Hamilton Form, Ltd.
7009 Midway Road
Fort Worth, TX 76118

Phone: 817 590-2111
Fax: 817 595-1110
sales@hamiltonform.com

PCI NPCA CPCI

Frequently Asked Questions

How should we detension a long-line self-stressing vessel-type form?

The industry-wide use of self-stressing vessel type forms has created problems for some prestressers. Generally, they attempt to speed up (lower labor man-hours) the stripping process but unknowingly create problems in the process. Too often we hear that the product is spalling, sticking, cracked or getting black marks.

To better understand the problem, one has to realize that when you stress a self-stressing form, it shortens. The form expands during the curing cycle. Then, during the detensioning operation, the bed will elongate due to the release of the prestress force. Hence, for a self-stressing vessel type form:

Rule #1 Keep the product in the location it is cast. This is easier said than done in some cases.

Rule #2 Fix the centerline of any long-line self-stressing form. This doesn't reduce the shortening/elongation, but rather splits one-half to each end.

Too often we hear that the strands are symmetrically cut at each end of the bed. Bang! Rule #1 is broken. The bed expands since all the prestressed force is released at the end and the products are trying to move toward the centerline of the bed.

In addition, when discussing detensioning procedures, we ask, "When do you release your strand depressors?" We normally hear, "That's the first thing we do in our detensioning sequence." Wrong! This creates an uplift on the product while still in a stressed (tension) mode.

Rule #3 Detension approximately 40% of the longitudinal strand prior to releasing the depressing load. Be sure to check with your plant/design engineer to determine this percentage (number of strands).

Example of suggested detensioning sequence for a double tee form with 10 strands per leg depressed at centerline using two cutting operators:

  1. Starting at both ends and proceeding to centerline at each header location, symmetrically cut 20% (2 strands per stem).
  2. Starting at centerline and proceeding to both ends, cut 20% (2 strands per stem).
  3. Release strand depressors (remove tapered pins if top depressor type).
  4. Starting at both ends and proceeding to centerline at each header location, symmetrically cut 30% (3 strands per stem).
  5. Starting at centerline and proceeding to both ends, cut remaining 30% (3 strands per leg).

Caution: This procedure should be verified by your engineering personnel. Following this procedure will insure that you have met the requirements of Rules #1 thru #3 and should minimize, if not eliminate, most common product stripping problems from occurring when using self-stressing vessel type forms.