Suspended ceilings and raised floors

Injuries to building occupants, financial losses, and disruption of operations are all reduced by bracing suspended ceilings. Typically hanging from many lengths of fine wire, a suspended ceiling consists of a grillage of light metal members which support ceiling tiles. During an earthquake an unbraced ceiling swings about like a pendulum. It crashes against structure and other elements often damaging fire sprinkler heads and triggering deluges of water. Ceiling tiles dislodge

▲ 11.21 Suspended ceiling damage. 1994 Northridge, California earthquake.

(Reproduced with permission from A.B. King).

▲ 11.21 Suspended ceiling damage. 1994 Northridge, California earthquake.

(Reproduced with permission from A.B. King).

▲ 11.22 Three methods for bracing suspended ceilings to prevent them swinging during an earthquake.

and fall, causing injuries and wrecking havoc on building interiors (Fig. 11.21).

Suspended ceilings, including the light-fixtures normally integrated within them, need bracing to prevent this uncontrolled swinging. Three techniques are illustrated in Fig. 11.22. The appropriate choice is based upon suspended ceiling manufacturers' technical information informed by full-scale testing and structural engineering calculations.

Figure 11.23 shows a method of bracing suspended ceilings to the floor soffit or roof structure above. This ensures good seismic performance. Contractors are reluctant to install bracing because of the extra effort required, so its necessity must be communicated clearly on architectural drawings, in specifications and followed through with site supervision to guarantee that it is correctly installed. The seismic integrity of a suspended ceiling depends largely on individual ceiling tiles providing in-plane diaphragm action (Chapter 4). The small clips that prevent tiles from uplifting and then falling are so easily omitted.

Light-guage steel braces at 45°

Floor slab

Figure 11.23 shows a method of bracing suspended ceilings to the floor soffit or roof structure above. This ensures good seismic performance. Contractors are reluctant to install bracing because of the extra effort required, so its necessity must be communicated clearly on architectural drawings, in specifications and followed through with site supervision to guarantee that it is correctly installed. The seismic integrity of a suspended ceiling depends largely on individual ceiling tiles providing in-plane diaphragm action (Chapter 4). The small clips that prevent tiles from uplifting and then falling are so easily omitted.

Light-guage steel braces at 45°

Floor slab

Section

Wire ties supporting ceiling weight

Section

Suspended ceiling runners and ties (to be continuous along lines of bracing)

Typical brace for every 25 m2 or depending on engineer's calculations

Typical brace for every 25 m2 or depending on engineer's calculations

Plan

▲ 11.23 Detail and plan of a braced suspended ceiling.

Protective bracing frame bolted directly to the floor around vulnerable equipment or plant rOi

Plant item

Pipe bracing

Duct bracing

Pipe bracing

Duct bracing

Services bracing

Services bracing

▲ 11.24 Examples of mechanical plant restraints.

▲ 11.24 Examples of mechanical plant restraints.

▲ 11.25 A braced roof-top water tank. Hospital, Whakatane, New Zealand.

These principles apply whether or not a ceiling is constructed from wood and plasterboard or is a proprietary modular system.

Raised floors that are installed in some office buildings provide space for, and allow access to, building services. Seismic restraint around the perimeter of the floors or cantilever action of their vertical supports provides resistance to horizontal accelerations. As for suspended ceilings, architects rely on a combination of manufacturers ' information and structural engineering advice when specifying a proprietary raised floor system.

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