lengths from it (more typical of hollowcore
or slab-type products).  Ideally, on a 500 foot bed they try to cast
about 498 feet of members, because if they only make a 100 foot beam,
they get to throw away 400 feet of strand.  Either way, they figure out
where to put the debonding tubes along the 500 feet of strand so they
end up at the ends of the members.

Debonding carries a few problems of its own.  It leaves a stress
discontinuity in a high-shear area, so shear is more of a concern than
in a continuously stressed member.  And, it eliminates the protection of
the concrete from the strand at the ends of the member.  In a bridge
beam or parking ramp member,  de-icing salts can then wick along the
strand the full length of the debonding, attack the strand and destroy
the end of the beam.

By the way, if you think about this a little, the need to debond at the
ends of some simple-span prestressed members might point out the folly
of trying to cantilever such members.  I've seen this called for in
hollow-core plank by engineers or architects who didn't know better.
The prestresser rolls his eyes, shrugs, and puts a lot of large rebar in
the top of the member.  It cracks when he picks it up off the bed and
droops noticeably when it's set.  Everybody gets all worked up.  He says
it's not his fault, which is true.  The only way to do it right is to
not do it at all--these plank need to be supported at their ends only.

Long answer to a short question--HTH.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********