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.

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 rigidity and strength. For example, proper fastening of intersecting walls usually reduces cracking of plaster at the inside comers.

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 generally 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 basement 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 publication,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.

Stairways and other openings that penetrate the floor structure should be located so as to interrupt as few members 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.

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 intervals.Floor joists are toenailed to the sill plate.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 conventional bolts often do.

Sill plates may be entirely eliminated where the top of a foundation of poured concrete (fig. 27B) or concrete block 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.

As noted previously, a foundation of pressure-treatedwood 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.

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

bearing plate at each end. Secure anchoring to the beam is important.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.

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.

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 offset the center support 1 foot from the centerline to provide 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. However,as discussed later, this is not necessary if off-center spliced joists are used.

For equal widths, the built-up beam is stronger than the solid beam.

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.

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 necessary 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 sideof wood beams framing into masonry (fig. 31). The topof the beam should be level with the top of the sill plates on the foundation walls.

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 generally 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 surfaceby 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.

Floor joists are selected primarily to meet strength and stiffness requirements. Strength requirements depend on the load to be carried. Stiffness requirements place an arbitrary control on deflection under load. Stiffness is also important in limiting vibrations from moving loads-often a cause of annoyance to occupants.Wood floor joists have generally nominal thickness of 2 inches and nominal depth of 8, 10, or 12 inches. The size required depends upon the loading, length of span, spacing between joists, and species and grade of lumber used. After the sill plates have been anchored to the foundation walls and the center beam installed, the joists are laid out according to the house design. The center-to-center spacings most commonly used are 24 inches or 16 inches.Span tables for floor joists, provided by the NationalForest Products Association or in local building codes,can be used as guidelines. Table 7 is a simplified version for joists spaced 24 inches on center and table 8 for 16 inches on center. The sizes shown in the table are minimal;it is sometimes desirable to use the next larger lumber size than that listed in the table.

Floor framing includes setting columns,beams,sills,headers,joist and applying the subfloor.These form a strong,solid base on which the interior and exterior walls rest,and carry other loads such dead loads,live loads,etc...The floor also forms a diapham to resist the lateral pressure from the earth and subsurface water that is pressing on the fondation wall,tending to push it inward.

FLOOR BEAMS; Usually the span from the foundation wall to the other is so long beams are installed to shorten the distance.Steel beams are often used,as well as solid and built-up wood beams.

Parallel Strand Lumber; Parallel strand lumber is used for beams,headers,columns,and post.It can be sawed,drilled,and nailed like solid wood.It has been accepted by building codes in the united states.

FACT:

Convection

Walls are the largest exterior surface areas of most homes and are also exposed to wind load and large amounts of air infiltration. Uninsulated or poorly insulated walls are ideal for convection heat transfer because of internal wall cavities. These cavities, which are more prevalent than most people think, are caused by little or no insulation or poorly-installed insulation. Exterior walls with an air cavity between their cold and warm side allow air currents inside the walls to carry warmth from one side of the wall to the other.

At the exterior walls of homes with wall insulation, the interior drywall surface is cooler at the studs because heat is lost through the wood to the exterior faster than through the insulation; heat loss to the exterior is even faster with metal studs because metal conducts heat faster than wood. The outside corners of house rooms are cooler because there is only wood framing at the corners, and no insulation. Similarly, in ceilings, drywall close to the building perimeter may be colder than interior surfaces. Where there are lower wall temperatures, in addition to increased relative humidity, there are also increased amounts of air movements due to convection. Cold air sinks and warm air rises. Where the exterior wall is colder, air is cooled and sinks. Air is full of particles, and in areas where there are increased air flows due to wall convection or heated air flows, there are increased rates of collisions of particles with walls.
If you can feel drafts around your socket and switch plates, then there is either no insulation, or your walls are so poorly insulated that the heat and cold air blow right around the insulation

Poorly insulated walls  the airflow is what freezes pipes and increase your home heating. In the summer Walls exposed to sunlight can get extremely hot. In an uninsulated or poorly insulated wall, heat can easily be transferred from the hot exterior wall to the cooler interior wall by convection, conduction, and radiation. Another form of air circulation consists of interior convective airflow. Because of a temperature difference between the room and the outer portions of the wall, air flow could be initiated if openings are in place, even with a perfect air barrier system located on the outside.
It could transport indoor moisture to a location below the dew point, possibly to an external air barrier element. The air pressures in this case are much smaller than what an air barrier system has to sustain, since the force is due to temperature difference alone. An element of low air permeance installed in a continuous manner on the interior side of the wall can minimize this phenomenon; it can be the air barrier system installed on the warm side, or a vapour barrier membrane. Installing the stud cavity insulation without leaving pockets of air is beneficial to minimize this phenomenon." 

 Some sources have suggested ventilating wall cavities to let moisture escape, especially where low permeability sheathing is used. However, studies in instrumented buildings have shown that ventilating actually increases the potential for condensation problems. Vents provided only at the top tend to draw more humid indoor air into the wall cavity, and thus provide more moisture to condense. Vents provided at both top and bottom allow cold air to pass through the wall, which may cool the sheathing surface below dew point temperature. While retrofit vents have sometimes alleviated paint peeling by letting moisture escape when the weather warms up, they also increase condensation potential. Even without vents, moisture is not trapped in the wall since it will eventually escape around and through the top plate.As a result of steadily rising energy costs, construction practice for light-frame wood structures has changed over the past few years. The use of 6-inch-thick walls and application of high-“R”-value, low-permeance sheathings to 4-inch walls has caused concern for the changing moisture patterns that may occur in walls.

The exterior wall system  used on most of todays homes provides many advantages.Exterior wall built in accordance with standard construction practices are vertical framing members arranged in a plane to define a wall. Exterior sheathing material, such as plywood or OSB, is then fastened to the framing members to define and to give stability to an exterior surface of the wall. To prevent air and moisture infiltration into the wall, the wall is then generally covered with a house wrap or vapor barrier. Foam board or generally rigid insulation is then fastened to the wall adjacent the house wrap. Brick veneer walls on holes require weep hole or weep wicks to promote air circulation for wall cavity ventilation.

With the rapidly increasing cost of energy used in home heating and cooling, a multitude of ways of insulating homes have been invented Most common is the batts of fiberglass,rigid polystyrene foam inserted into the wall cavities between the studs,or blowing cellulose, rock wool, or urethane foam into the wall cavities.In order to achieve an R-20 exterior wall insulation value, which is required by HUD Minimum Property Standards in new home construction, builders have increasingly started to use 2×6 construction framing instead of 2×4's in order to stuff another 2 inches of insulation into the wall cavity, which is a waste of good lumber.Builders have started nailing insulating sheathing over the exterior side of the studs. This insulating sheathing varies in thickness from 1/4 inch to 3 inches.Sheathing over the stud creates the problem of trapped water vapor within the wall cavities Any insulating material which absorbs moisture can lose insulating value because water is an excellent conductor of energy. In order to prevent this condensation, especially in colder climates, a polyethlene vapor barrier has to be installed on the warm side of the wall and then the wall cavities must be vented to outside air with vents.

Vapor Barriers are required.

Moisture control is required in all framed walls, floors, and roof/ceilings of the building thermal envelope. IRC section R318.1 states, “…a vapor retarder shall be installed on the warm-in-winter side of the insulation.” However exceptions to the code are noted:

1) In construction where moisture or freezing will not damage the materials.
2) Where the framed cavity or space is ventilated to allow moisture to escape.
3) In counties identified with footnote in Table N1101.2

Language in section 502.1.1 of the IECC also talks about “…other approved means to avoid condensation in unventilated walls…” The intent of the code is abundantly clear: To control moisture and prevent damage to materials. Obviously, trapping moisture between the foundation wall and a plastic wall membrane is not a good building strategy. While there are several smart ways to design a basement makeover (see FHB # ??), the point here is you are not restricted by the code. You do need to demonstrate to the code official that your design approach is sensible.

Light frame construction consists of joining load bearing framing members with sheathing to create building components such as floors, walls, and roofs, and  systems such as shearwalls, load bearing walls, Load Bearing Framing Members. Framing members, in the form of studs, joists, and rafters, provide reinforcing and support for vertical and horizontal sheathing, and are the main load bearing members conducting forces to the supports.Bracing is used to stiffen framed construction and make it rigid. Bracing is also used to resist winds, storms, twists, or strains. Good bracing keeps corners square and plumb. Bracing prevents warping, sagging, and shifting that could otherwise distort the frame and cause badly fitting doors and windows. Typically, Stud grade lumber is used for wall studs No.3 grade for roof and floor members.Lumber, whether kiln dried or air dried, should have a moisture content not exceeding 19 percent at the time of installation to minimize dimensional change as the building adjusts to its equilibrium humidity level. One way to accomplish this is to specify kiln dried lumber which carries a slight cost premium of about 10 to 15 percent, or to ensure that after manufacture, S-Green lumber has had time to season to the moisture content equivalent of S-Dry lumber. In ether case, lumber should be protected from wetting during construction.

There are high roof lines in todays roof framing steep pictch roofs rafters concentrate most of the load bearing at the bottom end of rafter at topplate,and partition walls,and load bearing walls. Framing with dimensional lumber (not trusses) is known as "Stick-freaming.".Most modern homes are engineered roof framing,or trusses.A typical tract house in most areas of the country would include "Stick-framing" in conjunction with trusses due to dormers,multi-gables,and other architectural interest features.The lower the roof slope,the more the framing members act like beams.The loads at both bearing ends become more equal.

Platform Framing;
Most all 2 story homes no are framed using platform framing.The exterior wall frame serves as the base on which sheathing and siding is applied and carries most of the roof load.It also carries the load of the second floor.Some of the interior partitions may support the ceiling framing while most serve as room dividers and as a base to carry the interior wall finish material.
There are several types of main structural members used in light framing construction to provide Residential - Envelope:
Dimension Lumber
Light frame trusses
Laminated veneer lumber (LVL)
Prefabricated wood I-joists
Parallel strand lumber (PSL)
Glulam
For residential applications,  building codes provide span tables which allow the selection of floor and roof members having suitable strength and deflection characteristics.

Lumber is classified into five major areas related to its intended use.These are structural,industrial-and shop,yard,select,and common lumber.

Structural Lumber-is two inches or more in thickness and width.It is machine stress rated for strength.

Industrial-and-shop lumber- is used for the manufacture of various wood products such as doors and windows.

Yard Lumber- includes grades  and sizes intended for construction and building purposes.It is classified as selected lumber.

Selected Lumber-has good apperance and finishing qualities,and are availiable in B&B,C,C&B,and D.

Common Lumber- is used for general construction and ultility purposes.It comes in grades No. 1,2,3,and 4.

Dimension Lumber is used for framing members such as joist,studs,rafters planks, and small timber.These are classified into four categories:structural framing,light framing,studs,and structural joist and planks.Structural light framing grades havegoodappaerance,but they have lower design value.Stud grade lumber has a high design value and therefore apporved for stud such as load bearing walls.

Plywood and oriented strand board(osb) is a nonveneerAPAperformance-rated panel made of compressed wood chips strand plies.These are used along with the structural wood framing to enclose and strengthen on walls and roofs.These structural panels are design to carry normal floor and roof loads over specfic spans.APA rated sheathing single layer is rated for flooring,roof sheathing,walls and exterior siding.These panels are rated to their ability to exposure to weather and moisture.Exposure1 panels are widely used in today constructed homes.Most performance-rated panels are exposure1.These performance rated panels have a identifying grade stamp.Span rating are found on these stamps.