Three Story Wood Frames
by Vincent Dunn Deputy Chief FDNY (ret)
Triple
Deckers wood frame building Fire and Collapse
Of
the five types of building construction (Fire resistive,non-combustible,ordinary,
heavy timber and wood frame) wood-frame construction presents the greatest
firefighting danger. In 1998 Two officers of the FDNY died when the second floor
of a three story wood frame building collapsed. They were thrown into the fire
on the first floor. In the 1980s during a two-year period, a chief and a company
officer were killed and an officer and nine firefighters seriously injured in
four separate collapses of wooden buildings.
To
understand how a burning wood-frame residence can collapse and how to extinguish
a fire burning within a wood-frame building, a firefighter must know how a
wood-frame building is constructed. The
four most widely used methods of wood-frame construction over the past two
hundred years are braced-frame, balloon frame, and platform frame, and
lightweight wood truss construction.
Braced-Frame
Construction
In
the eighteenth and nineteenth centuries, the first large wood-frame buildings
constructed along the East Coast, which still stand today, were of braced-frame
construction, sometimes called "post-and-girt" construction.
This type of wood-frame structure has a braced framework of vertical
timbers called "posts," which are positioned at each of the four
corners of the building, and horizontal timbers called "girts," which
are found at each floor level. These
large timbers reinforce the entire two-by-four-inch wood-frame structure and are
connected together by mortise-and-tenon joints. (The large timbers and the
mortise-and-tenon joints are indicators of braced-frame construction.) The ends
of the horizontal timbers are cut down to fit mortise openings which are cut
through the vertical timbers.
Balloon
Frame Construction
As
the population moved westward in the nineteenth century, the need for housing
increased, and cut and finished large timbers and skilled craftsmen became
scarce. A lightweight, quickly
assembled wood structure, which needed no large timbers, called "balloon
frame construction" replaced the Eastern braced-frame method of
constructing wood structures.
To
erect a balloon frame structure, four wood exterior walls are constructed flat
on the ground. Two-by-four-inch
wood studs, extending in one piece for the full height of the wall, form the
enclosing walls; the four walls are then lifted upright from the ground and
connected like a box at the corners. The
advantage of this type of wood construction is speed and the absence of large
timbers. The drawback is a vertical
void between the wall studs, which extends from the foundation sill to the attic
cap and allows hidden fire and smoke that penetrate the wall space to spread
vertically for two or three floors. This
unobstructed opening between each stud in the exterior wall, extending from the
foundation sill to the attic cap, is an indicator of balloon construction.
Platform
Construction
.The
platform construction method builds a structure one level at a time.
One complete level of two-by-four-inch wood enclosing walls are raised
and nailed together; the floor beams and deck for the next level are placed on
top of these walls. The next level
of two-by-four-inch wood enclosing walls are constructed on top of the first,
and the floor beams and deck for the next level are placed on top of these
exterior walls.
Lightweight
Wood Truss Construction
Lightweight
wood truss construction is the most common type of wood constrution built today.
It may replace platform construction, but, from a fire protection point of view,
it is inferior compared to platform construction. The sheet metal surface
fasteners used to connect the truss members is a defective structural connection
from a fire protection point of
view. The burning truss floors and roof can collapse within 10 minuetes of the
arrival of the firefighters. It is not possible to extinguish a fire in the
concealed spaces of lightweightwood
construction.
Wood
Floor Collapse
The 1998 collapse in a wood frame building that killed two FDNY fire officers was due to an illegal alteration of a floor system and fire destruction of the second story floor beams. A partition wall supporting the 2nd floor was removed during the alteration.
Wall
Collapse
The
walls of wood buildings collapse more freqently than the wood floors. Wood floor
beams may be 2 x 10 inches while the wood walls may only be 2 x 4 inches.
However when the walls collapse it causes the floors and roof to also fail.
Types
of Wall Collapse
There
are three ways in which a wood-frame building’s wall can collapse during a
fire: one wall may fall straight outward at a 90-degree angle, the entire
building may lean over and collapse on its side, or one or all four wood
enclosing walls may crack apart and fall in an inwardoutward collapse. A
three-story braced frame structure frequently falls in an inward/outward
collapse. The top two stories
collapse inward, back on top of the pancaked floors; the lower story collapses
outward on to the sidewalk.
Wall
Collapse Warning Signs
The
90-degree-angle wall collapse is often signaled by the corners of the falling
wall splitting apart from the remaining walls.
The lean-over collapse is often indicated by the burning structure slowly
starting to tilt or lean to one side. An
inward/outward collapse may not exhibit any structural warning at all-sometimes
the only indication that a collapse is imminent is a serious fire burning for a
long time on the lower floor. When
such a collapse occurs,
firefighters
report that they see no signs but that they hear a sudden, loud cracking noise
and feel a hurricane-like gust of wind on their backs as they turn to run from
the falling structure.
Of
the three types of wall collapses,
the inward/outward collapse is the most dangerous-because it is sudden, it gives
no visible warning signs prior to failure, and, unlike most other building
failures, it may involve the collapse of two, three, or four walls
simultaneously. During a fire in a
structure with masonry walls, it is rare that more than one wall will collapse
at one time . When a braced-frame wood building collapses, however, all four
walls may collapse at one time.
The
4 wooden structure collapses that occurred in the 1980s where walls failed,
an investigation showed they were located on corners and one was the end
building in a row of three which stood next to an open lot.
As the last building was unsupported at one end, it was, in effect, the
same as a corner building. All four
buildings were three stories high and a serious, long-burning fire had destroyed
the first floor of the structure.
Corner
Wood-Frame Buildings
Wooden
buildings constructed side by side receive support and stability from the
adjoining structure. If the lower
floor of a wood building burns and one of the wood bearing walls is destroyed by
fire, the structure will begin to lean to one side.
Adjoining structures built up against a wood building can prevent such a
fire-weakened structure from collapsing.
When
weakened by a fire on a lower floor, however, a wood-frame corner building will
collapse on its unsupported side into the street or an empty lot. The bearing
walls of a wood structure, unlike those in any of the masonry construction
types, are combustible and can collapse when exposed to fire. The side bearing
walls on the first floor of a three-story wood building are two-by-four-inch
wood studs spaced 16 inches on center, the same as the bearing walls on the
second and third floors. Though the
bearing wall studs of the first floor support more weight than the second- and
third-floor wall studs, there is no compensation for the increased deal load,
unlike some multi-storey, masonry bearing-wall buildings, in which the lower
levels of the bearing walls are thicker than the upper levels.
Therefore, if a fire weakened the bearing wall studs of all three floors
at the same time, the ground floor wall studs would fail first because they
support more weight than the second or third-floor bearing wall studs.
Based
upon these four New York City building collapses and other wood-frame building
failures, it is apparent that the height of the structure affects its stability,
Three-story wood-frame buildings collapse more frequently than one- or two-story
wood-frame buildings.
Causes
of an Inward/outward Collapse
Three
factors contribute to the inward/outward collapse of a bracedframe wooden
building: fire destruction of bearing walls, failure at the mortise-and-tenon
connection, and exterior wall overload. Unlike the exterior walls of the four
other basic construction types (fire resistive, non-combustible, ordinary
brick-and-joist, and heavy timber), the bearing wall of wood-frame construction
can be destroyed bv fire and can collapse when flames spread out of a window and
consume the outside or inside of this load-bearing wall. Burning wood-frame
buildings exhibit a rapid fire spread. When
the fire department arrives on the scene, both the wooden exterior walls and the
structure's interior are often involved with flame.
When wood buildings are built close together or when there is a common
roof space running through a row of wood houses, fire spread will be extremely
rapid and will probably involve more than one structure.
In addition to placing hose streams in the interior of the burning
structure, firefighters will need one or more hose lines to control exterior
fire spread along the outside combustible walls and to protect exposures from
radiated heat. A firefighter should know which of the four enclosing walls of a
burning wood building are the load-bearing walls that support the floors and
roof. Because these walls are
interconnected, the interior floors will collapse if the bearing walls fail
during a fire. Conversely, if the
interior floors collapse, they may cause bearing wall failure.
In
older urban neighborhoods, wood-frame buildings were built close together, with
the bearing walls usually the side walls and the non-bearing walls the front and
rear enclosing walls. This practice
has changed in suburban communities. Private
homes, built on large plots of land, are designed to have the larger area of the
building face the street front, so the front and rear walls are load-bearing and
the two side walls non-load-bearing. Condominiums
and row town houses have the same design.
During
a fire in a suburban row of town houses, if the floors inside collapse, the
front or rear walls may collapse outward. In
peaked-roof buildings, the bearing walls support the majority of the roof
rafters that are parallel to the ridgepole.
In flat-roofed wood buildings, the bearing walls are usually the walls
with the greatest dimension: the non-load-bearing walls have the shortest
dimension.
Mortise-and-Tenon
Joints
The structural framework of a braced-frame wooden building whichcollapses inward/outward consists of vertical timber corner posts and horizontal timber girders or girts at each floor level. The corner posts and girders are connected by mortise-and-tenon joints. When a braced-frame wood timber collapses, it fails at the weakest points--often the mortise-and-tenon connection. The mortise hole has removed the center section of the corner post timber and reduced its strength; the tenon end of the girder is only a fraction of the original girder's thickness and therefore has only a fraction of its strength. In addition to this design weakness, the connection can be destroyed by fire. Furthermore, unlike concrete and steel fastening, the wood mortise-and-tenon connection is susceptible to collapse by rotting. A vacant wooden building open to the elements can be quickly weakened by rotting structural components like the mortise-and-tenon connections.
Exterior
Wall Overload
The
exterior wall of a wood-frame building can be weakened by the weight of a metal
fire escape landing and ladder. This
heavy metal structure attached to the outside wall of a wood building is
anchored to two-by-four inch wall studs behind the wood sheathing.
The weight of the metal fire escape can exert a slight outward pull on
the wall studs to which it is attached for support.
This pull causes the wall studs to curve or bow slightly outward.
The load above, supported by the curved wall studs, is no longer
transmitted through the studs as an axial load (centered or evenly distributed)
but becomes an eccentric load (off-centered or uneven).
During a fire, the wall supporting a metal fire escape must be considered
a structural danger. The weight of the fire escape will accelerate the collapse
of a fireweakened wood wall.
There
are two types of masonry surfaces applied to outside walls of old wood
buildings: A brick-and-mortar veneer wall can be attached to the wooden
structure by thin strips of sheet metal, one strip every two square feet; or a
thick stucco coating, spread on wire mesh, can be nailed to the old wooden
surface of the building. These wall
surfaces increase the collapse danger during a serious fire in a wood-frame
building by adding considerable weight to the structure.
As much as eight pounds per square foot of stucco and wire mesh have been
found on a collapsed wall .
Brick
veneer not only overloads a wall but also hides major structural defects of the
wall. It can conceal an obvious
collapse warning sign, such as the wood walls splitting apart, or hide the
burning of the wood bearing wall behind it.
These masonry wall coverings also contain the heat and flame inside the
building, thus increasing the destruction of the structural framework.
Lessons
to Be Learned
1.
Burning wooden buildings of three or more stories collapse more
frequently than burning
one- or two-story wood buildings.
2. Wooden buildings located
on a corner plot or standing alone are more susceptible to collapse when exposed
to fire than wood buildings in the center of a row of similar buildings.
3.
When a serious fire burns out the entire first floor of a three story
wood building, there is a danger of collapse.
4.
Of the three types of wood-frame building collapses, the inward/outward
collapse is the most dangerous. It
gives no warning and can result in the simultaneous collapse of two or more
sides of the structure.
5.
Three contributing causes of wood-frame building collapse are fire
destruction of bearing walls, the mortise-and-tenon joint of a braced-frame
wooden building, and the overload of an exterior wooden wall.
6.
Large buildings collapsing on top of smaller buildings cause the smaller
buildings to collapse.