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

What is the difference between self-stressing and non-stressing forms?

In general there are two types of custom steel forms - self-stressing forms and non-self-stressing, or free-standing, forms.

Designing non-self-stressing forms includes designing the form skin, longitudinal skin stiffeners and transverse gussets such that the form will resist the hydrostatic pressure of the concrete. The deflections of the form must not exceed the allowable dimensional tolerances of the concrete product.

Self-stressing forms require that the forms be designed to resist the hydrostatic and compressive forces introduced when prestressing.

Twenty-five years ago, the standard design for self-stressing forms was to make use of compression bars to carry the prestress forces. The skin was designed to resist only the hydrostatic forces from the concrete.

After much thought and analysis, we determined that in some cases the compression bars could be eliminated by increasing the thickness of the skin and increasing the size and quantity of the longitudinal stiffeners. We called this a "stress-skin design" and began incorporating this design in 1978. We then contracted with Consulting Engineers Group (CEG) to conduct a load test to determine the actual stress levels in a double tee form using this design. Upon completion, CEG furnished us with a summary of the test results. This information allowed us to further refine our design of self-stressing forms. The elimination of stressing bars (when possible) allowed us to build more economical forms.

In some cases, such as inverted-tee beam forms, a combination of skin and stressing bars are used to carry the prestressing forces.

So what is a standard form design today? We have almost always used 3/16" thick gussets spaced 30" on center as our standard on most forms. However, we have some customers who ask for 1/4" gussets on 20" centers and 3/8" gussets where form sections bolt together. They want their forms to last! Other customers will tell us that they are purchasing a form to cast one job only. In these cases, gusset spacings of 40" centers will probably be adequate.

We frequently get requests concerning modifications required to cast a slightly different product using an existing form. This generally requires additional stressing capabilities. This frequently happens with stadium forms. Metromont Prestress, for instance, has used the same forms in casting several different stadiums. They have requested that we design their stadium riser forms with the intent of using them for multiple projects. This requires the use of design concepts that may make the form more expensive up front, but much more economical over the years.

In general, thicker form skin and closer gusset and stiffener spacings will result in a more durable and longer-lasting form.

When you contact our sales staff, let us know exactly what your form needs are for each particular project. Do you want the form to last 20 years, or will you be using it for six months with the intention of discarding it after the job is complete?

In summary, some form designs may vary from one manufacturer to the next. It's important for you to know what kind of form you need, how long you want it to last, and how much experience the form designer has. The old adage, "you get what you pay for," also applies to custom steel forms. The top six executives at Hamilton Form have over 175 years of combined experience in the precast/prestressed concrete industry. This experience translates into proven designs and valued engineering expertise for our customers.