T&R Seismic Calculator
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.
This guide allows a designer to calculate required bracing for suspended ceilings. The calculations are based on conservative assumptions. Reduced seismic bracing designs for individual sites may be possible if a suitably qualified Chartered Professional Engineer carries out a site-specific design. This guide should not be used as a calculation template for a PS-1; specific seismic design should be carried out for these cases.
This guide has been prepared by JSK Consulting Engineers for T&R Interior Systems with the usual care and thoroughness of the consulting profession. Interpretation and application of this guide is outside the control of the engineer and therefore is the users' responsibility. This guide does not constitute a producer statement or engineer's certification, and is not for use with trafficable ceilings or ceilings which support partition walls or any other service load.
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.
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, the University of Canterbury and T&R Interior Systems.
It remains the intellectual property of T&R Interior Systems and may not be used with other products.
Introduction to suspended ceiling seismic design
There are two main ways of approaching the seismic design of a suspended ceiling, each with their own benefits and drawbacks.
1. Perimeter Fixing
This approach to seismic restraint relies on connections to adjacent walls to transfer the horizontal seismic load back to the primary building structure.
The perimeter fixing approach relies on a fix-float philosophy, where the perimeter edges directly opposite a 'fixed' edge shall be 'floating'.
A fixed edge has the tee riveted to the wall trim and screws each side of the tee into the wall to provide a solid load path. The floating edge has seismic clearance between the ceiling and the wall to allow for movement of the building structure.
- Cheaper and faster installation
- Less plenum congestion
- Can only be used for small-medium ceilings, as the ceiling size is limited by the grid strength
- Walls must be capable of transferring the seismic load
Seismic joints may be used to expand the size of ceiling that may be edge restrained. A seismic joint essentially creates a 'floating' edge in the ceiling, allowing opposing ceiling edges to be fixed.
2. Back Bracing
This approach is used for larger ceilings, where the required ceiling tee lengths exceed the allowable lengths or where the walls around the ceiling cannot be relied upon to transfer the seismic load to the primary building structure.
Rigid bracing is used evenly throughout the ceiling to transfer the ceiling seismic load through to the overhead structure. This approach requires that all the perimeter edges are floating in order to prevent damage from inter-story drift.
- Suitable for large ceilings
- No load from the ceiling is transferred to perimeter walls. This is important if there are part height walls or bulkheads around the ceiling that have not been designed to carry the ceiling loads.
- Larger plenum heights require large floating edge clearances
- Usually more expensive than edge-restrained designs
- Harder to keep ceiling square as there are no fixed edges
An alternative to back bracing that may be used in certain situations is rigid hangers. This approach is suitable for ceilings suspended very close (50-500mm) to purlins, rafters/trusses, or floor joists. The wire hangers are replaced with heavy gauge steel angle hangers, which are used to transfer the seismic load back to primary structure.
Step One - Limit State Type
What is limit state design?
The limit state of a ceiling determines the level of seismic activity that the ceiling must withstand. A structure shall be designed and constructed in such a way that it will, during its design working life (with appropriate degrees of reliability) sustain all seismic activity that is likely to occur.
Serviceability Limit State One (SLS1)
No damage occurs and the structure and the non-structural components do not require repair after the seismic event.
Serviceability Limit State Two (SLS2)
The system is should only suffer minor cosmetic damage and should be able to be repaired within a short period of time. Typically only buildings with a specific post disaster function will require this limit state to be considered.
Ultimate Limit State (ULS)
Specifically, for earthquake actions this shall mean avoidance of collapse of the structural system or parts of the structure representing a hazard to human life inside and outside the structure necessary for the building evacuation.
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 lightweight fallen tiles 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?
Does the ceiling comply with all of the assumptions and limitations?
Assumptions and Limitations
- The design guide is only intended for use within NZ
- The building height must not exceed 40m
- The design working life of the ceiling is 50 years
- This guide only cover buildings of importance level 2 and 3
- For other importance level structures, specific seismic design is required
- Part Category 6 is not included in this generic design guide. If selected from the chart - then it is recommended that a suitably qualified engineer carry out a specific design.
- Only horizontal forces have been considered in this guide. Vertical forces may need to be considered on a site-specific basis
- The period of the part is less than 0.75s
- Part ductility is dependant on whether the design is SLS or ULS
- For ULS design ceiling ductility of 2 has been used as per the supplement to NZS 1170.5
- Class C soils have been assumed
- For perimeter fixed ceilings, a continuous ceiling dwang or cross nog is assumed for suitable attachment to the perimeter trim
- The maximum tee spacing is 1200mm in any direction
- The ceiling is non-trafficable
- The seismic loads transferred by the suspended ceiling should be confirmed by a qualified structural engineer
- Any additional body weighing more than 10kg is to be separately suspended and braced
- The guide is for use with CBI grid only
- Ceiling movement/damage should not cause an unusually high level of damage
- Fixing the trim into the wall requires tek screws to be within 25mm of the centre of the main tee, equally spaced on both sides
- Steel rivets are to be used for the tee connections to the wall trim
- All ceiling tiles are to be clipped into place, as per common suspended ceiling installation procedure, except for designated access points
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 suspended ceiling in this instance, the most stringent state which results in the shortest allowable tee length will apply.
Step Two - Seismic Weight
Calculate the total seismic weight based on the ceiling and service weights.
Enter or select the corresponding values in the column on the right and sum all the component weights to get a total seismic weight. This value will be used in the following steps this worksheet.
Show Common Tiles
|Design Distributed Load (min 3 kg/m2)|
|Total Seismic Weight Sw||
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.
Anything above IL3 will require a design by an engineer
Height of the highest point of ceiling above the floor (typically 2.4 - 3.5m)
Height of the reference floor above ground level. (E.g. 0m for the ground floor)
|Seismic Weight Sw|
|Total Seismic Force Sf
Step Four - Calculating the Grid Capacity
This section calculates the limiting tee length using the seismic force and the capacity of the grid tees.
Physical testing and calculations have given a capacity of the main and cross tees per metre. These limiting tee lengths will be used for both fixed two float two perimeter fixing and back bracing suspended ceiling seismic restraint.
The following steps calculate the maximum allowable tee lengths based on the capacity of the tees. Firstly, select the type of trim used, type of connection and the the corresponding capacity of the structural limit state.
Rivets are to be 3.2mm steel rivets or 3.2mm grade 5056 aluminum rivets.
Connection Type (wall angle)
If limiting tee length is not satisfactory, then increase the number of rivets or trim size to increase the allowable tee length. If none of these options work then seismic breaks or back bracing will be required.
When a 600 x 600 grid is required for design, the most common method for laying out the grid is to use main tees spaced at 1200mm, cross tees spaced at 600mm and then an additional cross tee to achieve 600 x 600 layout.
It is important that if the additional cross tee is added to achieve the 600 x 600 layout that the grid weight in the previous section has been increased. Also in determining the total grid capacity, the Main Tee is selected at 1200 mm and Cross Tee at 600 mm is selected. Selecting Main Tee at 1200 and Cross Tee at 600 allows for the correct capacity calculation of a 600 x 600 grid.
The 600 x 600 grid layout when the main tee is spaced at 1200mm with addittional cross tees making up with 600 x 600, has a reduced strength compared to laying out Main Tees at 600mm spacing.
Connection Location explain
If you wish to specify the location of the first tee connections, a note is to be added to the installation information stating that full lengths of main/cross tees are to be used around the fixed edges of the ceilings. Refer to the diagram below
- When yes is selected for the main tee direction, the limiting lengths are increased by 3.0m
Specify the location of the first Main Tee connection?
Is the ceiling raked?
|Limiting Tee Length||SLS||SLS2||ULS|
|Limiting Main Tee Length (max) Lmt|
|Limiting Cross Tee Length (max) Lct|
Enter the Maximum measured Tee Lengths as per plans supplied
If your allowable tee length is larger than the maximum tee length which you want to install, then the perimeter fixing method is appropriate and your seismic design for the suspended ceiling is complete. Click here to learn more about perimeter fixing.
If your allowable tee length is less than the maximum tee length which you want to install then several options are available to ensure the design is satisfactory.
The first option is to try and break the ceiling up through the use of Seismic Joints to ensure that the allowable tee length is not exceeded by the installation tee length.
The second option is to try Rigid Hangers. This section calculates if rigid hangers are suitable for providing dead load, live load and seismic restraint of the ceiling.
The third option is using Back Braces to transfer the seismic load of the ceiling to the structure, with floating edges around the sides of the ceiling.
Step Five - Calculating Rigid Hanger suitability
This section calculates if rigid hangers are suitable for providing dead load, live load and seismic restraint of the ceiling.
Rigid Hangers are suitable in the following situations:
- Low plenum height (less than 600mm)
- Purlins or timber joists above
- Edge restraining the ceiling is not suitable
- The plenum height is larger than 600mm)
- Concrete slab is above (back bracing is more suitable in this situation)
If rigid hangers are used, the suspended ceiling should have floating edges on all edges.
Height of connection to structure
This is the maximum height of the rigid hanger connection to structure. Measure from the connection to overhead structure to the ground level.
|Connection Height (m)|
Maximum Plenum Height
Specify the maximum plenum height so the seismic moment per hanger can be determined. This needs to be less than 600mm.
|Maximum Plenum Height (mm)|
The maximum allowable plenum height for rigid hanger ceilings is 600mmm. The seismic moment per hanger is then equal to the seismic load multiplied by the hanger spacing and the plenum height. Repeat this separately for ULS, SLS1 and SLS2 seismic forces.
|Choose Hanger Spacing|
|Design||Hanger Span (m)||x||Hanger Spacing (m)||x||Seismic Force (kg/m2) Sf||x||Plenum Height (m)||=||Seismic Moment per Hanger (kgm)|
As per AS/NZS 2785 3.2.2 (c), if the Dead and Live load per hanger is less than 50kg then 50kg shall be used as the design load. The allowable loads per hanger are shown in the Table below. If a hanger spacing of 2.4m is specified, hanger wires at 1200x1200mm spacing are required for static support of the ceiling.
|Hanger Type||Tensile Capacity||Bending Capacity|
|40 x 40 x 1.15 BMT Equal Angle||412kg||4.5kgm|
Hanger suitability check
The suitability check is then carried out as follows.
|Dead and Live load per hanger||/||Tensile Capacity||+||Largest Seismic moment per hanger (kgm)||/||Bending Capacity||<||Design Criteria|
If the design criteria is met, then rigid hangers may be used to adequately restrain the ceiling under dead, live and seismic loads.
Step Six - Selecting Back Bracing
Determine the maximum area of ceiling that each brace can support based on seismic force, brace type and the plenum height.
Height of connection to structure
This is the maximum height of the back brace connection to overhead structure. Measure from the connection to overhead structure to the ground level.
|Connection Height (m)|
|Brace Type Show brace types|
Plenum Height (mm)
Select the figure that corresponds to your actual measurement rounded up
Choose a Brace type first
|Brace Capacity (kg)|
|Area per Brace (brace per m2) Ab|
|Stud Type / Bracing requirement:|
Step Seven - Back Bracing Layout
Unless advice is given from a suitably qualified consulting engineer that the overall ceiling acts as a diaphragm, Option 1 is to be used for back bracing layout.
The goal of the back bracing method is to break the ceiling up into smaller areas, restrained around the edges by back braces. These back braced edges form 'braced tees'. How large these areas can depend on the seismic load - The larger the load, the smaller the area between braces.
As it can only be assumed that the ceiling acts as a diaphragm over small lengths the brace spacing along the braced tees may not exceed 2.4m.
This means that for tee spacing at 1.2m, every second tee must be braced at least once along the tee length. For tee spacing at 0.6m, every fourth tee must be braced at least once along the tee length.
The spacing of these braced tees is determined by either the minimum allowable tee length or the maximum allowable area per brace.
Back Brace Layout Spacing MT
Maximum Spacing for Main Tees (Y2)
Main tee diaphragm length (X1)
Maximum Spacing for Cross Tees (X2)
Cross tee diaphragm length (Y1)
The minimum allowable tee length (based on the grid capacity) calculated in the previous section at the end of Step 4, must be greater than Y 2 and X 2 or it becomes the limiting value for braced tee spacing.
Step Eight - Back Bracing Seismic Clearance
Determining a seismic clearance is to ensure there is sufficient clearance to allow parts to move relative to each other during a seismic event.
|Type of Design||Plenum height (mm)||Interstory Drift Factor||Required Seismic Clearance (mm)|
Round the required seismic clearance up to the nearest 5mm and ensure that this clearance is used on the floating edges required in the back braced design.
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
- Tile Weight
- Reflected ceiling plans
- A section showing plenum depths
This design is for 2 way exposed 24mm CBI grid only and cannot be used with any other manufacturer's grid