T&R Seismic Calculator - Suspended Plasterboard Grid System
Because every building is different, there is no standard seismic restraint solution to address site, location, form and function. The scope of seismic restraint and related engineering work that will be required will not be known until the ceiling design is completed. The T&R Seismic System will provide a solution for buildings with an Importance Level of 3 and below. A suitably qualified Chartered Professional Engineer will be required for Importance Levels 4 & 5. It is imperative that mechanical services, sprinkler systems, electrical and suspended ceiling design are all co-ordinated at appropriate stages.
While full compliance with seismic requirements will add cost, it will limit damage, reduce repair costs and reduce the time to re-occupy post event. Furthermore it is now a legislative requirement for Code of Compliance Certificates and Health and Safety Laws. The new laws, affect those who are upstream from the workplace (for example designers, engineers, manufacturers, suppliers or installers). Specifically they have a duty to ensure, so far as is reasonably practicable, that the work they do or the things they provide to the workplace don't create health and safety risks.
Usage Notes:
This seismic design guide is provided to determine the installation requirements and details for the ceiling system. The calculations are based on conservative assumptions. Reduced seismic bracing for individual sites may be possibly if a suitably qualified engineer carries out a site specific design.
This guide has been prepared by JSK Consulting Engineers for T&R Interior Systems with the usual care and thoroughness of the consulting profession. On the basis of the assumptions and limitations presented in the guide, application of the guide is up to the users discretion and outside the control of JSK Consulting Engineers.
If the building is outside the assumptions and limitations detailed then a suitable site specific seismic design should be performed by a qualified Chartered Professional Engineer. This guide should not be used as a calculation template for a PS1; site specific design should be carried out for these cases also.
Allowance for relative motion between the ceiling and structure must be provided by floating edges. If the perimeter bracing method is used then two perpendicular edges must be fixed with the remaining two floating. If back bracing to the upper structure is used, then all edges must be floating. Floating edges must also be provided around rigid or separately braced items that pass through the ceiling. The amount of clearance should be checked by an engineer on a case-by-case basis.
See the assumptions and limitations notes.
Consult a structural engineer for the expected earthquake deflections of the structure.
© The T&R Seismic System has been developed in conjunction with JSK Consulting Engineers and T&R Interior Systems.
It remains the intellectual property of T&R Interior Systems and may not be used with other products.
Step One - Limit State Type
Determine the type of design for the installation.
Is the suspended ceiling and/or elements which directly interact with the ceiling required to be returned to an operational state within an acceptably short time frame in order for the structure to be occupied?
As per the suppliment to NZS 1170.5, this category can apply to ceilings which interact with fire suppression systems, emergency lighting installations and other parts.
Note that this applies for all parts and components that are essential for a building to be occupied. These would include; fire protection systems, critical plumbing systems, electrical systems and lifts. For all structures these will be elements required to be returned to an operational state within an acceptably short time frame (hours or days rather than weeks) in order for the structure to be reoccupied.
For example reinstatement of stopped edges at corner junctions may be considered a viable option within the time frame indicated to allow reoccupation of a office but may be unsuitable for an operating theatre
Does the suspended ceiling, when considered as a whole, weigh more than 7.5kg?
For the ceiling to not be considered ULS design it must weigh less than 7.5kg
Is the suspended ceiling installation at a height of 3m or greater?
For the ceiling to not be considered ULS design, it must be installed at a height less than 3m
Would collapse of the suspended ceiling block emergency egress routes?
Could fallen ceiling tiles block emergency egress routes?
Obtain site specific design advice from an appropriately qualified engineer
Your Limit State Type is
As there are two limit states which apply to the ceiling in this instance, the most stringent state which results in the shortest allowable lengths will apply.
Step Two - Suspended Ceiling Weight
Calculate the total seismic weight based on the ceiling and service weights.
Enter or select the corresponding values and sum all the component weights to get a total seismic weight. This value will be used in the following steps of this worksheet.
| Grid Weight (kg/m2) | Main Tee (mm) Cross Tee (mm) | 0.73 |
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| Lining Weight (kg/m2) |
Show Common Linings
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| Lining Thickness (mm) ERROR: The cross tee grid spacing exceeds the manufacturers recommendations for plasterboard thicknesses of less than 13mm. Reduce cross tee spacing to 400mm |
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| Services (kg/m2) | Luminaires | |||||||||||||||||||||||
| Insulation | ||||||||||||||||||||||||
| Other | ||||||||||||||||||||||||
| Design Distributed Load (min. 3 kg/m2) | ||||||||||||||||||||||||
| Total Seismic Weight Sw | kg/m2 |
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Note: If the selected plasterboard thickness is 12mm or less then the cross tee spacing of 400mm should be used.
Step Three - Seismic Actions
Calculate seismic force based on seismic zone, height above ground level, and building importance level.
Locate the area for which the suspended ceiling will be installed. Find the Site Specific Zone Factor by locating the line closest to the area of installation, or the shaded area it is within, and tapping it to show the rating.
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Zone Factor Show Zones |
0.01 |
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| Importance Level Anything above IL3 will require a design by an engineer more info |
2 |
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| Height Factor Ceiling connection height above ground floor more info |
1 |
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| Seismic Weight Sw | ||
| Total Seismic Force Sf kg/m2 |
SLS =
SLS2 = ULS = |
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Step Four - Overall Ceiling Capacity
4.1 Limiting Tee Length Based on Edge Connection Capacity
The following steps calculate the maximum allowable tee lengths based on the capacity of the connection between the edge trim and the wall substrate. The allowable line load is based on the spacing between fasteners around the perimeter of the ceiling. For spacings below 600mm, a continuous dwang will be required around the perimeter of the ceiling. For additional information, refer to the T&R Suspended Plasterboard Typical Edge Details.
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Wall Substrate Fastener Spacing (mm) |
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| Maximum Length of Main and Cross Tees (m) |
SLS SLS2 ULS |
0 0 0 |
4.2 Overall Ceiling Capacity Based on Diaphragm Capacity
This section calculates the capacity of the grid based on the diaphragm capacity of the plasterboard ceiling. The lining manufacturer's recommendations should be followed however if the recommended fastener spacings around the perimeter of each sheet are less than those required below, additional fasteners may be required for a specific seismic design. For additional information, refer to the T&R Suspended Plasterboard Ceiling Layout Drawing.
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Plasterboard fixing spacing around sheet perimeter (mm) |
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| Maximum Length Tees (m) | Main Tee | Cross Tee | |
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SLS SLS2 ULS |
0 0 0 |
0 0 0 |
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4.3 Selecting the Limiting Main and Cross Tee Lengths
This section selects the limiting tee lengths for ULS, SLS and SLS2. The limiting tee length for the main and cross tees should be the smallest of the tee lengths calculated above in sections 4.1 and 4.2. Additionally all plasterboard ceilings are limited to a maximum allowable tee length of 12m due to the installation of control joints
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Maximum allowable tee length for perimeter fixing (m) |
Cross Tee Length Ct | Main Tee Length Mt |
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SLS SLS2 ULS |
0 0 0 |
0 0 0 |
If your allowable tee length is larger than the maximum tee length which you want to install, then your seismic design for the ceiling is complete.
If your allowable tee length is less than the maximum tee length which you want to install then back bracing options are available to ensure the design is satisfactory.
Use Back Braces to transfer the seismic load of the ceiling to the structure, with floating edges around the sides of the ceiling.
Step Five - Back Bracing
Determine the maximum area of ceiling that each brace can support based on seismic force, brace type and the plenum height.
Type A Brace
Figure 5.1: Installation arrangement of Type A brace.
The brace is constructed of 64 x 33 x 0.5mm steel stud compression post, with 2x 64 x 33 x 0.5mm steel studs at 45 degrees in each direction.
Type B Brace
Figure 5.2: Installation arrangement of StratoBrace (Type B) back brace.
The brace is constructed of various steel stud compression posts with 2x steel studs at 45 degrees in each direction, depending on the plenum height. Instead of using the steel studs fastened directly onto the grid, a StratoBrace bracket is used.
Brace Capacity
| Brace Type Show brace types | |
| Plenum Height (mm) Ph | |
| Brace Capacity (kg) | |
| Bracing Requirement |
Area Per Brace
| Motion limiting factor | X | Brace Capacity (kg) | / | Seismic Force (kg/m2) | = | Area per Brace (m2) Ab |
| 0.8 |
Note that for raked ceilings, StratoBrace back bracing cannot be used. Type A bracing can be specified or contact a suitably qualified consulting engineer if specific back bracing design is required. Note that these figures are based on the StratoBrace specifications, ensure that when StratoBrace is used the installation recommendations and guidelines are followed.
If further refinements to the back bracing design are required or a greater capacity of back bracing needed, contact a suitably qualified consulting engineer.
All fasteners into the building structure are sufficient for the loadings imparted from the back bracing, as long as the manufacturers specifications are followed during the installation of the fastener.
For connections into structure which are not detailed in the table please discuss with a suitably qualified consulting engineer.
Step Six - Back Bracing Layout
Calculate the minimum number of back braces required.
Minimum number of back braces required
| Total Ceiling Area(m2) Ca | / | Area per Brace (m2) Ab | = | Minimum Number of braces #b |
The plasterboard lining rigidly holds the grid together, creating a diaphragm that transfers the seismic load evenly through the system into the structure. Therefore the back braces are to be laid out evenly over the ceiling based on the allowable area per brace.
The number of braces calculated represents the minimum number of braces required to transfer the seismic load from the ceiling back to the structure. If the ceiling has many corners, discontinuities, breaks, or unique shape then it is likely that additional braces will be required. Following the brace placement guidelines should result in the ceiling being adequately braced.
Brace placement steps:
- Place braces near ceiling corners.
- Place braces near discontinuities / breaks in the ceiling.
- Place braces near the ceiling edges.
- Evenly distribute the remaining number of braces throughout the ceiling.
- Check that the area that is supported by each brace does not exceed the maximum area per brace. If the area per brace is exceeded then additional braces must be added.
Step Seven - Determining the Seismic Clearance
The required inter-storey drift values need to be determined to ensure that there is sufficient clearance to allow parts to move relative to each other during a seismic event.
| Type of Installation | Type of Design | Clearance Required (mm) Sc |
| Perimeter fixing | SLS ULS |
10 10 |
| Back Braced | SLS ULS |
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For larger plenum heights the required seismic clearance may be impractical, for site specific installations contact a suitably qualified consulting engineer.
You Will Need...
- Building Location
- Ceiling Height
- Grid Spacing
- Luminaire Weight and Spacing
- Reflected Ceiling Plans
- A section showing plenum depths
This design is for T&R Suspended Plasterboard Grid System only and cannot be used with other manufacturer's ceiling products.