Office Building Construction
OFFICE BUILDING CONSTRUCTION
Toronto area office building construction varies significantly according to structure size, its locale (downtown and suburban), age of building, zoning controls, occupants’ needs and functionality, and internal/external design. These are further complicated by unique circumstances, such as incorporating existing historical structures and complying with heritage requirements. The following description details methods used in office building construction, but these same techniques generally apply to other buildings discussed in later chapters, including multi-use residential buildings.
STRUCTURAL DESIGN AND LAYOUT
Office buildings can range from single or two storey suburban professional buildings to high-rises in prime downtown location. The construction process is complex, particularly in larger buildings, given the need to co-ordinate extensive structural, mechanical, environmental, lighting and finishing activities. Costs associated with office building construction vary based on specific design criteria.
Construction standards are subject to the Ontario Building Code requirements. These requirements are focused on structural strength and integrity of the building, fire resistance, acoustic separation, adequate means of egress in and out of the structure, assurance of adequate sanitary conditions, and appropriate construction materials to prevent heat loss (insulating materials), water intrusion (basement damp-proofing) and moisture (vapour barriers). Beyond minimum standards established by the Code, the structure may have added elements and standards established by the owner and architect to achieve a uniform element of design throughout the building. Normally, the final design is only altered with the approval of the building owner or the managing representative of the particular commercial project.
Architects are commonly involved in commercial buildings to assist with preliminary designs, drawings, specifications and technical drawings. These professionals also typically co-ordinate overall project management with consulting engineers, landscape architects and other involved in the construction process, be it for a new building or restoration/alteration of an existing structure.
Small commercial buildings may use wood (including manufactured wood products) for framing, similar to residential techniques. Wood framing is usually the most economic method of building a structure. Wood floors and demising walls must meet building code requirements regarding structural loads and fire separations between individual units, and between units and common areas such as stairways.
Fire protection rating is achieved primarily through the use of drywall (gypsum board). Structural and fire rating requirements, including specifications for first and higher floor stud bearing walls, beams and joists are set out in the Ontario Building Code. Masonry (concrete block) walls may be used in combination with wood framing in larger commercial buildings to more effectively separate units for add ed structural mass, as well as fire requirements. The walls effectively separate a large structure into successive fire resistant compartments. Stairways and common area walls may also be concrete block or poured concrete (e.g., structural spans for larger openings within the masonry walls).
The rigid structure of most larger office buildings consists of steel, concrete or a combination of the two. High-rise steel structures are supported by a substantial steel skeleton consisting of columns and I-beams, upon which floor and wall materials are secured. Steel can be divided into heavy steel framing for structural components in high-rise buildings and lightweight steel, which has gained popularity for interior framing purposes in for low-rise office buildings, but is more widely associated with prefabricated steel buildings.
Concrete building have some popular, particularly given the 9/11 terrorist attacks on the World Trade Center and resulting degradation of the steel framework under intense heat. Steel softens at high temperatures and must be properly encased using some form of passive, fire resistant material. Designers for recently-constructed or newly planned office towers now seek enhanced protection. Concrete case-in-place (sometimes referred to as concrete framed) buildings, for example, have impressive resistance to explosions or impact and can endure very high temperatures for a long time period. Further, building cores are being encased in thick concrete for added protection in relation to primary services, elevators and stairwells in the event of fire or terrorist attack.
However, steel proponents quickly point out that proper design and structural considerations can effectively minimize fire/terrorist concerns. In addition, steel structures have proven very effective in handling wind load and seismic activity (earthquakes) given the inherent ability to bend. Lastly, the off-site fabrication of structural steel members enhances efficiencies during construction and increasingly-sophisticated milling procedures allow for flexibility in building design and aesthetics.
Reinforced concrete buildings can be broadly categorized into cast-in-place (poured in place) or precast and deliverered to the job site for installation. As with steel, significant advancements in forming procedures now provide for creativity and ingenuity in concrete building designs. Cast-in-place reinforced concrete has proven effective from a cost perspective. Interestingly, many modern buildings combine steel and concrete to gain the benefits of both.
The building core in a multi-story building is the central or arterial component integrating functions and service needs for occupants. Such areas are normally composed of toilet facilities, elevator banks, janitors’ closets, utilities, smoke shafts, mechanical facilities and stairwells.
Registrants will encounter both side-core or centre-core plans ( or variations, such as low-rise, elongated buildings with multiple cores). Centre-core floor design are better suited for multiple tenancies, while side-core arrangements provide a larger open space on individual floors for larger tenants. The popularity of central core construction derives from its overall design efficiency. The traditional centre-core building attracts tenants who prefer perimeter window areas relegated to middle management and executive offices, with internal areas used for other staff levels, reception areas, aisles and hallways. High-rise office tower in downtown locations are predominantly centre-core plans.
Side-core and off-centre building designs are frequently found where extensive numbers of people perform similar functions with no requirement for executive offices or preferential locations near windows. These cores have become increasingly popular with the advent of high tech call centers and large centralized customer service organizations.
The size and shape of the floor plate in a new building will vary depending on occupant needs. For example, an owner/user (e.g., a corporate headquarters) will have specific requirements versus the developer of a speculative building anticipating tenant needs. A key factor in floor plate design is leasing depth, particularly in speculative building construction; i.e., the distance from either the centre core (in the case of a single tenant) or centre core plus corridor (in the case of a multi-tenant configuration) to the outside wall.
Floor plate design and size are also impacted by considerations beyond developer and user. For example, zoning requirements, including setback requirements, may limit both floor plate size and shape. The Ontario Fire Code can also influence floor plate design in terms of distance to emergency stairwells and locations of such exits within the structure.
Floor height is also a consideration from both costing and user perspectives. Additional costs are incurred when floor-to-floor height increases, particularly in office towers. However, such may be unavoidable for both functional and aesthetic reasons. With larger structures, a greater floor height allows more sunlight to enter the structure, provides more air space and ventilation (particularly in high tech needs including extensive use of computers), and provides greater clearances to accommodate high intensity lighting systems now widely used in modern office towers.
For clarification, floor-to-ceiling height is commonly defined as the distance from the top of one floor to the top of the next floor. This space includes (from the bottom up) the finished floor (which typically has space provision for cabling to serve office needs) resting on the floor plate, the physical space for personnel, desks and so forth, and the depth of the dropped ceiling to accommodate HVAC, plumbing, ductwork, fire protection, lighting and other mechanical system serving the office area.
NUMBER OF FLOORS
Developers seek to maximize total square footage in new buildings, while adhering to zoning requirements. Higher totals can be achieved through increasing the number of floors. Traditionally, steel beam construction and concrete flooring have been most popular, but these designs typically require a greater depth of construction to accommodate both the beams and slab systems.
Concrete floor plates are gaining popularity in many types of high-rise structures, as this system helps maximize the sandwich space (i.e., the space between the finished floor and the dropped acoustic ceiling). Alternatively, the ability to minimize overhead space for various services can also translate into more floors within a building, assuming that zoning requirements are based on total building height restrictions. Even without adding floors, more judicious use of ceiling service areas can translate into lower overall floor-to-ceiling heights and consequent savings on exterior finishing of the building façade.
Finishes vary somewhat by type of construction and project size. Smaller commercial buildings with exterior wood framing usually install brick veneer and/or siding (vinyl or aluminum), as is the case with residential structures. Concrete buildings may be simply painted or a plastic wall coating applied for greater durability. Alternatively, stucco is a popular choice, as well as various engineered, composite ext3erior panels. Concrete building can also be visually enhanced with decorative face brick or stone.
Curtain walls are widely used in Canadian office towers and other commercial structures. These self-supporting, continuous cladding systems typically condidst of glass inserted in aluminum or stainless steel framing. Curtain walls are designed to span multipule floors, while allowing for building movement and expansion, effectively sealing the structure’s exterior and providing thermal efficiency for heating and cooling the building.
Curtain walls are readily identifiable by their thin horizontal and vertical mullions into which glass is inserted. Most curtain wall installations are accomplished by assembling all mullions and fasteners to the building skeleton and then inserting glass frames (or other components, such as louvers for ventilation) on a one-by-one basis (commonly referred to as stick walls). Alternatively, curtain walls can also be pre-assembled as modular units and lifted into place (sometimes referred to as unitized walls), or some combination thereof. Early systems often has problems with water penetration, condensation forming on interior surfaces or ice forming on external components, but these issues have been largely eliminated in quality curtain wall construction.
Energy-efficiency advancements in curtain wall technology have resulted in considerable heating and cooling savings, not to mention increased comfort for occupants. The double wall provides two glass panels with an air space in between. While different types of double walls exist, the general principle lies in creating an air compartments between them (e.g., cooled air to lower the interior glass wall).
The alternate approach, increasingly popular in Canada, involves high performance glass with special coatings to minimize glare and reflect long-wave radiation. Low-eglass, for example, contains a think metallic layer that allows filtered sunlight into the structure during winter months, while retarding outward flow of head. Some high performance windows may have inert gas to fill between glazings (e.g., argon or krypton) to reduce heat and cold transfer.
LIFE SAFETY SISTEMS
A life safety system, installed within a building, provides for the life and safety of occupants and, more specifically, the integrity and operation of support systems during an emergency. The scope of life safety systems and associated regulations are governed primarily by the Ontario Fire Code.
Such systems include the use of alarms, emergency communication systems, strategically places firewalls, automatic door closures, emergency lighting, emergency ventilation, auxiliary power and sprinkler systems. The life safety system will depend on the size of the structure, the particular uses of that structure and regulatory controls.