A prestress floor system for wood floor framing structures places
the structure's joists under a laterally directed compressive force
while the joists are maintained in their relative positions by blocking
elements. The compressive force is provided by a cable anchored
at one end to an outermost joist and anchored at the other to a
spring that is, in turn, anchored to an outermost joist. The resulting
structure has increased vertical stiffness and reduces the amount
of deflection, vibration, and noise experienced during loading.
1. A load carrying structure, for supporting vertical loads, comprising:
a plurality of substantially parallel spaced-apart joists positioned
between a first joist and a last joist that support sheets comprising
a ceiling or floor;
spacing means for maintaining the lateral spacing of the joists
while they are under laterally directed forces; and
pressing means for pressing the joists laterally toward each other
under a predetermined force transmitted through the spacing means
and for reducing vertical deflection while the joists are under
2. A load carrying structure as recited in claim 1 wherein the
spacing means comprises a plurality of blocking elements located
between adjacent joists.
3. A load carrying structure as recited in claim 2 wherein the
pressing means comprises a spring anchored to the first joist and
a cable anchored to the last joist at one end and to the spring
at its other end.
4. A load carrying structure as recited in claim 1 wherein the
pressing means comprises a spring anchored to the first joist and
a cable anchored to the last joist at one end and to the spring
at the other end.
5. A load carrying structure as in claim 1 wherein the pressing
a cable coupling the first joist with the last joist; and
tensioning means for maintaining the cable under tension.
6. a load carrying structure as recited in claim 5 wherein the
tensioning means includes adjustable anchoring means for coupling
the cable to one of the first and the last joists and for providing
adjustable amounts of tension thereto.
7. A load carrying structure as recited in claim 5 wherein the
tensioning means comprises:
a spring coupling the cable and the last joist; and
a bolt coupled to the spring and anchoring the spring to the last
joist, the bolt passing through the last joist through an aperture
and adjustably fastened by a nut to an opposite side of the joist
so as to permit adjustment of the tension on the spring.
8. A load carrying structure for supporting vertical loads, comprising:
a plurality of spaced-apart longitudinally extending joists including
a first joist, a last joist, and at least one joist positioned therebetween,
adapted to support planar elements defining a ceiling or floor;
a continuous cable acting under tension for generating a compressing
force directed substantially only laterally across the joists from
said first joist to said last joist; and
blocking means acting under compression for opposing the compressing
force and keeping the joists spaced apart in static equilibrium.
9. A load carrying structure as recited in claim 8 wherein the
cable is attached at one end to the last joist and a spring attached
at one end to the first joist, wherein the free ends of the cable
and spring are attached to each other.
10. A load carrying structure as recited in claim 8 wherein the
blocking means comprises a plurality of blocking elements spanning
11. A load carrying structure as recited in claim 10 wherein the
blocking elements are arranged in two parallel rows that straddle
12. A load carrying structure as recited in claim 10 wherein the
cable passes through the midpoint of each joist.
13. A load carrying structure as recited in claim 10 wherein the
cable passes through a first side of the first joist and couples
to an opposite side of the first joist, the cable applying the compressive
force to the opposite side, further comprising:
a plate coupling the cable to the opposite side of the first joist,
wherein the plate overlies a blocking element area; and
a spring connected to the cable and selected to have a force that
is less than a crushing capacity associated with the joists and
the blocking elements multiplied by an area of the plate that overlaps
the blocking element area.
14. A prestressed floor structure comprising:
a plurality of substantially parallel, longitudinally extending
joist members defining a generally rectangular flooring area, a
first outside joist member comprising one side of the area and a
last outside joist member comprising an opposite second side of
the area, with a plurality of said joist members positioned in between
the first and last joists;
a coil spring anchored to the first joist member;
a cable that is passed through each joist member except for the
first and last outside members and that is anchored at one end to
the last outside joist member and anchored at its other end to the
coil spring, the cable having a length such that the spring is stretched;
a plurality of blocking elements placed between adjacent joist
members, extending substantially parallel to the cable.
15. A prestressed floor structure as recited in claim 14 wherein
the cable passes through the midpoint of the joists.
16. A prestressed floor structure as recited in claim 14 wherein
the blocking elements are arranged into two parallel rows that straddle
17. A prestress floor structure as recited in claim 14 wherein
the spring force of the coil spring is selected to create a force
on the joist members equal to a predetermined amount that is sufficient
to maintain the relative spacing of the joist members while they
are subjected to a vertical load.
18. A prestressed floor structure as recited in claim 14 wherein:
the cable passes through the midpoint of the joists;
the blocking elements are arranged into parallel rows that straddle
the cable; and
the spring force is transmitted through the blocking elements by
a compressive force from the cable and spring.
19. A prestressed floor structure as recited in claim 14 wherein
the joists are of a laminate construction.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to building structures and, more
particularly, to load carrying wood frames for supporting floors,
ceilings, and the like.
2. Description of the Related Art
Wood structures predominate in residential and light commercial
construction. In the case of wood floor framing, the structure typically
comprises a series of parallel, spaced-apart wooden spanning members
or joists. These joists can be provided in a number of sizes, such
as those known in the United States as two-by-twelves, which have
standard dimensions of one and one-half inches thick by eleven and
one-fourth inches high, and are allowed by most building codes in
the United States to have a length of slightly over 21 feet. A range
of other sizes is available, including joists known as two-by-eight
up through two-by-fourteen. When the span length exceeds 16 feet,
vibration and vertical deflection upon loading can become quite
noticeable. Even floor systems that are properly constructed and
structurally safe can exhibit much movement and resulting noise,
which can be quite disconcerting and psychologically troublesome
for any occupants.
In the United States, the wood joists are typically installed at
a 16-inch spacing, and therefore a typical 20-foot by 20-foot room
will require fifteen to sixteen joists. Large 4-foot by 8-foot sheets
of plywood are attached to the upper surface of the joists using
nails driven through the sheets into the joists, thereby forming
the floor subsurface to which flooring, tile, or carpeting can be
laid. Blocking, or smaller wood segments that span the distance
between joists, is installed below the seams of adjacent plywood
sheets to provide a nailing surface.
Loading of the floor, such as when a person walks on the floor,
causes vertical deflection and resultant noise and vibration. This
occurs because, with 16-inch joist spacing, substantially all of
the load is supported by a single joist, which cannot support the
pressure sufficiently to eliminate the deflection. Conventionally,
the extra vertical stiffness required to stop the deflection is
provided by doubling the number of joists, or using an 8-inch joist
spacing. This doubles the number of joists needed and therefore
doubles the joist costs and dramatically increases the amount of
time required for construction.
Even if not present initially, the vibration and vertical deflection
upon loading can spontaneously occur, or become worse, with age.
This occurs because lumber initially has approximately a 19% moisture
content when the structure is erected, and later dries out and stabilizes
at approximately a 9% moisture content, thereby shrinking the wood.
This can change the relative dimensions of the structure, even pulling
against the nails used to erect the structure, causing open spaces
to appear and pull the members apart. Thus, when the floors are
subject to loading, as when someone walks over them, the members
are moved relative to each other, causing vertical deflection and
vibration, and often squeaking as well.
Methods other than doubling the number of joists can be used to
control the vertical deflection and squeaking of floor systems.
Constructing smaller spaces, such as by using joist spans having
a length that is less than the maximum allowed by building codes,
decreases the amount of vertical deflection. Often, however, this
conflicts with the architectural design for the completed structure.
If the original span length is to be kept, designers can choose
alternate construction materials, such as truss joists, steel beams,
and concrete slabs. Unfortunately, these materials are much more
costly than comparable wood floor systems.
From the foregoing, it should be apparent that there is a need
for a wood floor system that provides reduced vertical deflection
upon load and controls vibration and squeaking, that allows for
shrinkage of the wood floor structure members, and does so without
the high cost and complex assembly of floor systems constructed
from materials other than wood. The present invention meets these
SUMMARY OF THE INVENTION
The present invention provides a building structure comprising
a prestressed floor system in which the joists are prestressed by
a combination of pressing elements that exert a laterally directed
force pressing the joists toward each other and spacing elements
that oppose the force and keep the joists in their relative positions.
This couples the joists together such that substantially more than
one joist supports a point load, such as a person walking on the
floor. That is, all of the joists contribute to supporting the load,
and it has been found that three to five of the joists might actually
experience some minimally observable deflection as a load is applied.
The pressing elements are advantageously inexpensive and simple
to manufacture, and the spacing elements are preferably readily
available at job sites. The pressing elements and spacing elements
are easily installed, and provide a prestressed floor system with
increased vertical stiffness and reduced deflection, vibration,
and squeaking under loading, at much cheaper cost than alternative
materials or double joists. The spacing elements can advantageously
comprise blocking placed between adjacent joists, spanning the distance
from one joist to the next. The pressing elements can advantageously
comprise a cable with attachments at two outside joists, passing
through a hole in each of the remaining joists, thereby pressing
the joists toward each other and placing them under compression.
In particular, the spacing elements can comprise blocking cut from
the same material as the joists, and the pressing elements can advantageously
comprise a cable and spring arrangement, in which a spring is attached
at one end to an outside joist while the cable is attached to an
opposite outside joist and then is passed through a hole in each
of the remaining joists in a straight line, to be attached to the
free end of the spring. Once the spring and cable are attached the
spring is stretched, and therefore the cable is placed under tension.
The force of the spring is transmitted through the spacing elements
across all the joists in the structure, placing them under compression.
The spring force is selected such that, as the wood dries and shrinks,
the cable is kept taut and the spacing elements are kept in position
between the joists.
Most building codes and regulations allow holes of up to one-inch
diameter in wood joists, for plumbing and electrical connections.
The holes necessary for the cable can be of much less than one-inch
diameter, and therefore the integrity of the joist is not compromised.
The joists experience zero stress and zero shear forces along the
neutral axis at the joist vertical and longitudinal midpoint. Therefore,
the holes in the joists through which the tension cable is passed
are advantageously located at each joist midpoint. Placing the hole
for the cable at the midpoint has no effect on the strength of the
joist, and is preferred.
The spacing elements can be advantageously arranged in two parallel
rows, straddling the cable. This provides the necessary increased
stiffness and lateral rigidity without localized bending of the
joists when the cable places them under compression. The spacing
between the rows of blocking is dictated by the spring force used
and the working area between the joists. The spring transmits its
force to the joists and blocking elements through a pressure plate,
to which the spring is attached. The size of the plate determines
the spacing between the blocking elements, and is determined by
calculating the crushing capacity of the blocking elements. In particular,
the crushing capacity of the wood must be greater than the actual
stress applied. The crushing capacity of wood that is typically
used in wood frame building structures is 650 lbs. per square inch.
It should be appreciated that, while the spring force is necessary
for placing the floor structure in a prestress condition, it is
not necessary for support of the structure. That is, if either the
spring or cable were to break, the remaining wood floor structure
would still stand. This is in contrast to many concrete structures,
for example, in which prestress is necessary for the structure to
Materials other than standard wood lumber can be used to create
a building structure in accordance with the present invention. For
example, the joists can be manufactured having a plywood laminate
construction, such as the joists made by Truss Joist, Inc., typically
known as "TJI Joists." Such joists comprise elongated
laminate members separated by a relatively thin plywood web, thereby
creating a cross-sectional shape similar to that of an I-beam. Alternatively,
the web can be of an open web design, in which the laminate members
are separated by diagonal cross-members, or girders. Because TJI
joists are of a laminated construction, they are much stiffer, and
can be made much larger, than conventional wood joists. Thus, TJI
joists can be made longer than standard wood joists, and the structures
that can be erected using the present invention in this way can
be quite large.
The present invention provides a prestress wood floor system that
is easily constructed at nominal cost when compared to conventional
wood floor framing. Spring forces and cable lengths can be standardized
for room sizes, and cables can be cut to length at a job site. The
resulting floor structure has increased vertical rigidity and reduces
squeaking. In addition, the structure has the added benefit of increased
lateral stiffness. The floor is maintained under a prestress condition
even as the wood dries and shrinks, because the spring force takes
up any slackness in the cable.
Other features and advantages of the present invention should be
apparent from the following description of the preferred embodiment,
which illustrates by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a floor system constructed in accordance
with the present invention.
FIG. 2 is a detail plan view of the floor system illustrated in
FIG. 1 showing the attachment of the cable to a joist.
FIG. 3 is a detail plan view of the floor system illustrated in
FIG. 1 showing the attachment of the spring and cable to a joist.
FIG. 4 is a cross-sectional view of the cable attachment through
a wood joist.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The following description of the present invention is not to be
taken in a limiting sense, but is made merely for the purpose of
illustrating the general principles of the invention. The following
detailed description is of the best presently contemplated modes
of carrying out the present invention.
A prestressed wood floor system 10 is illustrated in FIG. 1 and
comprises a plurality of elongated, parallel joists 12 spaced apart
laterally from each other. The joists are illustrated in plan view
and can be any one of several materials, from standard wood joists
in a variety of sizes, including what are known as two-by-twelves
(actually 11/2 inches by 111/4 inches), to joists having a composite,
laminate construction comprising elongated spans joined by a web,
which can be laminate or open web. In FIG. 1 the free ends of the
joists are attached to cross-wise spans or blocking elements 14
often simply referred to as blocking. These elements create a floor
frame to which a plurality of plywood sheets are attached, thereby
forming a floor surface 16. When a load is placed on the floor,
such as when a person walks across it, a certain amount of vertical
deflection will typically occur as the joist beneath the person
attempts to support the load. In accordance with the present invention,
the floor system is placed under a prestress condition to distribute
the load across three to five joists or more and to thereby minimize
The prestress is achieved with a cable 18 that is passed through
each joist at its midpoint, and is used to apply a compression force
transmitted through all the joists. The cable 18 is approximately
1/4 inch diameter. Holes of up to one-inch diameter are allowed
by most building codes and regulations, for plumbing and electrical
connections, and will not compromise the strength of a joist. Even
so, the holes necessary for the 1/4 inch cable can be of much less
than one-inch diameter, for example 3/8 inch, and therefore the
joist need not be weakened. Furthermore, the joists experience zero
stress and zero shear forces along the neutral axis at the joist
vertical and longitudinal midpoint. Therefore, the holes in the
joists through which the cable is passed are advantageously located
at each joist midpoint. Placing the hole for the cable at the midpoint
has no effect on the strength of the joist.
The cable 18 is anchored at one end to a first one of the outside
joists 12a by a stud 20 and is attached at the opposite end to a
spring 22 which is attached to a second outside joist 12b by a
stud 24. The stud can be adjusted to stretch the spring, placing
the cable under varying amounts of tension. A blocking element 26
is located so as to bridge the space between each pair of adjacent
joists and couple the force from the cable through the joists. The
blocking elements are arranged into two parallel rows that straddle
the cable 18. The blocking elements 26 oppose the force of the spring
and cable, and therefore are placed under compression.
The combination of the blocking 26 the cable 18 and spring 22
place the joists 12 in a state of equilibrium in which the joists
will substantially maintain the relative spacing between each other,
and will minimize the amount of vertical deflection they experience
while being subjected to a load. The combination has been found
to reduce vertical deflection for a given load by as much as 50%
when compared with conventional wood floor systems. The additional
cost for a typical 20-foot by 20-foot room is expected to be almost
one-tenth the additional cost for doubling the number of joists,
which would be the conventional way of obtaining similar results.
As an added benefit, the structure in accordance with the present
invention has increased lateral stiffness.
The attachment of the cable 18 to the first outside joist 12a is
shown in greater detail in FIG. 2. The end of the cable is attached
to a stud 20 comprising a stud terminal body 28 a locking cone
30 and a threaded stud bolt 32. During assembly of the structure
10 at the job site, the cable 18 is inserted into the tapered end
of the terminal body and into engagement with the cone 30. The stud
bolt 32 extends from the tapered end of the cone and is inserted
through a hole 34 in the joist. A steel plate 36 is placed against
the outside surface of the joist and the stud bolt is passed through
a hole 37 in the plate. A fastening nut 38 is threaded onto the
bolt and fixes the stud bolt in position, gripping the cable more
tightly as the nut is turned and tightened toward the cone 30 thereby
placing the cable 18 under tension as described further below.
The stud 20 is readily available and, for example, comprises an
"EASY-RIG" stud terminal, Model Number ER7-8 obtainable
from MacWhyte Marine Products. The cable also is readily available,
and preferably comprises a 1/4 inch outside diameter stranded steel
cable. The steel plate 36 is preferably 3/8 inches thick and is
6-inches by 6-inches square. The size of the plate and the spacing
between the blocking elements 26 is determined by the crushing capacity
of the wood, as described further below.
The attachment of the cable 18 to the spring 22 and of the spring
to the second outside joist 12b, is illustrated in greater detail
in FIG. 3. Prior to delivery at a job site, the cable is swaged
into a large ring eye 40 having an eyelet 40a and a shank 40b that
is passed through the midpoint of the joist 12c that is adjacent
the second outermost joist 12b. By swaging the cable the ring eye
40 prior to delivery and use of the cable, there is no need for
field swaging at the job site. This simplifies the construction
process. A first hooked end 42 of the spring 22 is passed through
the eyelet of the ring eye 40. The opposite end of the spring has
a second hook 44 which is passed through the eyelet 46 of the stud
bolt 24. The threaded shank 47 of the stud bolt is passed through
the outermost joist 12b and a steel plate 48 and is secured by
a nut 50. Tightening the nut draws the eyelet 46 of the stud bolt
toward the nut. Because the cable 18 is securely tightened at its
opposite end to the first outermost joist 12a by the stud 20 tightening
the nut 50 stretches the spring 22 and pulls against the cable,
placing the cable under tension. The tension on the cable can be
calibrated to a fixed amount of spring stretching, such as by measuring
the eyelet-to-eyelet distance or the coil-to-coil distance. In this
way, the cable tension can be adjusted to a predetermined amount.
As noted, the spacing between the blocking elements 26 and the
size of the steel plates 36 and 48 are determined by the crushing
capacity of the lumber used to construct the structure. In practice,
the lumber typically used in wood frame construction has a crushing
capacity of 650 pounds per square inch. For the loads that will
be endured by the floor structure 10 it should typically be designed
to accommodate a 2500 lb. load. The crushing capacity must be greater
than the actual stress applied to the wood, and therefore the spring
force in pounds divided by the blocking element area covered by
the steel plates must be less than 650 pounds per square inch. For
example, for a crushing capacity of 650 lbs. per square inch, and
a 2500 lb. load, a steel plate acting on the blocking elements 26
over an area of six square inches would provide 3900 lbs. of stress
that can be accommodated. This is greater than the actual load,
and comfortably exceeds the maximum margin of safety typically required
by building codes in the United States.
The steel plates 36 and 48 can conveniently provide six square
inches of coverage for the blocking 26 by covering a one-inch by
six-inch area along the joist. FIG. 4 most clearly shows the relative
spacing between the blocking and the coverage of the blocking through
a joist by one of the steel plates 48. The area of the steel plate
that overlaps the blocking elements 26 represented by dotted lines
in cross-section, is an area one-inch wide and six-inches tall indicated
by the cross-hatching. Because the square plate is six-inches by
six-inches, and the plate covers one inch of each blocking element,
a four inch face-to-face spacing between the blocking elements is
indicated. The steel plate size and area of overlap were experimentally
determined to be sufficient to achieve the desired results with
a safety margin.
When a wood floor structure is first erected, the lumber has a
moisture content of approximately 19%. As the lumber ages, it gradually
decreases in moisture to stabilize at a moisture content of approximately
9%. Thus, a joist having a length of 20 feet will shrink approximately
5/8 inches. It is this shrinkage that is partially responsible for
the vertical deflection, vibration, and noise under load that is
often exhibited by wooden structures. The shrinkage changes the
relative position of the joists and subfloor sheets, pulling against
the nails that hold the elements together and creating gaps that
can close when subjected to a load. This closing causes vertical
deflection, vibration, and squeaking.
The spring 22 effectively controls the effects of shrinkage, and
keeps the cable 18 taut under tension because the spring force is
selected to compensate for the shrinkage. As the wood shrinks, the
spring 22 will be stretched less and therefore the spring force
will decrease. For example, where a spring force of 2500 lbs. is
desired, the spring should be stretched initially so as to provide
a spring force of approximately 3500 lbs. After the wood has reached
its stable moisture content, the wood will have shrunk and the spring
will have been stretched less to exhibit the desired spring force
of 2500 lbs. Where laminate construction joists are used, the spring
forces will likely be higher. The higher spring force is necessary
because laminate joists are typically of longer spans than wood
joists, and therefore must support a greater weight. The potential
for vertical deflection upon load will be greater, and therefore
a greater compressive force acting on the joists is necessary to
The present invention provides a wood frame structure that has
been found to reduce vertical deflection by half when compared with
conventional wood frame floor structures. To achieve an equal stiffness
with conventional flooring, it would be necessary to double the
number of joists, significantly increasing the cost of a typical
20-foot by 20-foot room. In accordance with the present invention,
a compressive force is applied to the joists, and the distribution
of load is increased from one joist to three to five joists or more.
That is, all of the joists assist in supporting the load through
the cable and blocking. The cable used to apply the compressive
force can be provided in standard lengths, with the cable being
cut at a job site to suit the particular dimensions of a given structure.
Resistance to the compressive force can be provided by blocking
elements that are created from the same materials used for the joists,
and that therefore are readily available at the job site. The cable
tension can be provided by a spring, which allows the structure
to adjust itself so that the compression force applied will reach
the desired level after the wood has reached its final moisture
The present invention has been described above in terms of presently
preferred embodiments so that an understanding of the present invention
can be conveyed. There are, however, many configurations for wood
floor systems not specifically described herein, but with which
the present invention is applicable. For example, additional mechanisms
for applying cable tension might occur to those skilled in the art.
The present invention should therefore not be seen as limited to
the particular embodiments described herein, but rather it should
be understood that the present invention has wide applicability
with respect to wood floor systems and frame structures. Such other
configurations may be achieved by those skilled in the art in view
of the description herein.