April 24, 2017
By Jeff Halashewski, Dipl Arch Tech
Over the past year, this author has witnessed an increase in the design community’s awareness of the seismic requirements set out for Canadian jurisdictions. However, for both new buildings and existing construction, the destructive effects of earthquakes can be of significant concern. Damage to inadequately restrained key systems within buildings can be extensive. When a major component, such as a generator, is knocked off its supporting structure, the fall can threaten both life and property.
The cost of properly restraining such equipment is insignificant compared to the associated costs of replacing or repairing the components, along with the expense of system downtime due to damage to the building services and businesses. When thinking of it from this angle, whose responsibility is it to restrain a generator—the building owner or tenant?
This author’s previous article for Construction Canada explored why proper restraining of nonstructural components can reduce the threat to life and minimize long-term costs due to damage and associated loss of service. (In the November 2016 issue of Construction Canada, this author discussed the basic concepts that feed into larger discussions regarding ‘domestic’ earthquakes, nonstructural components, and methods of selecting seismic restraints. Click here.) It stressed the importance of minimizing risks at the design stages, which should include a well-written specification that can be taken to the delegated design engineer or person producing the shop drawings for the given component.
In this follow-up article, the author explores how this information can actually be implemented, working through the processes of setting up the performance requirements through to submittals, fabrication, inspections, and quality assurance/control (QA/QC). The goal is to be able to easily understand what is required as part of the specification for any given building.
What is a seismic restraint?
By definition, a restraint is a method to mechanically control movement, whether via dampers, bracing, containment, immobilization, or suppression. Seismic forces relate to vibrations produced by an explosion or even by the natural environment—for example, a building on the banks of a fast-moving river will experience vibrations within. With respect to ‘seismic restraints,’ then, design professionals have no control over the first word, and therefore must focus on the second—controlling the parts of the building that will move.
Seismic restraints can fit into many categories, but the most common are:
Each type of these assemblies has its pros and cons, but it all comes down to the intent and the seismic force potential on the component requiring restraint, either being surface-mounted, suspended, or supported off another nonstructural component. The materials used can vary, ranging from those like concrete, steel section, springs, cables, rods, and premanufactured solutions.
Designing seismic restraints
There are three ways in which design can be managed. The ultimate decision depends on the complexity of the nonstructural component, which can vary in shape, weight, and sensitivity to forces acting on it.
1. Design assist
A seismic consultant hired by the project architect is responsible for the project’s design. He or she co-ordinates closely with the architect, structural engineer, landscape architect, and civil engineer to ensure each individual system is designed to meet the project intent. In the construction phase, there will be a requirement for delegated design for the seismic requirements of the components.
2. Assisted design stamped by engineer (shop drawings)
The shop drawing is the manufacturer’s or contractor’s drawn version of the information produced from the construction documents. It normally shows more detail than the construction documents, and is drawn to explain the fabrication and/or installation of the items to the manufacturer’s production crew and contractor’s installation crews.
3. Delegated design
The intent of delegated design is to account for professional engineering responsibility for design, review, and acceptance of components of work that form a part of permanent work in accordance with the current building code. It is assigned to a design entity other than the consultant. This includes design requiring:
Regardless of the path chosen or required, when designing for seismic restraint hardware, there are four aspects of the system to consider:
There is also a pre-designed solution, which involves taking materials from catalogues and web-based databases. However, these solutions are not intended to alleviate the engineer’s responsibility—they should only serve as a guide. All information for calculations must be made available on the structural drawings or in the specifications in each of the technical sections requiring seismic restraint hardware design.
Seismic design requirements
Part of the seismic design requirements is to identify if there is any need per the building code. Following the simple process of figuring out the requirements in a code matrix can make this process a lot easier. Figure 1 identifies the items from the building code that must be considered if the building is required to have nonstructural components designed for seismic restraints.
There are four Importance Categories to consider.
1. Low Importance
Examples of this category include low-human-occupancy buildings with one person or less per 40 m2 (430 sf) of floor area (e.g. structures found on a farm) and low-hazard industrial-occupancy warehouses (i.e. Group F, Division 3), where structural failure causing damage to materials or equipment does not present a direct threat to human life.
It is important for authorities having jurisdiction (AHJs) to be aware when the Low Importance Category is being assigned, since it enables relaxation of some code requirements. In some cases, this relaxation is inappropriate. For example, an equestrian riding facility that also has provision for permanent or temporary grandstands is an Assembly use; it should not be categorized as Low Importance.
2. Normal Importance
All buildings not classified as ‘Low,’ ‘High,’ or ‘Post-disaster’ are ‘Normal.’ It is important to remember, though, there is a broad range of buildings, so questions may arise for some facilities not specifically listed, such as wind turbines, private versus public bus terminals or airports, or private clinics and non-emergency treatment facilities unlikely to be considered essential to provision of services to the public in a disaster. Discussions with the owner, structural engineer, and AHJ may be required to appropriately assign the Importance Category.
3. High Importance
This category applies to schools, community centres, and industrial or storage facilities having hazardous or toxic materials. It is not limited to the specific facilities noted, and might apply to a college, sports facility, arena, or large place of worship. The code uses the term “likely to be used as post-disaster shelters,” but it should be noted this is not the code-defined term “Post-disaster Importance” (shown below), but a lower category of importance that requires professional judgment to assign.
4. Post-disaster Importance
Post-disaster buildings are essential to the provision of services in the event of a disaster. Examples include:
The site class (A to F) relative to substrate type (e.g. rock, hard, or soft soil) is one of three factors required for the calculation of the seismic hazard index. It is assigned by a geotechnical engineer following soil tests.
A shear wave velocity test may be required by the geotechnical engineers to assign a site class better (i.e. higher) than can be ascertained without the test (e.g. C rather than D, or B rather than C). The shear wave test would be an additional cost over simple borehole analysis, but may ultimately save the project considerable cost.
Seismic hazard index
While the building’s structural design involves complex seismic restraint calculations, a reasonably simple formula is used to determine the seismic hazard index. The seismic hazard index formula is:
IE Fa Sa(0.2)
IE = Earthquake Importance Factor for the structure (taken from National Building Code of Canada [NBC] Table 220.127.116.11.);
Fa = Acceleration-based site co-efficient (NBC Table 18.104.22.168.B); and
Sa(0.2) = five per cent damped spectral response acceleration (NBC SB-1 Table 1.2, Column 17).
If the value of the seismic hazard index is equal to or greater than 0.35, it triggers the need to restrain architectural elements like suspended ceilings, parapets, ornamentations, and masonry veneer connectors, as well as mechanical and electrical systems and equipment in all buildings.
When specifying for any nonstructural component necessitating seismic requirements, it is imperative the performance requirements are identified for the engineer preparing the shop drawings so the most appropriate design for the component can be provided.
Another important aspect to the requirements is the design intent. For example, if the restraint assembly is exposed to public view, the architect might want the restraint hardware to have an architecturally exposed finish to highlight the exposed condition. Within the specification section, there should be content requiring the intent of architectural finish, identified by a detail located on the drawings showing the intended configuration of the restraint system.
|Once it is known which building components require seismic restraints to conform to the authority having jurisdiction (AHJ) and building code, there is an easy process to follow.
Installation and review
In the case of delegated design, the delegated design engineer will review the site-installed components prior to signing back responsibility to the prime consultant.
Testing and certification of seismic restraint systems
Building codes require systems and components that have seismic certification to be properly documented and labelled.* When proposing to provide nonstructural components for a given project, the manufacturer must provide the project team with project submittals, including a Certificate of Compliance (CoC) for seismic certification. This CoC should properly document the limitations of the certification, mounting restrictions, and attachment considerations. In addition, certified components must be provided to projects with a seismic label to show the certification limits and mounting restrictions for the given restraint system.
* For more, visit www.seismicapprovals.com/certification/certificates-and-labels.
Suspended, acoustic-tile ceilings should be designed and installed in accordance with current Ceilings and Interior Systems Construction Association (CISCA) requirements. For Seismic Design Categories C, D, E, and F, bracing needs to be provided at regular intervals to resist code’s design forces and limit the vertical and lateral movement.
Suspended ceilings with areas less than 13.4 m2 (144 sf), which are surrounded by walls or soffits that are laterally braced, are exempt from seismic design requirements.
Where ceilings are unbraced or splayed, wired bracing is used to resist seismic forces and limit lateral deflections. Clearance of about 25 mm (1 in.) should be provided around all penetrations through the ceiling sprinkler drops. (If flexible sprinkler drops are used and have been certified to accommodate 25 mm of movement, then this clearance requirement may be waived.)
Independent support of light fixtures, diffusers, cable trays, electrical conduit, and other ceiling appurtenances must be offered.
For ceilings directly hung to structural framing or furring with materials applied to structural framing, the fastening must be able to resist the vertical seismic design forces and weight of the ceiling and its light fixtures, sprinklers, and HVAC appurtenances.
Non-loadbearing interior walls
Non-loadbearing walls and partitions must be designed to resist out-of-plane seismic design force. (For more information, seek out U.S. Federal Emergency Management Agency [FEMA] E-74, Reducing Seismic Risk for Nonstructural Components [6.3−Interior Partitions], by clicking here.) This design force will be based on the weight of the partition framing, finishes, soffits, connected casework or equipment, and ceilings for which it is providing bracing. Out-of-plane design force will not be more than 480 Pa (10 psf). Superimposed axial load, exclusive of sheathing materials, should not be more than 1460 N/m (1077 pound-force foot). Partitions must also be designed to accommodate interstorey drift.
Lateral bracing to the structure is not required when partitions do not extend to the underside of the structure, and their:
For all other conditions, supplemental bracing or framing must be provided to resist the out-of-plane seismic force. Such bracing or framing is to be independent of splayed wire ceiling bracing.
The wall bracing or framing also needs to be designed for compatibility with ceiling requirements, fire ratings, and architectural treatments. It is critical to know whether the partition wall is being used to provide lateral restraint for other nonstructural items. One must check the walls and other lateral restraints at the top are adequate to resist the additional load.
Exterior masonry veneer
There is rising awareness of the importance of seismic design of masonry veneer walls. Although veneer wall cracking failure may not jeopardize a building’s structural integrity, it can be costly to repair and lead to life-safety hazards.
The veneer must be fastened to the substrate to accommodate out-of-plane seismic design force and deformation of supporting framing. The anchored veneer must also be detailed to prevent moisture penetration from weather that could corrode anchors. (For more information, see Acrefine Engineering’s “Specification 9500: Specification Requirements for Nonstructural Building Components.”)
Storage racks and shelving
For light-duty storage racks and shelving, one must provide restraints to resist seismic design forces in any direction. Where this restraint is provided by anchorage to a wall, the latter needs to be verified to ensure it has adequate strength to resist anchor demands.
Backing plates or blocking should be installed as required to deliver loads to primary wall framing members. Nothing should be directly anchored to gypsum board, plaster, or other wall finishes that have not been engineered to resist imposed loads.
With respect to industrial storage racks, the racking should be designed in conformance with building code requirements and the AHJ. In Canada, there may be jurisdictions that accept Rack Manufacturers Institute’s (RMI’s) ANSI MH 16.1-2008, Specification for the Design, Testing, and Utilization of Industrial Steel Storage Racks, and the seismic design requirements of American Society of Civil Engineers (ASCE) 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structure. Where design criteria conflict, the more stringent shall apply. However, some jurisdictions, including City of Vancouver, will explicitly not accept these design guide standards.
On a side note for warehouse racking, the slab may be required to be certified prior to the AHJ completing the permit review or acceptance. This means it has to withstand loads of up to a certain design weight. The racking designer will provide the loading criteria to the proposed end-user for design purposes, and verify with the base building structural engineer to obtain approval for the proposed loads.
Getting certification after the building has been built may incur a charge for the certification of the slab. For new construction, this should be a requirement—even in ‘non-seismic areas,’ the owner needs to keep the file readily available to tenants upon request.
With any building process, it is an important step to go through the local code requirements. In a lot of cases (i.e. in areas outside of active seismic events), there will not be a requirement to design for a seismic restraint system for nonstructural building components. However, it is important to go one step further and confirm with the local AHJ whether the proposed building will be designated as ‘post-disaster.’ In these instances, the local authority will have a minimum requirement to provide seismic restraints, even if it is outside an earthquake-prone area.
A good example of this can be found with Alberta Infrastructure, which has indicated, starting in 2018, all post-disaster buildings will require all overhead mechanical and electrical, and designated floor-mounted components, to be designed with seismic restraints by a professional engineer experienced in that field.
In the end, it is the design/construction professional’s responsibility to look after the nonstructural building components for life safety of the occupants. Further, the survivability of those components within the building after the seismic event is key for productivity. The longer the building is shut down, the more money lost by all parties relying on it.
(The author drew on information provided in product literature from USG and Bailey Metal Products Limited in developing this article. It also relies on information from CSA Group’s Technical Committee on Seismic Risk Reduction of Operational and Functional Components [OFCs] in Buildings, S832-14, along with the Ontario Association of Architects (OAA) Practice Tips 35.)
Jeff Halashewski, Dipl Arch Tech, is a specifications writer for the Edmonton office of DIALOG. He is a member of the CSC Edmonton Chapter executive, and also a member of the Canadian Association of Earthquake Engineering (CAEE). Halashewski has 17 years of experience in institutional, commercial, and industrial buildings, and has worked on projects that involved seismic restraint of nonstructural components. He can be reached via e-mail at email@example.com.
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