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Urban Search & Rescue: Understanding the 5 Building Classifications

Updated: Mar 28

When it comes to Urban Search & Rescue (US&R) and structural collapse scenarios, responders need to be aware of the various building classifications. These classifications differ from those typically encountered in fire service training, as they focus more on determining the appropriate shoring methods to be used during rescue operations. In this first article of a series, we will provide an overview of the six building classifications commonly referenced in US&R and Structural Collapse courses. The subsequent articles will delve deeper into the components, expected performance during disasters, and challenges faced by responders for each structure type.

1. Light Frame Construction (Wood and Light Metal Stud)

Light Frame Construction buildings utilize closely spaced wood or light gauge metal studs for bearing walls, along with joists for floors and rafters for roofs. The roof is generally composed of wood or metal rafters or trusses spaced between 16” to 32” on center (o.c.), with sheathing that may be spaced or solid boards laid straight or diagonally, or plywood. Floors in these buildings consist of wood or metal joists or flat trusses spaced, usually 12” to 24” o.c., with sheathing made of wood boards laid straight or diagonally, or plywood. Newer constructions may feature concrete topping over plywood sheathing.

Exterior walls in Light Frame Construction buildings are typically built with wood or metal studs spaced 16” to 24” o.c. Sheathing materials include wood boards laid straight or diagonally, or plywood, while smaller and older buildings use lath and plaster or gypsum board. Interior walls and columns typically feature wood or metal studs spaced 16” to 24” o.c., with sheathing types similar to exterior walls. Larger buildings may include column and beam framing in addition to the stud bearing walls. These structures are characterized by their flexibility in response to lateral forces from earthquakes and strong winds and are often used for single-family and multi-unit residential buildings, as well as low-rise commercial, institutional, and light industrial buildings.

2. Heavy Wall Construction (Exterior walls of Reinforced Masonry, Unreinforced Masonry, and Tilt-up Concrete)

Heavy Wall Construction buildings are characterized by their heavy, fire-resistant exterior walls, made of Reinforced Masonry (RM), Unreinforced Masonry (URM), or Tilt-up Concrete (TU), and lightweight wood floors and roofs. The roof of a URM building typically has wood rafters or nailed wood trusses made from 2x and 1x members, sheathed with straight 1x wood sheathing. Bowstring trusses and other truss types are also used for main roof supports, with 2x joists and 1x straight sheathing. Tilt-up buildings usually feature panelized systems with long-span glued-laminated (glulam) wood beams, 4x purlins, 2x sub-purlins, and plywood sheathing or other lightweight roof systems.

URM buildings usually have 2x or 3x wood joists with straight 1x wood sheathing for floors, while Tilt-up buildings often employ large wood joists or flat wood trusses with plywood sheathing. The exterior walls of URM buildings have 9” thick parapet walls, with an additional 4” of thickness for each story in height. Tilt-up walls are reinforced concrete, typically 6” or greater in thickness, cast flat on site in approximately 24’ widths, and then tilted into position. Interior columns and walls in URM buildings may have wood stud walls, large wood columns, and beams, while Tilt-up buildings usually have steel pipe/tube columns spaced in a 24’ on center by 50’ or similar spacing, with a uniform structural grid.

Occupancy types for these buildings can range from multi-family residential and institutional structures to office, commercial, educational, or industrial and warehouse buildings.

3. Heavy Floor Construction (Cast-in-Place Concrete)

Heavy Floor Construction buildings have cast-in-place concrete floors and roofs, which provide them with increased fire resistance, thermal mass, and structural stability. The roof structure of these buildings is usually made of cast-in-place concrete, which can be either reinforced flat slab, waffle slab, or one-way or two-way ribbed slabs. The roof system may also include precast concrete double tees or hollow core slabs, supported by cast-in-place beams or walls. In some cases, a lightweight roof system such as steel or wood framing may be used in combination with the cast-in-place concrete floors.

The floors of these buildings are also made of cast-in-place concrete, which can be flat slab, waffle slab, or one-way or two-way ribbed slabs. These floors are supported by cast-in-place concrete columns or walls. Interior walls can be made of cast-in-place concrete, reinforced or unreinforced masonry, or metal or wood studs with sheathing. Exterior walls may consist of cast-in-place concrete, precast concrete, masonry, or metal or wood studs with sheathing. These buildings are typically used for commercial, institutional, and residential mid- to high-rise buildings, parking structures, or industrial facilities.

4. Steel Frame Construction

Steel Frame Construction buildings utilize steel columns, beams, and open-web steel joists to create a rigid frame system that can support heavy loads and span greater distances. The roof system of these buildings is generally composed of open-web steel joists, steel beams, or steel trusses, supporting steel decking, which is topped with a built-up roofing system or other lightweight roofing materials. Long-span systems such as space trusses or arches may also be used for large clear spans.

The floor system of Steel Frame Construction buildings typically consists of open-web steel joists or steel beams supporting steel decking, which is topped with a concrete slab. Long-span floor systems like double tees or hollow core slabs may also be used. Interior and exterior walls can be made of metal or wood studs with sheathing or precast concrete panels. Curtain wall systems or glass and aluminum storefront systems can also be employed for the building façade. These buildings are commonly used for commercial, institutional, and residential high-rise structures, as well as for large industrial and warehouse facilities.

5. Pre-Engineered Metal Buildings

Pre-Engineered Metal Buildings are prefabricated structures that consist of rigid steel frames, often with metal or wood purlins and girts, and metal siding and roofing. The roof system of these buildings is usually made up of metal decking supported by steel purlins, which are, in turn, supported by the rigid steel frames. The roofing material is typically a metal sheet, which can be corrugated or standing seam.

The floor system of Pre-Engineered Metal Buildings is typically a concrete slab on grade or a combination of cast-in-place concrete and steel decking supported by steel joists or beams. The walls of these buildings are usually made of metal siding, which can be insulated or non-insulated, and can be attached directly to the rigid steel frames. Some buildings may use masonry, precast concrete, or metal or wood studs with sheathing for exterior walls. Interior walls can be made of metal or wood studs with sheathing or masonry. These structures are often used for warehouses, industrial buildings, aircraft hangars, or other large single-story facilities.

6. Precast Concrete Construction

Pre-cast concrete construction involves the use of modular pre-cast concrete components, including floors, walls, beams, columns, and other sub-components that are connected on site. These structures are typically built with a regular grid of columns and beams, and often have concrete or masonry shear walls to provide "Box Type" resistance to earthquake forces and strong winds. The critical interconnections between pre-cast components are vital for structural stability, as inadequate connections have led to widespread collapse problems in past earthquakes.

Roof and floor components in pre-cast construction commonly use pre-tensioned steel cables for reinforcement. Exterior columns and walls may incorporate pre-cast concrete columns or pre-cast concrete frames, as well as cast-in-place concrete shear walls. A grid of pre-cast concrete or steel columns is typically used to support interior beams and girders. Pre-cast concrete buildings can range from single-story to high-rise structures and may include commercial, office, multi-use, parking structures, and large occupancy facilities. Highway bridges and overpasses can also be built using pre-cast concrete segments or pre-cast beams combined with cast-in-place concrete slabs.

Understanding these six building classifications is crucial for responders working in US&R and structural collapse scenarios. As we continue with our article series, we will explore the unique challenges each structure type poses and their expected performance in disaster settings.

In our next article, we'll talk more specifically about types of forces that need to be understood and how they can impact construction members.

Source: Urban Search & Rescue Program, California Fire Service and Rescue Emergency Mutual Aid System, Revised April 2019.