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RE: Concrete plank - weld plates

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Double tees have different recommendations than plank do. They should not be fastened at the bottom of the stems. They need to be fastened at the flanges, as you have stated.
 
 
-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Saturday, January 07, 2006 10:39 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Concrete plank - weld plates

I am not sure that the welding at alternate ends really works - I inspected a damaged structure that used double tees with the bottom of web welded at alternate tees and there was severe damage at many weld plates.  The double tee roof was in the southwestern US, with the top directly exposed to hot sun by day and cool nights.  The detailing was clearly intended to allow for expansion-contraction but many weld plates had pulled out of the wall. 
 
We considered connecting the base of the tees only along one wall - but that of course creates problems with diaphragm action.  In reviewing recommended detailing for double tees, we concluded that the tops of the tees should have been attached to the walls rather than the bottom of the tee webs.  One possibility is to use a detail that allows axial movement but resists lateral movement along one wall. 
 
I have used hollow core planks in roofs attached at each end without problems - they typically were covered by roofing materials, which I think helps reduce some of the thermal movements.
 

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

 


From: Kestner, James W. [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Thursday, January 05, 2006 8:59 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Concrete plank - weld plates

We are not talking about large movements or sliding, we are talking about infinitesimal movements, stress buildup and stress relief. Since the weld plates are unyielding, an unacceptable stress level builds up until something ruptures (either the plank or its support). The concrete has the ability, thru creep movement and warping, to relieve these stresses (without cracking).
 
Jim K.
 
 
 -----Original Message-----
From: Kevin Below [mailto:kevinbelow(--nospam--at)videotron.ca]
Sent: Thursday, January 05, 2006 9:22 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Concrete plank - weld plates

********** "The structure can still be made stable if lateral forces are able to be adequately transfered to all weld plates thru the grouted joints between planks and/or the topping." ************
 
Jim, I don't understand.  I think you are saying that welding at alternate ends allows the unwelded end to move and avoid the stresses due to creep.  This would mean that the joints between adjacent planks would allow sliding between planks, and that the topping would crack along the joint line.  if the grouted joints and the topping can transfer the shear between planks in the diaphragm, then how can they allow the sliding ?

Kevin Below 

 

-----Original Message-----
From: Kestner, James W. [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: 5-Jan-06 09:59
To: seaint(--nospam--at)seaint.org
Subject: RE: Concrete plank - weld plates

The plank manufacturer is correct! This seems to go against your intuition of fastening everthing down and the more the better. In the 50's, many precast buildings (with multiple bays) had structural problems where plank were welded down at both ends. 
 
This is generally not too much of an issue with temperature but much more a problem with axial shortening. Any member that is pretensioned or post tensioned will undergo long term creep (axial shortening) because of the high stress in the tendons. If welded down at both ends, the stress will build up enough to damage one or more ends of the plank or its supports.
 
Precasters and engineers have learned (the hard way) to weld plank at only one end and allow the movement to occur at the other. The technique that is typically used (and seems to work) is to weld alternate ends of adjacent plank. The structure can still be made stable if lateral forces are able to be adequately transfered to all weld plates thru the grouted joints between planks and/or the topping.
 
I hope this helps!
 
Jim K.
 
 
 -----Original Message-----
From: hadiprawira djohan [mailto:hadiprawira(--nospam--at)yahoo.com]
Sent: Thursday, January 05, 2006 8:18 AM
To: seaint(--nospam--at)seaint.org
Subject: Concrete plank - weld plates

Fellow Engineers,
 
From what i encountered in the past, If a plank is resting on top of a steel beam, the plank is ussualy tack welded to the top flange at both end of the concrete planks.
 
Until recently, a plank manufacturer argue that the plank is only need to be welded at one end, in order to allow expansion and contraction of the building, which may result in the disloding of the weld plates and spalling of the plank.
 
I disagree with his argument, considering that concrete planks are located inside the building, where temperature is constantly regulated, thus no extreme expansion and contraction can occur.
 
Besides, 3" weld plates at every 48" plank section is not that rigid, If one is to argue that welding plank at both sides will create a very rigid structure, which may not perfrom too well in the seismic event. 
 
Would anyone please tell me what i am missing here? Thank you.
 
 
djohan.
 


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