Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Overhead anchors

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Title: Overhead anchors

The subject of "overhead" applications for anchors carries a lot of baggage, especially here in the western U.S. where there is long-standing concern for fire performance. Most ICBO evaluation reports for anchors contain language restricting both mechanical and chemical anchors from uses "in conjunction with" fire-resistive construction unless "proper consideration is given" to fire conditions. As far as I am aware, no such restriction exists for cast-in anchors in the code (UBC, IBC), and ICBO/ICC does not have criteria by which the performance of an anchor under fire conditions can be evaluated, so the origin of this restriction is somewhat cloudy. Nevertheless, I generally interpret it to mean that anchors should not be used to support fire-resistive construction (i.e., where failure of the anchor would compromise the fire-resistive system) unless the anchors are protected or are otherwise designed with consideration of the effects of fire. I do not believe that using anchors to hang mechanical and architectural systems from a slab should be affected by this restriction, although some building officials may feel otherwise. I am currently pursuing a clarification of code intent through ICC in this regard.

Hilti performs extensive fire testing on most of its anchor systems (i.e., tests on tension-loaded anchors installed in the underside of a slab in a burn chamber and subjected to a standard time-temperature curve) at the University of Braunschweig, iBMB, one of the largest fire test facilities in Europe, so data regarding the behavior of anchors under these conditions is available. Note that in most cases, steel failure (i.e., thread softening) governs the behavior, although adhesive anchors with normal to shallow embedment will often exhibit bond failure. The data is typically expressed in terms of loads corresponding to 30, 60, 90, 120 and 180-minute fire exposure, using a safety factor of 1.0.

The more relevant condition to be concerned with when placing anchors in the underside of a structural slab is the potential for cracking in the slab and its effect on the tension capacity of the individual anchors. For this reason I would generally recommend against the use of smaller diameter adhesive anchors with shallow embedment in overhead applications, and I'd also recommend against the use of displacement-controlled expansion anchors, such as drop-ins, in most cases. (Particularly when seismic loading is involved.) The optimal solution for anchorage in tension zones is a well-designed torque-controlled expansion anchor or undercut anchor. Creep should not be a problem for these anchor types assuming they are properly installed and function correctly.

A standard for qualification of mechanical anchors is available that addresses many of the concerns raised in this list. ACI 355.2 calls for extensive testing of anchors in a variety of conditions to verify long-term performance in direct tension applications where cracking of the concrete may occur. This standard is referenced from ACI 318-02 Appendix D (anchor provisions) and will by extension be referenced from the 2003 IBC and NFPA 5000.

John F. Silva, SE
Hilti, Inc.