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Re: 33-ft simple span slab

[Subject Prev][Subject Next][Thread Prev][Thread Next] Ben,

A 3" plank with 1/2" strands at 6 1/2" on center and placed with 1" cover from bottom will have a strand eccentricity from the centroid of the section of 1/4". The uplift moment from this along the length of the plank is only about 3 kip ft.

This will provide no precamber.

The deflection in your construction condition will be about 1 1/2" at the quarter point of the 33' length assuming midspan shoring (2 * 16.5' spans continuous with a 3" plank supporting the load) and a construction load of 30psf. This allows for cracking but no long term effects as it is a short term condition.

After the shoring is removed the extra deflection caused by the shoring reaction being reapplied to the 33' span, assuming no cracking, would be about 2" so the total short term deflection due to self weight after the shores are removed is going to be in the order of 2" at midspan and more at the quarter points, about  3 1/4" based on 1 1/2" + 3/4 of 2" (assuming no cracking which is very unconservative as shown below).

The stress in the bottom of the section at the quarter points in the shored condition is about 800psi so this section is cracked. This increases by about another 900psi at the quarter point after releasing the shoring, resulting in a tension stress at the surface of 1700psi at the quarter point of the 33' span.

It is very cracked and the short term deflection will be significantly more than the above 2" - 3".

The total long term deflection will be much larger as the compression stresses involved are very large and a kcs factor of 2  for long term effects is not applicable..

If you look at the construction condition stresses, ductility and ultimate strength, I would suggest not standing under it while they are pouring the topping.

At 06:37 PM 13/05/2005, you wrote:
Gil,
 
The 3" plank is pretensioned with 1/2" strands at 6 1/2" on center and placed with 1" cover from bottom. The eccentricity produces about 2" prestressing camber. This 2" camber can be maintained by mid-span shoring.
 
Ben
 
----- Original Message -----
From: Gil Brock
To: seaint(--nospam--at)seaint.org
Sent: Thursday, May 12, 2005 5:59 PM
Subject: Re: 33-ft simple span slab

Ben,

Only the 3" precast slab is pretensioned, not the whole slab. Is there 2" precamber in it?

Everything built on the floor will experience the full deflection as the precamber is only a visual thing.

You must be putting in a lot of strand to stop cracking under service load for this slab to keep it uncracked. The service stress from the gravity loads is going to be in the order of 2200psi tension (if I have converted 15 - 16MPa correctly). So you need about 12-13MPa (about 1800-1900psi) of precompression plus bending stress in the 3" slab from the prestress. Are the strands placed at minimum cover to the bottom of the 3" slab or placed concentric? If concentric there is no precamber and I would be worried about it snapping in half if they are very eccentric at this level of compression.


At 01:19 PM 13/05/2005, you wrote:
Gil,
 
I have no problem justifying it by calculations in meeting building code. What haunts me is the long-term serviceability. Keep in mind the slab is pretensioned and I don't expect any cracking under service load. The 50mm immediate deflection you mentioned will be probably canceled out by the upward prestressing camber.
 
Ben
 
 
----- Original Message -----
From: Gil Brock
To: seaint(--nospam--at)seaint.org
Sent: Thursday, May 12, 2005 4:11 PM
Subject: Re: 33-ft simple span slab
Ben,
I am not a believer in the approximate methods in the codes for deflection calculations either because they are grossly inaccurate and not conservatively so in many cases. But I am a believer in proper deflection calculations which allow for cracking and long term effects logically as our software does, not simply as fudge factors.
However, with a short term uncracked deflection of about 50mm, I would not even worry about calculating the cracking and long term effects. The slab is not pushing the limits, it is way past them.
You can use any arguments you like about the slab never being loaded etc but the end result is that you have presumably been contracted to provide a design to the local design rules which normally requires deflection checks for the design loads. That is what the client is paying for.
Hope your insurance is paid up and there is no clause in it voiding the cover because you deliberately ignored standard design procedures and logic.

At 10:16 AM 13/05/2005, you wrote:
Gil,
 
I know the 8" slab is mathematically pushed to its limits for the span in terms of serviceability and strength by calculations. Long-term deflection is a complicated thing and I am not a die-hard believer of the formulas. But I am interested to know the practical results of any similar applications.
 
You raised a valid concern about vibration. Theoretically the slab system has a frequency of around 3 Hz. However, being a dormitory building floor slab, there are many non-structural partition walls running floor-to-floor height and the vibration will be greatly mitigated.
 
Ben Z, P.E.
 
 
 
----- Original Message -----
From: Gil Brock
To: seaint(--nospam--at)seaint.org
Sent: Thursday, May 12, 2005 12:31 PM
Subject: Re: 33-ft simple span slab
Ben,
I would be very worried about deflections, vibrations and everything else. It is too thin. Do the calculations.
At 06:18 AM 13/05/2005, you wrote:
I am pondering on using  8" composite slab (3" pre-tensioned plank with 5" topping) for a 33-ft simple span floor slab in a dormitory building.  The plank will be shored at mid span before topping. My only concern is the long-term deflection. Has any one had similar experience?
Thank you
Ben Z, P.E.
Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022               Fax +61 7 3807 8422
email:            gil(--nospam--at)raptsoftware.com
email:            sales(--nospam--at)raptsoftware.com
email:            support(--nospam--at)raptsoftware.com
webpage:          http://www.raptsoftware.com

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022               Fax +61 7 3807 8422
email:            gil(--nospam--at)raptsoftware.com
email:            sales(--nospam--at)raptsoftware.com
email:            support(--nospam--at)raptsoftware.com
webpage:          http://www.raptsoftware.com

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022               Fax +61 7 3807 8422
email:            gil(--nospam--at)raptsoftware.com
email:            sales(--nospam--at)raptsoftware.com
email:            support(--nospam--at)raptsoftware.com
webpage:          http://www.raptsoftware.com

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022               Fax +61 7 3807 8422
email:            gil(--nospam--at)raptsoftware.com
email:            sales(--nospam--at)raptsoftware.com
email:            support(--nospam--at)raptsoftware.com
webpage:         http://www.raptsoftware.com