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 do you know if you are exceeding the capacity of your forms?

We frequently receive inquiries concerning increasing the maximum allowable force on a self-stressing form. Sometimes this is a result of increasing strand quantities and sometimes it is the result of a design change to a larger diameter strand.

For years,the largest strand diameter was 1/2" and the design load was 28.9 kips per strand. With the introduction of low-lax and 1/2" special strand,the loads were often increased to 31 kips and even to 34 kips. These relatively small increases normally fell within the design factor of safety of the form. Now, however, with the rapid growth in the use of 0.6" strand, the additional forces may well exceed the form stressing capacity.

Therefore, it is imperative that producers contact their form supplier if they intend to use any strand larger than that for which the form was originally designed. For instance using 0.6" strand in a form that was designed for 1/2" strand might very well exceed the overall capacity or cause local buckling at the ends of the form, resulting in a form failure and possible injury to personnel.

Also, there are certain conditions to be aware of that could result in the unforeseen reduction of maximum stressing capacity of a form. Consequently, any reduction in the form stressing capacity could be very crucial when evaluating the impact that increased strand size or quantities could have on the ability of the form to carry additional load. Some factors to be aware of are:

  1. Severe corrosion of the form skin and/or understructure.
  2. Uneven bearing of jacking plates at the end of the form.
  3. New or enlarged holes in the jacking plates that were not approved in advance by your engineering department or form supplier.
  4. Forms that have not been properly anchored to the foundation to prevent uplift and at the same time allow the form to expand and contract longitudinally.

Evaluating the potential increased capacity of a self-stressing form can only be based on assumptions that the form has been properly maintained and in good working condition. All forms should be periodically inspected to ensure they are kept clean and properly maintained so they will continue to perform as intended. In our next newsletter we will discuss the basic requirements of good form maintenance.