Building Framing Types
There are five basic structure or framing types and many alternative structural systems.
Wall Bearing. This type relies on masonry walls to support floor and roof structural members and is generally one to two stories in height (many stories require necessarily thicker first-floor walls). Wall-bearing construction is rather easy and often is used in fairly simple structures with no expected major modifications, but floor plans typically are inflexible, and construction is vulnerable to inclement weather.
Reinforced Concrete. This type is naturally fireproof, a key used in high-rise type structures with stringent fire safety and exit codes. It differs from wall-bearing framing, with freestanding columns braced at each floor by horizontal beams. Skeletal framing for high-rises carries almost unlimited loads (from building weight dead load and live load of contents, people, and wind), and has wide floor layout flexibility. Reinforced concrete frames often are poured floor by floor onsite. Precast concrete framing is formed, cured, and shipped to the site and commonly pre-stressed with steel cables under tension to give tensile strength to floors, with connecting joints welded to concrete-embedded steel plates. It is preferred in some uses and common in multi story garage construction. Fiber-reinforced concrete has greater structural integrity; its character varies with type of concrete, fibers, geometry, distribution, orientation, and density.
Structural Steel. This type is similar to concrete framing (post-and-beam design, freestanding structure before exterior wall placement) but can be erected faster, is lighter (but not as rigid), often has lower initial costs, and is easier to work with in bad weather. It must be fireproofed (additional cost) for certain occupancies and multi-stories but can collapse suddenly in a fire if its temperature rises too high. Steel structures are shop fabricated and field erected, ranging from simple buildings to sports superdomes and high-rises but usually not buildings where rigidity is important (e.g., hospitals, classrooms, research labs).
Combination of Types. Some simple one-story or two- story buildings combine framing types. Some areas are highly suitable for wood frame construction and siding (e.g., California, Oregon, Washington).
Tensile Structures. This type has steel or concrete supports covered by a tensile fabric system. Fabrics carry tension-generated loads while structural supporting elements carry all compression loads, usually supported by compression or bending elements. Large arena or public space roofs often use this type.
Footings and Foundations
Spread Footings. These simple concrete footings rest on the ground to support concrete foundation walls or grade beams and are commonly used on low-rise buildings (three to four stories create big single point loading pressures). Foundation settlement and cracking typically occur if spread footings are in poor soil.
Pilings. Treated timber, steel, or concrete pilings are a common (and perhaps best) substitute for spread footings. They are more expensive and cannot be revised, replaced, or maintained without huge costs but sometimes are justified over the building life. Friction piles carry loads from friction between pile surface and surrounding soils; point-bearing piles often sit on a stiff stratum (e.g., shale, rock, gravel) that carries large unit pressures. Precast pilings are placed in groups (some at slight angle for better stability), capped with a concrete top, and driven into the ground by a large free-falling (or double-acting hammer) weight. Piles also can be drilled and concrete poured in place (caissons), using reinforcing steel cages if needed.
Monolithic Concrete. Many concrete framed buildings use floor systems poured in place as part of the overall structural system; they usually are solid and stable and reduce sound and vibration transmission.
Precast Concrete. Many companies produce precast floor units (set in place by a crane and anchored to supporting beams). They are quickly installed in any weather, offer better quality control, and reduce noise (similar to monolithic concrete) but are not optimal for odd-shaped buildings and openings. Some floors have hollow cores to reduce weight; some are pre-stressed concrete to overcome concrete’s natural weakness in tension, supporting longer beam, floor, or bridge spans.
Steel Systems. These systems use lightweight steel joists or trusses with spot-welded decking, usually topped by steel-welded wire fabric and a thin layer of concrete to add strength and stability (often with compartments or cells for wiring). Steel systems can be erected quickly, usually are more economical than concrete systems, reduce noise and vibration transmission, and improve fire resistance.
Composite Systems. This system combines steel beams and poured concrete flooring as a single unit to carry loads, securing each beam’s top flange to the concrete slab and encasing it in concrete (or using a method similar to construction for steel systems). It is lighter than a concrete floor but has the same advantages. In general, floor systems with greater mass better reduce noise and vibration transmission. Sometimes, the floor system must meet specific design needs (e.g., specialized lab equipment). Some equipment might require a slab-on-grade floor with an appropriately designed mounting pad or piling.
Exterior Wall Types
Solid Masonry. Used for centuries, masonry walls are still in common use, with facings of brick or stone and backup concrete blocks or lightweight cinder masonry, often with steel reinforcements and concrete in vertical cavities. They are 12 to 16 inches thick and easy to maintain but have poor insulating values.
Masonry Veneers. This common system has an exterior brick or stone wythe (facing of one thickness) solidly and permanently anchored to wood or metal stud framing. It can be heavily insulated in stud spacing or cavities. Water entering the wall core must be swiftly and surely routed to the exterior.
Curtain Walls. This wall is suspended from floor to floor on a structural frame and does not carry dead load (opposite of wall-bearing). The system anchors metal extrusions in a grid of mullions with windows and insulated panels. Designs, colors, and materials are nearly unlimited. Curtain walls perform well and are easy to maintain (high U-Values) but must be well caulked to avoid leakage in heavy rains or winds.
Precast Units. Used extensively for university projects, curtain walls faced with precast concrete (any shape and texture) are usually an easily transported and erected size. They can be embedded with steel anchors and backed with steel studs and gypsum board for insulation. Prefabricated stone and brick panels that can be installed in large sections also are available.
Wood Facades. These facades (e.g., redwood, cedar, fir) normally are used on wood-framed structures in one of many shapes (e.g., lap or shiplap siding, shakes, board and batten, tongue and groove).
Miscellaneous Types. Tilt-up concrete wall panels (poured onsite, cured, tilted in place, anchored) are used sometimes for warehouses and other simple structures. If large temperature changes occur in short time spans, expansion and contraction characteristics of materials (and any sealants) must be assessed. Seismic design standards and building codes vary substantially by state but must be considered.
Roofs generally are either sloping (many older buildings) or flat or nearly flat (many new buildings). (See Figure 2.13.)
Built-Up Roofs. Labor-intensive installation requires quality control and planning for weather. Four or five layers of alternating saturated felt and one of four asphalt grades depending on roof slope (or coal tar pitch on older roofs with minimal slope for water drainage) are flood-coated with asphalt and embedded with gravel.
Built-up roofs are flexible, adapt to near-flat areas, and are guaranteed for 15 to 20 years.
Single-Ply Roofs. Continuous rolls of a (single or laminated) material sheet are cut to fit in the field and placed over insulating roof substrate. Single-ply roofs are grouped by installation type, material type, chemical composition, or manufacturing process; the most common are EPDM and TPO which are more popular because of their stronger seams and low albedo, diffusing sunlight and better meeting LEED certification.
(1) Single-ply fully adhered roofs are attached to insulation by hand-applied or sprayed contact cements. Partially adhered ones use mechanically attached plates or fasteners over the roof deck (can fail if not properly installed). Adhered sheets are easy to maintain and repair; rips and tears are apparent. (2) In single-ply ballasted roofs, a single-ply membrane is layered onto the roof deck with direct adhesion only at roof edges or Rounded, smooth, clean rocks hold it down but tend to hide problem areas.
Other Roofing Types. Steep slope roofs use shingles or standing-seam metal sheets. Spray-on foamed coatings insulate and waterproof. Such roofs are for re-roofing, not new construction. Insulating concrete forms and reinforced concrete combine into an insulating substrate (ICF deck) for many roof types.
Roofing Protection. If regular access is needed to maintain equipment on the roof, walkways or stepping stones must be installed, keeping penetrations to a minimum and coordinating with mechanical systems.
Window frames are made primarily from metal, wood, or sometimes plastics. Wood frames require more maintenance (primarily painting unless treated with factory-applied plastic facing) but save more energy than metal windows; better wood windows are more appealing and last longer. Most schools use long-lasting aluminum or alloy frames that do not rust or rot, are extruded in complex shapes for weather stripping, and require little maintenance. Manufacturers offer air infiltration tests that can be compared. Some expensive windows prevent interior frost buildup, and good windows allow installation of factory-sealed double glazing. Multistory structures can need windows that hinge or pivot for cleaning. Window insulating properties can vary substantially (e.g., metal frames with thermal breaks, Low E glass)
Doors and Frames
Institution doors and frames are usually wood (preferably solid core) or hollow metal (fabricated from sheet steel and strong and durable). They are custom fabricated, ADA compliant, and reinforced to fit all hardware types, styles, and sizes (preferably the same high-quality keying system for all buildings to improve security, lower maintenance costs, and reduce number of keys). Doors should be installed before masonry so that frame anchors can be built into joints. Doors at heavy-traffic entrances must be durable; some doors are integral parts of a code-required fire-rated wall system (labeled as meeting UL standards).
Roof and exterior wall insulation is made from many types of materials: (1) loose fill that does not settle and is not edible by termites and rodents, for hollow cells in masonry units; (2) batts, often fiberglass or another inorganic material, effective in joist or stud spaces when anchored permanently; (3) boards, made of one of a permanent (e.g., organic, inorganic, plastic, synthetic) material not attractive to insects or rodents, for roof insulation and between masonry wythes (including preformed tapered flat-roof boards); (4) poured-in-place, lightweight, flexible concrete-like boards for sloped roofs, with cure time before installation of roofing or vapor barrier; or (5) lightweight materials. Resistance to heat flow is classified by R-value, based on material’s physical characteristics and insulating value.