DESIGN AND PERFORMANCE ISSUES RELATING TO COMMERCIAL OFFICE BUILDINGS 6 6.1 INTRODUCTION Commercial office buildings represent a large building segment and house the core of American business operations. Corporate headquarters, banks, law firms, consulting firms, accountants, insurance companies, non-profit organizations - the list is almost endless - use office space in buildings around the country to house their operations. As these companies make decisions about the buildings that they construct or office space that they lease, seismic considerations can easily be factored into the decision process. The following are some unique issues associated with commercial office buildings that should be kept in mind during the design and construction phase of new facilities: Protection of building occupants is a very high priority. Occupants are predominantly work-force, with high daytime "8 am to 5 pm" occupancy. Most office building occupants are generally familiar with the characteristics of their building; a small percentage of occupants may be disabled to some degree and visitors will generally not be familiar with the building. Office buildings change their interior layouts frequently, to respond to tenant needs, fluctuations in work-force or organizational changes. Ensuring the survival of business records, whether in electronic or written form, is essential for continued business operation. Closure of the building for any length of time represents a serious business problem. 6.2 OWNERSHIP, FINANCING, AND PROCUREMENT Commercial buildings may be owner operated, particularly if owned by national or global corporations, but many are developer owned (at least initially) housing tenant (lease holder) operations. In many instances the developer and building designers provide an empty "shell," which is fitted out according to the tenants' planning, spatial and environmental needs; design and construction is generally undertaken by the tenant's consultants and contractors. This tends to split the responsibility for interior nonstructural and other risk reduction design and construction measures between the building designers and contractor, and a multiplicity of tenant designers and contractors. Financing for these facilities is typically through private loans. The effective life of an office building is 20 to 30 years, after which major renovation and updating is normally necessary. Interior renovation is usually on a much shorter interval, particularly for rental office structures. 6.3 PERFORMANCE OF OFFICE BUILDINGS IN PAST EARTHQUAKES The seismic performance of modern office buildings designed to recent codes (adopted since the late 1970s) has been good as far as providing life safety. However, the recognition by building owners that satisfactory life-safety code- level performance may still encompass considerable damage (see Figure 6- 1), along with repair costs and possible business interruption of the building for weeks or even months, even in a moderate earthquake, suggests that some performance-based design strategies may be useful. Where severe structural damage has occurred in commercial office buildings, it has generally been to older buildings, often the result of configuration irregularities. Figure 6-2 shows an older medical office building, which had a vertical irregularity that caused one floor to pancake during the 1994 Northridge earthquake in Southern California; a failure resulting from inadequate attachment of heavy nonstructural walls in an older 5-story office building is shown in Figure 6-3. Newer office buildings have also been damaged, most notably the more than 100 welded steel moment-frame buildings (healthcare and residential structures as well as commercial, higher education and industrial buildings) that failed during the 1994 Northridge earthquake. The damage occurred primarily at welded beam-to-column connections, which had been designed to act in a ductile manner and to be capable of withstanding repeated cycles of large inelastic deformation. While no casualties or collapses occurred as a result of these failures, the incidence of damage was sufficiently high in regions of strong motion to cause wide-spread concern by structural engineers and building officials. Initial investigations showed that in some cases, 50% of the connections were broken and very occasionally the beam or column was totally fractured. Possible causes focused on incorrect connection design, incorrect fabrication, poor welding techniques and materials, struction techniques. As a result, a large research program was initiated, sponsored primarily by FEMA, to identify the problems and arrive at solutions. Many structural specimens were tested in university laboratories. New guidelines for these types of structures have been developed (SAC, 2000a, b), but remedial measures have resulted in more costly designs and extended approval procedures, with the result that many engineers have avoided welded steel moment-resistant frames in recent projects. 6.4 PERFORMANCE EXPECTATIONS AND REQUIREMENTS The following guidelines are suggested as seismic performance objectives for commercial office buildings: Persons within and immediately outside the building must be pro- tected to at least a life safety performance level during design-level earthquake ground motions. Persons should be able to evacuate the building quickly and safely after the occurrence of design-level earthquake ground motions. Emergency systems in the facility should remain operational after design-level earthquake ground motions. Emergency workers should be able to enter the building immedi- ately after the occurrence of design-level earthquake ground motions, encountering minimum interference and danger. 6.5 SEISMIC DESIGN ISSUES The information in this section summarizes the characteristics of commercial office buildings, notes their relationship to achieving good seismic performance, and suggests seismic risk management solutions that should be considered. Seismic Hazard and Site Issues Unusual site conditions, such as a near-source location, poor soil characteristics, or other seismic hazards, may lead to lower performance than expected by the code design. If any of these other suspected conditions are geologic hazards, a geotechnical engineering consultant stabilization, foundation and structural design approaches should be employed to reduce consequences of ground motion beyond code design values, or costly damage caused by geologic or other seismic hazards (see Chapter 3 for additional information). If possible, avoid sites that lack redundant access and are vulnerable to bridge or highway closure. Structural System Issues Office buildings are typically low- to mid-rise in suburban locations and occasionally high-rise in downtown locations of larger cities or in satellite suburban office complexes. Office buildings are intrinsically simple, and often are of simple rectangular configuration, not least because economy is usually a prime concern for commercial structures. Thus, their seismic design can be economical and use simple equivalent lateral force analysis procedures with a good probability of meeting code performance expectations as far as life safety is concerned. The protection of nonstructural components, systems and concepts requires structural design to a higher performance level. Configuration irregularities may be introduced for image reasons or site constraints in odd-shaped urban lots, and the structural design may become more complex and expensive. To assist the protection of nonstructural components, special attention should be paid to drift control. The need for planning flexibility requires minimization of fixed interior structural elements and a preference for column-free space. Need for flexibility in power and electronic servicing has resulted in increasing use of under floor servicing to work cubicles, and structural systems have been developed to provide this. Office buildings typically employ steel or reinforced concrete frames to permit maximum planning flexibility. Steel or reinforced concrete moment frames provide maximum flexibility, but tend to be expensive in high and moderate seismic zones. New guidelines for the design of welded moment-frame connections, noted above, have increased the cost of these types of structural system, increasing the already common use of steel braced frames. Elevator cores duct shafts and toilet rooms, being permanent, can be used as shear walls if of suitable size and location. Since these elements are much stiffer than a surrounding frame they may be a source of stress concentration and torsion, if asymmetrically located. If severe asymmetry of core locations is essential for plan-ning reasons, the cores should not form part of the lateral- force resisting system. Nonstructural System Issues The extensive use of frame structures for commercial office buildings, together with the tendency for them to be designed to minimum code standards, has resulted in structures that are subject to considerable drift and motion (sway). The result has been a high level of nonstructural damage, particularly to partitions, ceilings and lighting. This kind of damage is costly and its repair is disruptive. In addition, storage units, free standing work stations and filing cabinets are subject to upset. Excessive drift and motion may also lead to damage to roof- top equipment, and localized damage to water systems and fire suppression piping and sprinklers; thus the likelihood of water damage is greater. The responsibilities within the design team for nonstructural component support and bracing design should be explicit and clear. The checklist for responsibility of nonstructural design in Chapter 12 (see Figure 12-5) provides a guide to establishing responsibilities for the design, installation, review and observation of all nonstructural components and systems