Case studies of central office cable tray support failures

Concrete anchor devices are an extremely important consideration when designing for network reliability. Most telephone central office cabling is supported on overhead cable racks or trays. Stored program control switching systems (SPCS) frames and modern transport equipment frames tend to be approximately 7 feet (178 cm) tall. Express cable trays to the power plant and main distributing frame (MDF) areas are usually supported by an auxiliary framing grid suspended from the ceiling slab. From a structural and earthquake protection viewpoint, both the equipment frames and the overhead structures are now separate and independent systems. This is a superior design plan, especially where building vibration or seismic activity is a consideration. Since metallic cabling, especially copper power cables, are quite heavy, all support elements must be capable of handling massive loads safely and reliably. Virtually all of the weight is borne by cast-in-place structures such as ceiling inserts, and embedded channel struts, or by anchors which are drilled and set by a variety of fastening systems. Two cable tray collapse failures in eastern Pennsylvania underscore the fact that some anchor designs are more prone to failure than others. This paper describes case studies of the two failures, and their root-cause investigations. Additionally, the paper discusses the results of dynamic pull-out tests, performed both in the laboratory and the field, on several traditional and newer style anchor designs, exploring their merits and drawbacks