Q: Although we aren't having any trouble with our form ties, I wonder how safe they really are? Is there a standard factor of safety, and who decides what it is?
Design professionals plan structures using established safety factors, many of them clearly defined in codes and design standards. In construction, there are many cases where safety factors have not been defined so precisely by outside authorities. Much has been left to the judgment and experience of contractors and materials suppliers. Load ratings for formwork accessories sometimes fall into this category. Frequently the manufacturer determines the ultimate or breaking load for a particular product, assumes an appropriate factor of safety, and arrives at a load rating for the product. Typically, manufacturers have assigned form ties a safety factor of 1.5. Thus, if a standard snap tie breaks at a load of 4,500 pounds, it is given a safe load rating of 3,000 pounds.These customary load ratings must be reexamined in the light of recommendations recently adopted by the American Concrete Institute (ACI) Committee 347, Formwork for Concrete. ACI 347's new report shows minimum safety factors of 2 for almost all formwork accessories--ties, anchors, and hangers. Some form anchors are even required to have a safety factor of 3.
The form tie is a tension member adapted to holding forms securely in place, resisting the lateral pressure of unhardened concrete. ACI Committee 347 recommends using a safety factor of at least 2 for all form ties.
To comply with these recommendations, you'll have to read the fine print in the catalogs. Usually the safety factor used in arriving at the load rating is given. Often it is 1.5, although some sellers provide ratings based on both 1.5 and 2. Regrettably, we have even seen one catalog that showed safety factors as low as 1.25.To convert a load rating from a safety factor of 1.5 to 2, multiply by 0.75. Or if the catalog gives an ultimate load, just divide that by 2. Here are a few of the common ratings upgraded to safety factors of 2 and 3. For embedded anchors, you also must check the concrete strength needed to develop the capacity of the anchor.There is another safety concern for the form designer. Form ties are by definition tension members. They hold forms together by resisting lateral pressure of concrete, and published load ratings are generally for tension loads only. Unfortunately, real-world applications don't always conform to the definition. Bending and shear loads applied to form ties which weren't designed for them have been involved in some tragic failures. Manufacturers need to provide information on shear capacity of their products. Users need to be alert to field conditions, designing for loads other than tension where necessary. We mention the ACI recommendations here because for some years American Concrete Institute, through its formwork committee, has been recognized as a consensus-standards maker for formwork. The American National Standards Institute (ANSI) regulations for safety in concrete and masonry work cite ACI. Existing Occupational Safety and Health Act (OSHA) regulations for concrete refer to ANSI standards, thus indirectly citing ACI. The upcoming OSHA revision appears to be silent on specific factors of safety. but cites ACI 347 as a helpful reference.
|ULTIMATE OR BREAKING LOAD, LB.||SAFE LOAD RATINGS, LB.|
|safety factor 1.5||safety factor 2.0||safety factor 3.0|