Designing suspended ceilings for Seismic

In the event of an earthquake, our seismic-rated ceiling systems help support life safety and the preservation of property. Combining seismic tested perimeter and separation joint solutions, we are able to design a complete solution that not only performs but also reduces installation time, speeds up inspection, saves on material and labor costs and improves the aesthetics of the finished ceiling. In Australia, buildings commonly use two types of suspended ceiling – perimeter fixed and floating.

Vertical restraint is often provided by the force due to gravity on the suspended ceiling itself. However, rigid vertical struts may also be required, depending on the seismic zone and the ceiling-to-structure heights.

Joints in the main tees should not be placed within 150 mm of any vertical hanger, strut or bracing connection point.

Seismic Design Process

The design team should be responsible for coordinating the design and installation of all sub trade equipment in the plenum and ceiling. The preferred approach, to ensure optimal coordination and productive use of resources, is for seismic design of non-structural components to be completed prior to the project going out to tender. The design team needs to specify the seismic grade of the ceiling and the associated seismic restraint requirements in the tender documents along with:– Bracing Layout– Edge/Perimeter details– Service Integration– Suspension points identified that are clear of services.

Seismic Design Coordination

Designers of other components in the plenum may not have full understanding as to the requirements of the suspended ceiling system. Design coordination is possibly the single biggest issue that faces suspended ceiling installation. Seismic Design coordination of mechanical services and other plenum located elements, including lights, air conditioning, cable trays, fire suppression systems etc, with the suspended ceiling is essential. In general, the coordination of proper seismic design prior to issuing tender documents will result in:

– More accurate costing being achieved

– Better installation process

– Fewer site variations

– Lower overall cost

– Less time delays during installation

– Fewer site conflicts

Perimeter-fixed ceilings

Perimeter-fixed ceilings are suspended using vertical wire or cable hangers and laterally restrained by anchoring the edges of the ceiling to the surrounding wall, frame or other structural element. Ceilings of this type are not normally braced.

Perimeter fixed suspended ceilings are anchored around the edges of the ceiling.

A perimeter-fixed ceiling should be anchored at the ends of two orthogonal edges of the ceiling grid, maintaining a 10–15 mm sliding clearance at the opposite edges. The free ends must be able to move (slide) against the wall but still be sufficiently restrained to prevent the grid or tiles collapsing. This is typically achieved with a channel moulding or proprietary fixing.

Perimeter fixed ceilings should be fixed along two orthogonal edges and free on the others.

A common cause of damage to perimeter-fixed ceilings in Christchurch was inadequate fixing of the ceiling perimeter to the wall. This was particularly due to undersized or insufficient rivets to connect the grid members to the wall support.

Floating ceilings

If perimeter fixing is insufficient for the seismic demand, the ceiling must be braced to the structure using another method. Floating ceilings are suspended using vertical hangers and braced to the underside of the floor above to provide lateral restraint. They are not connected to the perimeter wall or frame. The gap between the ceiling and the surrounding wall has to be sufficiently large that the ceiling is not damaged by impacts against the wall during an earthquake.

Vertical suspension may be provided using wire or cable hangers, or vertical struts constructed from angles, metal studs or proprietary compression posts anchored to the ceiling grid and the building structure.

Floating suspended ceilings are suspended from above.

Lateral bracing in a floating ceiling can be provided by using rigid braces, cable braces (splay wires) combined with vertical struts, or a combination of these techniques. In each case, the seismic braces should be designed to enable the suspended ceiling to carry the seismic demand load .

Rigid braces

Rigid braces should be installed in pairs orthogonal to each other and in combination with a vertical hanger or strut. They should be at a maximum angle of 45° from the plane of the ceiling grid. The braces should be as close as possible to the same vertical plane as the main and cross tees.

Rigid braces may connect to vertical struts or the ceiling grid, but braces that connect to the ceiling grid should be fixed within 50 mm of a vertical strut connection point. If four rigid braces are used – two in each orthogonal direction – the vertical strut may be omitted.

Bracing requirements for strut-connected braces in a floating suspended ceiling system

Cable and wire braces

Cable-only and wire-only braces are not as rigid as angle braces, but their performance is improved by using them in conjunction with vertical struts.

Cables braces should be connected between the main tee and the building structure at a maximum angle of 45° from the plane of the ceiling grid. Four orthogonal cables should be installed, one pair on either side of the vertical strut. Each pair should be connected to the main tee as close to the vertical strut as possible and the cable fixings in each pair should be no more than 100 mm apart.

Bracing requirements for cable braces in a floating suspended ceiling system.

Hangers

Hangers must maintain a horizontal clearance of 50 mm from restrained components and services, and 150 mm from unrestrained components and services in the ceiling space.

A bridging system should be used where the correct clearances cannot be achieved or suitable hanger points cannot be accessed because of obstructions in the ceiling space.

Hangers should be spaced at centres of no more than 1.2 m and be within 9.5° of vertical (1:6), unless using counter-sloping wires. Floating terminal ends of each main and cross tee should be supported with a perimeter wire located within 200 mm of each wall.

Hanger wires should be 2.5 mm in diameter, with top and bottom connections secured using at least three complete tight turns extending no more than 75 mm along the cable.

Supported components

Both perimeter-fixed and floating suspended ceilings must be designed to support ceiling-mounted distributed services up to a load of 3 kg/m² under seismic conditions. Where service loads are greater than 3 kg/m², the ceiling designer should be advised. For ceilings longer than 2.4 m, the maximum allowable deflection is L/360.

A suspended ceiling must not be used to support plant or equipment that weighs more than 10 kg. Any such plant or equipment in the ceiling space must be separately supported and also maintain the relevant clearances.

Where a suspended ceiling is used to support equipment, it should be positively fixed to the ceiling suspension system, not supported by the ceiling panels or tiles.

Flexible connections should also be used between ceiling-supported equipment and ducts, pipes or cables that are supported by the structure.

Lighting fixtures

All lighting fixtures that are mounted on a suspended ceiling, including detachable accessories (such as diffusers and light controllers), should use a positive locking mechanism to prevent them inadvertently disengaging during an earthquake.

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