BUILDING CONSTRUCTION - tmfire.ustmfire.us/BUILDING CONSTRUCTION.pdf · 1 BUILDING CONSTRUCTION An...
Transcript of BUILDING CONSTRUCTION - tmfire.ustmfire.us/BUILDING CONSTRUCTION.pdf · 1 BUILDING CONSTRUCTION An...
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BUILDING CONSTRUCTION
An intimate knowledge of building construction is essential for the safety and
efficiency of Firefighters, Captains and Chief Officers. Possessing the knowledge of
construction types and the effect the fire will have on various construction methods are an
integral part of successful firefighting efforts, and substantially increasing the safety of
Firefighters working on scene.
It is not necessary for a firefighter to know how to build a wall or building from
scratch, but it is essential that they understand the strengths and weaknesses of a structure
and the probable effects fire will have on that structure.
The components in any structure are designed with one purpose, to counter the
effects of gravity. All buildings are designed to transfer the weight of the structure and
its contents, thru the structure to the ground. All structures will eventually collapse. Fire
has the effect of speeding up that collapse. Understanding how the components will fail
can tell Firefighters the approximate time a building can be expected to collapse under
various fire conditions.
Buildings can be basically divided into four styles, masonry, frame, metal, and
concrete.
MASONRY
Masonry buildings can be classified into two types. Unreinforced and reinforced.
Up until the late1940’s, most masonry buildings were constructed of brick. These
structures usually had no reinforcement in the brick. Mortar between the bricks was
usually substandard, containing a high amount of lime and little if any Portland cement.
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Unreinforced masonry can usually be identified by a “king’s row”, usually every six
courses, used to hold the bricks together structurally.
As unreinforced masonry increases in height the thickness must also increase to support
the weight of the bricks above. Because of this, usually unreinforced masonry is limited
to a few stories. To span openings in unreinforced masonry, pored concrete or arched
lintels are used to span door or window openings. Steel and wood lintels were also used.
Roof or floor members are usually set into beam pockets in the wall or nailed to rim
joists bolted thru the wall. Beam pockets are usually unrestrained and very often fire cut
to help prevent wall collapse in the event of fire or structural collapse.
Joist
Fire Cut
Furring Strip
Fin. floor
Metal Strap
Plaster
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Hazards of unreinforced masonry are that walls can collapse easily if roof or floor
sections fail and push or pull on the wall sections. Parapets are common in these
structures and pose substantial collapse potential from hose streams and the weight from
cantilevered overhangs or facades, as they pull on an unreinforced masonry wall having
little lateral strength.
REINFORCED MASONRY
Reinforced masonry is usually associated with block construction. In the late
1940’s concrete masonry units (CMU) or cinder block begin to replace brick
construction. Lime mortar is usually no longer present with block construction. After the
mid 70’s, block construction has the advantage of having steel reinforcement throughout
the structure in the form of steel lintels, grout cells and bond beams, greatly reducing the
potential for catastrophic collapse if properly constructed.
LintelBlock
ReinforcedSteel
PouredConcrete
WallOpening
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FRAME CONSTRUCTION
Frame construction involves building wall, roof, and floor structures from lumber.
Wall members are 2x4 or 2x6 dimensioned lumber 16” or 24” on center. Headers
composed of solid wood beams, box beams, or lightweight truss assemblies span window
and door openings. The exterior can be wood siding, stucco, brick or rock veneer.
Roof structures can be classified in two styles, conventional and lightweight
construction.
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CONVENTIONAL FRAMING
Conventional framing was the primary method of framing roof structures prior to
1960. Conventional framing consists of 2x4 or 2x6 roof rafters and ridge beams. King
posts help to support the roof structure. Collar ties run horizontally connecting the roof
rafters, helping to prevent the roof from spreading laterally.
In conventional framing, interior walls running in the direction of the ridge are
usually bearing walls. The ceiling joists are not part of the roof structure. Roof sheathing
prior to 1960 is 1x solid sheathing, 30 style buildings usually contain skip sheathing.
Planks
Beams
Post
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LIGHTWEIGHT CONSTRUCTION
In 1959 gusset plates were patented. This revolutionized the construction
industry by allowing lightweight trusses to become commercially available. Lightweight
trusses are an engineered product and are designed for a specific structure. Lightweight
trusses are designed as a series of triangles, each component of the lightweight truss is
dependent on the other for its support and strength.
2x4
2x6
Optional Overhang4
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3/8" W.P. Plywood Gussets
Members are held together by gusset plates or gang nails. In lightweight trusses
the top cord is under compression, the bottom cord is under tension.
12x
1/2 Span
Lightweight trusses, unless specifically designed, are designed to be supported by
the ends only. The bottom cord is unsupported. At this writing, lightweight trusses were
being designed for up to 128’ of clear unsupported span.
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Lightweight trusses come in different styles. Some of these are peaked truss,
scissor truss, bow, and parallel cord.
In the late 50’s plywood became commercially available, replacing 1x material
for sheathing, decreasing cost and labor.
In the early 80’s OSB (oriented strand board) becomes readily available. With
that came TJI’s or truss joist I beam. TJI is a wood I beam used in floor and roof
systems.
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TJI’s are composed of laminate top and bottom cords usually 2x2 with the center
webbing of OSB. The wood I beam concept has taken the mass out of older solid beam
components, removing the mass equates to lower building costs.
Structural Element
Non Structural
Solid Beam TJI
In tests, an equal size TJI is superior in strength to an equal size solid timber.
Unfortunately for firefighters it is true only until the material burns.
Framing multiple stories is accomplished two ways. The older method was called
balloon framing. This involved straight runs on sidewalls usually with no fire stops from
first to second floor. Interior floors were generally attached on the outside wall with a
rim joist. In older cities, pancaking of floors occurs when floors are impacted or burned
and the rim joist is sheered of the sidewalls.
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Rafter
Stud
1"x4'Let-InBrace
Let-InRibbon
Sub-Floor
Sill
Foundation
Ledger
Girder
Joist
Single Plate
PartitionStud
Double Plate
Joist
In platform framing each floor level is built before the next floor is built, virtually
eliminating fire travel, because of the fire stopping, and lessening the potential for a
pancake type collapse.
ROOF STYLES
There are numerous roof styles constructed using either conventional or
lightweight construction. In the Phoenix area the most common seen are.
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Gabel, common in ranch style. These can be constructed using conventional or
lightweight truss.
Hip, common in ranch style, can be conventional or lightweight truss.
Dutch Gabel, common in ranch style, can be conventional or lightweight
truss
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Saw tooth roofs are generally seen in industrial applications. Saw tooth roofs are
generally well constructed using larger lumber or steel trusses. The hazard is these
buildings have been neglected and are reaching advanced age.
True mansard, not common in Phoenix area but can pose a similar problem as hip
or gable roofs.
Roofing Material
Wood Sheathing
Rafters
Glass
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False mansard, attached to the outside of buildings, generally not fire stopped.
Basically a horizontal chimney. There can be breeches thru the buildings walls or
parapets into the roof structure. Any fire has the potential for rapid extension in a
mansard building.
Bow strung, is the older method of spanning large areas without interior
columns. Generally, larger dimensional construction plated and bolted together. Attic
spaces in Bow strung buildings can be 10’-20’ high. Most Bow strung buildings are
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approaching 50 – 80 years old. Originally, extremely strong and solidly built, fatigue and
neglect have taken a tole on many of the Bow strung buildings in the Phoenix area. Many
have been retrofitted with tie rods along the lower cord to increase their strength. Bow
strung trusses are usually placed 15’-25’ on center with 2x10 or greater perlins
connecting each truss, covered by 1x sheathing. In addition, most Bow strung buildings
are constructed using unreinforced or under-reinforced masonry walls. In a fire situation
the large attic space can conceal a large amount of fire for and extended amount of time
before detection. The failure of one truss has the potential of failing an extremely large
amount of roof area, along with collapsing other trusses in a domino effect. Collapse of
the front or rear of the building is probable because of the attachment of the perlins to the
front and rear facades. This is compounded by the placement of windows and doors in
the front and rear of the structures further weakening those areas.
Wide span, are gable style truss roofs presenting all the hazards of the Bow
strung.
Flat roof, commonly seen in residential and commercial occupancies, can be
constructed in several techniques. Conventional framed, constructed of larger
dimensional solid lumber, generally spanning small to medium buildings.
TJI, lightweight constructed TJI’s can span unsupported nearly 50’ and greater.
TJI’s when exposed to fire can fail rapidly. As one TJI fails the weight is transferred to
other members that cause a domino effect, failing large areas fairly rapidly. TJI’s are
easy to fire block across the trusses, but the space between the trusses can transmit fire
rapidly in that enclosed space. TJI’s can be used for both roof and floor joists.
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Parallel cord truss, is a lightweight truss where the top and bottom cord are
parallel and connected with gusset plates. All features of a lightweight truss apply.
Generally supported on the ends and intended to be unsupported in the center.
Parallel cord bar truss, constructed with either steel or wood top and bottom cord.
Top Chord(Compression)
Plywood RoofingMaterial
Metal Open Webbing
Bottom Chord(Tension)
Webbing can be relatively lightweight steel, capable of spanning large areas. These
trusses are supported by the top cord only. Commonly found in “Big Box” style
buildings.
Roofing Material
Concrete
Wire MeshCorrugated Metal
Many commercial bar trussed buildings may be covered with a lightweight
concrete or Q-decking material.
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A corrugated Q-decking material is placed over the trusses. Reinforcing wire is
added and lightweight concrete is pored over the metal decking. A waterproof material is
then placed over the concrete.
Top Chord(Compression)
Plywood orQ-Decking
Roofing Material(If Applicable)
Bottom Chord(Tension)
Unsupported
Open Webbing
Open Web
Steel Bar
Weld
Angle Irons
Another example is drywall or plywood sheets placed over the Q-deck. Then
foam panels or spray foam is applied, followed by the application of a waterproof
covering. These roofs can be confusing as fire will travel thru the voids and the roofs
will often appear compromised while probing.
Panelized roofs are commonly found in tilt slab buildings and allow large spans
with relatively few columns. The panelized roof generally is divided with large glue
Corrugated Metal
DrywallBuild -up
Roofing Material
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laminated beams. Perlins will be of a smaller dimension spanning a smaller area; each
additional segment is decreased in dimension and span until the smallest lumber may be a
2x4. Areas of the main girder may be very spongy feeling due to the small dimension
lumber in the final panel. Sheathing on panelized roofs is commonly only ½” plywood or
OSB. Under fire conditions, these roofs can be expected to fail rapidly because of their
lightweight makeup.
Roofing Material
4"x8" Plywood
Joist
Beam
Purlin
Metal Hanger Detail
13' to 40'
24"
8'
Metal buildings or metal clad buildings are normally wood or metal framed in
construction, covered with steel, aluminum or fiberglass panels. They are usually
commercial industrial buildings. They are easily recognized by the corrugated
appearance to the exterior. Skylights are commonly incorporated into the roof structure
and may be difficult to detect from the exterior, especially at night. The lightweight
construction and early failure under fire conditions, makes roof and interior operations in
a well-involved structure extremely hazardous.
Light Panel
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Tilt slab is a method of forming large buildings. The walls of these buildings are
constructed of onsite pored, reinforced concrete that are lifted into place and welded
together with steel plates that are imbedded into the panels. Roof and floor structures are
usually lightweight construction; panelized roofs often compose the roof. These
buildings are normally commercial and can be quite large. Occasionally seen in these
buildings are roof assemblies composed of pre-stressed concrete I-beams. Vertical
ventilations are ruled out if this structure is present.
Post and beam construction are usually associated with buildings requiring a great
deal of strength and stability. These buildings can be constructed of steel girders or
poured reinforced concrete. A series of columns tied together by beams form the
supporting framework. Most of these buildings are large industrial buildings. Roof
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structures in these buildings can be of various types running from saw tooth to panelized
depending on the date of construction.
A hazard to older post and beam construction are that wall panels between the
posts are typically unreinforced masonry. Although the structural components are
extremely strong the unreinforced masonry panels can fail independently of the main
structure when subjected to fire and or lateral movement.