4.2 Design Considerations
The selection of design solutions must be consistent with the scope and objectives selected for the project. Some design solutions can be implemented without consideration of the building code and without engineering expertise. Other design solutions rely on building codes and standards, such as ASCE/SEI 7-10, ASCE/SEI 31-03, and ASCE/SEI 41-06, that all contain elements of the performance-based design methods discussed above. If engineering consultants are engaged to provide design solutions, the selection of seismic force levels, design coefficients, and design methods depends upon the performance objectives selected.
Specific design solutions for nonstructural items fall into three broad categories:
- NON-ENGINEERED (NE): These are typically simple, generic details or common sense measures that can be implemented by a handy worker or maintenance personnel using standard items from any hardware store. Many of these solutions apply to contents that are not directly covered by building code provisions. As an example, Chapter 6 contains a detail showing the general configuration for anchoring a bookcase to a stud wall (see Figure 188.8.131.52-4) and identifies the parts needed but does not explicitly indicate the size of the angle bracket or screws needed; this is left to the handy worker based on the size and weight of the particular bookcase and the type and spacing of studs. Some of these types of solutions have failed in past earthquakes, usually due to undersized bolts and hardware or because bolts have failed to engage a structural member. As a result, non-engineered solutions are generally not appropriate for hospitals or other facilities that have chosen operational functionality as a performance level objective.
- PRESCRIPTIVE (PR): Prescriptive design details are available in the public domain that have been engineered to meet or exceed code requirements for a set of common conditions and can be used directly in many situations. One prescriptive detail included in Chapter 6 is the anchorage detail for a residential or small commercial water heater (see Figure 184.108.40.206-6). This detail is applicable for the anchorage of a water heater, up to 100 gallons, attached to a wood stud wall. The detail calls out the required hardware and the size and spacing of fasteners.
While there are only a limited number of these details currently available, we anticipate that more such details will be developed as engineers, architects, and specialty contractors become more familiar with the new code requirements for nonstructural components. Some of the prescriptive details have been developed by or for the Office of Statewide Health Planning and Development (OSHPD), the entity in California responsible for overseeing hospital design.
- ENGINEERING REQUIRED (ER): These are nonstructural anchorage details specifically developed by a design professional on a case-by-case basis for a specific set of conditions. First, the owner and design professional need to agree on the desired level of protection for the anticipated level of shaking, only then can the design professional develop details consistent with the objectives. Design methods and design coefficients are selected based on the performance objectives as discussed above. An anchorage detail designed for a lateral force of 1.0 g will generally be more robust and more costly than one designed for a lateral force of 0.1g. Higher design forces and more complex engineering methods may be required to meet higher performance objectives.
As part of the design process, it may be important to consider a number of issues:
- Interaction of nonstructural components. Many nonstructural systems are interconnected or interdependent; items in close proximity can impact one another and tall or overhead items can fall and damage items below. Lights, ceilings, diffusers, ducts, piping, sprinkler heads, and variable air volume boxes may all share the plenum space above the ceiling and it may be challenging to find ways to keep them separated and to provide independent support for all of them.
- Interaction of nonstructural and structural components. Nonstructural components may be damaged by the deformations of structural components. Items that cross seismic separations between buildings, connect at adjacent floor levels, or are located in base isolated structures have special design considerations based on the expected deformations of the structural system.
- Strength of structural components. Since nonstructural components typically anchor to structural slabs, walls, and framing, it is important that the capacity of these components be checked for adequacy when tall and heavy items are being anchored to them.
- Location. Design forces are typically higher for items located in mid- and high-rise buildings and on roofs. The location of the item in the building may influence the design.
- Primary vs. secondary effects of failure. If failure of an item may result in the release of water or hazardous materials such as toxins, chemicals, or asbestos, it may warrant additional attention to address these damaging secondary effects.
- System performance. Fire protection systems, emergency power generation systems, and computer and communication networks are systems that depend on the functionality of multiple components; the failure of any part might compromise the functionality of the system. All related components must be checked if the system is required for functionality.
- Emergency egress. Items located over exits, in stairways, and along exit corridors may warrant special attention in order to ensure the safe exit of building occupants.