QT WEB SITE – OPEN LETTER TO OUR CUSTOMERS
Our customers have enjoyed great success with Quick Tie products. Correspondingly, there has been a considerable amount of effort by certain of our competitors, and others, to criticize the Quick Tie system, including comments regarding purported “problems” in the “skipped story” hold-down systems in general and cable hold-down systems in particular. This open letter has been written to clarify how the Quick Tie system works and to counter the misstatement’s we’ve encountered regarding our product.
The misstatement - pre-stressed cable end wedge connections can loosen under pulsing loads
Our cable end connections are swaged on (not wedged in like concrete pre-stressing cables) and the pre-stressing force is applied by tightening a nut on the threaded end of the swaged connection until a calculated elongation has been achieved. This connection is not subject to loosening under pulsing loads as has been implied.
The misstatement - a pre-stressed cable will loosen up over time
The mechanism which causes a wire rope to lose some of its tension force is called relaxation. Based on regular testing of our product, this tension loss occurs over a period of approximately 40 days and causes a loss of between 15 and 25 percent. This loss is included in our pre-tensioning calculations
The misstatement - this system doesn’t adequately compensate for building shrinkage
The pre-tensioning calculations for our cable also include an allowance in the total required cable elongation to account for anticipated shrinkage of the building. The Quick Tie system has increasingly been selected in lieu of traditional floor-to-floor strapping (which has no shrinkage compensation), to provide a tight, movement resistant wall. The amount of shrinkage that a building will experience is subject to many variables and the industry has not standardized this design parameter. .
The misstatement - external (wind) uplift loads will add to the cable pre-stress force and overload the cable
When the Quick Tie cable is pre-stressed to a certain tension, it puts the wall system into compression. When an external (wind) load is applied, it doesn’t add to the cable force but it actually subtracts from the wall’s compression. This process satisfies the balance of forces and the cable tension remains substantially the same while the external force reduces the wall compression toward zero.
The misstatement – a cable system will experience excessive deflection under an external (wind) force
Because the pre-tensioning is properly designed, the residual cable tension (after relaxation and wall shrinkage losses have occurred) is approximately equal to the force anticipated for the hold-down. Until the external force reaches the residual tension, there is no increase to the cable tension force and extremely little, if any, additional elongation of the cable. Some indications of excessive deflection in wall tests using cable hold-downs we’ve heard discussed are most likely attributable to a pre-tensioning level that was not sufficient to balance the required hold-down force.
The misstatement – a “skipped story” hold-down has a single point of failure and, therefore, does not provide redundancy
Virtually all walls which utilize hold-down systems ultimately have a single point of failure at the base connection (where the force is generally the largest). Most structures achieve redundancy through the use of multiple shear walls. However, one of the most positive means of assured performance is the fact that 100% of Quick Tie’s cable assembly anchors are “proof tested” by the pre-tensioning process which applies a considerable initial overload (designed to take care of the relaxation and shrinkage losses) to each and every Quick Tie. All components of our system including the swage connections and epoxy anchorage are thus tested by this method.
The misstatement – a “skipped story” system is unstable during construction
Construction stability “means and methods” generally fall in the domain of the contractor and Quick Tie makes it clear in our submittals that the building hold-down stabilizing force is not satisfied until the cables are installed. However, when contractors consider the many benefits of our system – overall efficiency, economy and speed of installation - they are increasingly choosing the Quick Tie solution.
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