stawianie domów na zacios 3.pdf

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Chapter 3
FRAMING AND CLOSING IN
Page
Floor framing ....................................
38
bracing ( 71 ), Installation of
Air infiltration barrier materials (
),
Posts and girders (
39
),
72 ).
Built-up wood beams (
), Steel
41
Ceiling and roof framing ...........................
I-beams (
), Beam-joist installation (
12
41
41
),
),
Roof designs ( 72 ), Manufactured
wood roof trusses (
),
Framing details for plumbing, heating,
and other utilities (
73 ), Ceiling joists
and rafters (
78 ).
Roof sheathing ..................................
49
), Bathtub
83
framing ( 49
), Cutting floor joists (
49 ),
Plywood ( 83 ), Structural
flakeboard ( 84 ), Board
( 84 ), Plank roof
decking ( 86 ), Fiberboard roof
),
).
Stairways .......................................
50
Landings ( 54 ), Framing for stairway
opening (
),
Roof coverings ..................................
87
).
Built-up roofs ( 94 ), Other roof
coverings ( 94 ), Finish at the
Floor sheathing ...................................
61
Plywood ( 61 ), Reconstituted wood
62 ).
Skylights .....................................
94
Exterior wall framing ............................
62
Requirements ( 62 ), Platform
construction ( 62 ), Second-story
framing ( 64 ), Window and door
37
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Framing and Closing In
The sections contained in this chapter address the tasks
related to erecting the structural framing for the house
and creating an enclosure that provides some degree of
protection from the elements.
Figure 26 –Common nails.
Recommended Nailing Practices
Wood members are most commonly joined together
with nails, but on occasion metal straps, lag screws,
bolts, staples, and adhesive can be used. Proper fastening
of frame members and covering materials provides rigid-
ity and strength. For example, proper fastening of inter-
secting walls usually reduces cracking of plaster at the
inside comers.
The recommended number and size of nails, shown in
the technical note on nailing schedule, is based on good
nailing practices for the framing and sheathing of a well-
constructed wood-frame house. Sizes of common wire
nails are shown in figure 26.
Houses that are located in hurricane areas should be
provided with supplemental fasteners called hurricane
straps or tiedowns to anchor the floor, walls, and roof to
the foundation. Wind, snow, and seismic loads are one of
the special topics discussed in chapter 8.
because wood beams may shrink and foundation walls
will not. In beams and joists used in floor framing, mois-
ture content should not exceed 19 percent; about 15 per-
cent is a much more desirable maximum. Dimension
material can be obtained at either of these moisture con-
tents, when specified.
Floor Framing
Floor framing consists of columns or posts, beams, sill
plates, joists, and subfloor. Assembled on a foundation,
they form a level anchored platform for the rest of the
house and a strong diaphragm to keep the lateral earth
pressure from pushing in the top of the foundation wall.
The columns or posts and beams of wood or steel that
support the joists over a basement are sometimes replaced
by frame or masonry walls when the basement area is
divided into rooms. Floors of the second story are gener-
ally supported on load-bearing walls in the first story.
Wood-frame houses may also be constructed over a crawl
space with floor framing similar to that used over a base-
ment or on a concrete slab as shown in the section on
foundations.
Grades of dimension lumber vary considerably with
wood species. For the specific uses described in this pub-
lication, material is divided into five categories. The first
category is the highest quality, the second is better than
average, the third average, and the fourth and fifth for
more economical construction. Joists and beams are
usually of a species of second category material, while
sills and posts are usually of third or fourth category.
(See technical note on lumber grades.)
Stairways and other openings that penetrate the floor
structure should be located so as to interrupt as few mem-
bers as possible. Stairways should be oriented parallel to
floor joists so that only one joist need be interrupted with
24-inch on-center joist spacing. Wherever possible, the
stair opening should be coordinated with a normal joist
location on at least one side. Stairways should never
interrupt a structural beam or bearing wall when it can
be avoided.
Factors in design
An important consideration in the design of a wood
floor system is wood shrinkage. When wood with a high
moisture content is used, subsequent shrinkage can result
in cracks, doors that stick, and other problems. This is
particularly important where wood beams are used,
The stairway design should be completed before floor
framing begins, because the stairwell opening must be
38
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Sill plates may be entirely eliminated where the top of a
foundation of poured concrete (fig. 27B) or concrete
block (fig. 28B) is sufficiently level and accurate. Joists
may bear directly on a solid concrete wall or on a top
course of solid concrete block. They may also bear
directly on cross webs of hollow core block or on cores
that have been filled with mortar. Where the sill plate is
omitted, anchorage of the floor system may be provided
by anchor strap devices, as described above. The straps
should be spaced to coincide with joist locations so that
each may be nailed directly to the side of a joist (fig. 28).
framed at the time the floor is constructed. The rough-
framed opening for a stairwell should be 1 inch wider
than the desired finished stairway width. The length of
the opening must be accommodated to tread run and stair
rise, which in turn are governed by total rise.
Other openings such as those for clothes chutes and flue
hole should also be located to avoid interrupting framing
members. Two-foot on-center spacing of joists generally
provides ample clearance for such openings.
Sill plate
As noted previously, a foundation of pressure-treated
wood does not require a sill plate or special anchor
devices. Floor joists bear directly on the top foundation
wall plate and are toenailed to provide anchorage.
A wood-frame floor system should be anchored to the
foundation to resist wind forces acting on the structure.
This is usually done with a 2- by 6-inch sill plate attached
to the foundation by ½-inch anchor bolts at 8-foot inter-
vals. Floor joists are toenailed to the sill plate (fig. 27A).
The sill plate may also be attached with anchor straps that
are embedded in the foundation in the same manner and
at the same spacing as anchor bolts. These devices do not
require holes in the sill plate; metal straps are simply bent
up around the plate and nailed. Anchor straps are less
exacting and do not interfere with other framing as con-
ventional bolts often do.
Posts and girders
Wood posts or steel columns are generally used in the
basement to support wood or steel beams. Masonry piers or
wood posts are commonly employed in crawl-space houses.
Steel pipe columns can be used to support either wood
or steel beams. They are normally supplied with a steel
Figure 27 – Anchoring floor system to poured concrete foundation wall:
39
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Figure 28 – Anchoring floor system to concrete block foundation wall:
For maximum benefit in reducing joist spans, beams
and bearing walls should be located along the centerline
of the structure. In some cases it may be desirable to off-
set the center support 1 foot from the centerline to pro-
vide for even-length joists; for example, in a 30-foot-deep
floor system, displace the centerline to 14 and 16 feet
from the two sides instead of 15 feet from both. How-
ever, as discussed later, this is not necessary if off-center
spliced joists are used.
bearing plate at each end. Secure anchoring to the beam
is important (fig. 29).
Wood posts should be solid, pressure-treated, and not
less than 6 by 6 inches in size for freestanding use in a
basement. When combined with a framed wall, they may
be 4 by 6 inches to conform to the width of the studs.
Wood posts should be squared at both ends and securely
fastened to the beam (fig. 30). The bottom of the post
should rest on and be pinned to a masonry pedestal 2 to 3
inches above the finish floor.
The center beam usually bears on the foundation at
each end and is supported along its length by columns or
piers. The spacing of columns or piers is adjusted to the
spanning capability of the beam for a particular design load.
Center beams
Wood-frame floor construction typically employs a
beam or girder to provide intermediate support for the
first floor. In two-story construction, the beam generally
supports the second floor as well via a load-bearing wall
extending along the center of the first story.
Two basic types of center beams-wood and steel-are
commonly used. The decision on which to use should be
based on a comparison of the total installed cost of each,
including intermediate support columns or piers, and foot-
ings. Other considerations include delivery, scheduling,
and ease of construction.
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Figure 29 – Steel post support for wood or steel beam:
For equal widths, the built-up beam is stronger than the
solid beam.
Built-up wood beams
Built-up beams are constructed by nailing three or four
layers of dimension lumber together. The built-up beam
may be made longer than any of the individual members
by butting the ends of the members together. These butt
joints must be staggered between adjacent layers so that
they are separated by 16 inches. In addition, the built-up
beam must be supported by a column or pier positioned
within 12 inches of the butt joints (fig. 31).
Typical allowable spans for built-up wood beams are
shown in table 5. Dry lumber should always be used to
avoid settlement problems caused by shrinkage of the
built-up beam and the joists it supports. It is not neces-
sary to use a wood plate over wood beams, because floor
joists can be nailed directly to the beam.
Ends of wood beams should bear at least 4 inches on
the masonry walls or pilasters. When wood is untreated, a
½-inch air space should be provided at each end and side
of wood beams framing into masonry (fig. 31). The top
of the beam should be level with the top of the sill plates
on the foundation walls.
Figure 30 – wood post support for wood beam:
Steel I-beams
Steel I-beams are often used because they have greater
strength and stiffness than wood beams, which enables
them to carry a given load over a given span with a beam
of lesser depth and thus provides greater headroom or
reduces the requirement for additional supporting posts.
Allowable spans for steel I-beams are shown in table 6.
However, steel beams require an additional supplier,
which can complicate delivery schedules. They are also
heavier and more difficult to handle in the field. The total
cost of a steel beam, including columns or piers, is gener-
ally greater than that of a wood beam.
Where steel beams are used, a wood plate 2 by 4 or 2
by 6 inches across is usually attached to the top surface
by bolting or by driving nails part way into the sides of
the plate and bending the protruding nail shanks over the
edges of the beam flange. Floor joists are then toenailed
to the beam plate to anchor to the floor and to provide
lateral bracing for the beam. A beam plate is not required
if the floor joists are secured by other means.
Beam-joist installation
In the simplest method of floor framing, the joists bear
directly on top of the wood or steel beam. The top of the
beam coincides with the top of the foundation or anchored
sill, if the latter is used (fig. 31). This method assumes
Wood center beams are of two types, solid or built-up.
The built-up beam is preferable because it can be made
up from dimension material that is drier and more stable.
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