Closure Strips

Forum for discussion of structural detailing and construction practice of post-tensioned structures.

Closure Strips

Postby Mark Plog on Thu Jan 24, 2008 8:31 pm

1) When placing a closure strip at the 1/4 span point of a tendon profile is it better to center it's location at the 1/4 point or place the edge of the closure strip at the 1/4 point so as to maximize the distance from the closest supports?

2) For the side of the closure strip closest to the supports should the tendon profile be a drop cantilever from the high point or should the parabolic profile be maintained? It would appear unrealistic to try and keep a parabolic shape for such a short distance (4-5 ft typically) particularly in the banded direction.
Mark Plog
 
Posts: 1
Joined: Thu Jan 24, 2008 8:25 pm

Re: Closure Strips

Postby Dr_Bijan_Aalami on Fri Feb 08, 2008 3:26 am

Where spans are essentially of equal length, positioning the closure strip at ¼ span length from the support leads to a location of low moment with a typical tendon profile being at the centroid of slab. The proper profile for tendons that terminate at the two sides of a closure strip is to follow the path as if there were no interruption through the strip. Hence for the case of tendons in shape of reversed parabola in typical spans, the short side of a closure strip will have a profile that is similar to the typical span, and extends just passed the inflection point to anchor at the mid-depth of slab.
Dr_Bijan_Aalami
 
Posts: 158
Joined: Mon Jan 07, 2008 12:30 am

Re: Closure Strips

Postby Jvaldes on Mon Nov 14, 2011 9:17 pm

Dear Dr_Bijan_Aalami,

I would like to better understand the way a closure strip should be modeled on ADAPT PT. From my understanding, as soon as you incorporate a closure strip on the slab, each side of the separated slab would work independently, therefore two models are required having cantilevered ends where the closure strip begins/ends. Nonetheless when the closure strip is placed at L/4 the lowest moment happens in the final condition when the slab acts continuous; so is it possible to model the entire design strip with out the pour strip; and the pour strip is just a detail located where the lowest moment occurs or 2 analyses are a better representation of the real condition. Or maybe there is better way to model it, your advice is greatly appreciated.

Thanks,
Jvaldes
 
Posts: 1
Joined: Sat Nov 12, 2011 12:36 am

Re: Closure Strips

Postby Dr_Bijan_Aalami on Mon Dec 05, 2011 4:39 am

You are right in that for regular geometry and uniform loads the moment at the ¼ span location does not govern the design. Hence, assuming that the two facing ends of a closure strip are propped prior to casting the closure, you suggest that the structure can be modelled , analysed and designed in its entirety for the final geometry.

Within the approximations inherent in extracting a design strip and modelling it in ADAPT-PT, your suggestion is a reasonable approach. However, when using the current ADAPT-PT, tendons will be modelled passing through the closure gap. This impacts the analysis results and the final design. As a first step, you need to design the closure strip region as RC, using the design values from the program, recognizing that the design values were obtained assuming continuity for tendons.

The new release of ADAPT-PT allows you to terminate the tendons on each side of the closure strip, and automatically designs the gap as an RC member. It eliminates the approximation in the current version for design of closure strips.
Dr_Bijan_Aalami
 
Posts: 158
Joined: Mon Jan 07, 2008 12:30 am


Return to Construction and Detailing


cron