EDT 300 - Sill and Floor Construction1 Weekend Cabin Retreat Project Sill and Floor Construction...

EDT 300 - Sill and Floor Construction 1

Weekend Cabin Retreat ProjectWeekend Cabin Retreat ProjectSill and Floor ConstructionSill and Floor Construction

Sacramento City CollegeEDT 300

Kenneth Fitzpatrick, P.E.

2 EDT 300 - Sill and Floor Construction

ObjectivesObjectives Recognize platform and balloon

framing. Plan the appropriate floor support

using joists or trusses for a structure.

Determine proper joist sizes using a typical span data chart.

Describe the components of a floor system.


ObjectivesObjectives

Demonstrate an understanding of the principles involved in post and beam construction.


Types of FramingTypes of Framing

Methods of floor framing vary from one section of the country to another.

Builders in a given area may use different methods, based on personal preference and experience.

The basic types of floor framing are platform and balloon framing.


Platform FramingPlatform Framing

Platform framing is popular for several reasons. It can be used for both one- and two-story

structures and is easy and fast to construct.

Shrinkage is uniform throughout the structure. A firestop is automatically provided.

Construction is safe because the work is performed on solid surfaces.



In platform framing, the sill is the starting point in constructing a floor.

A sill is the lowest member of the frame of a structure, Rests on the foundation supports the floor joists or the uprights

(studs) of the wall. The sill in most residential

construction is a 2 x 6. (actual dimensions 1 1/2" x 5 1/2").



Platform framing utilizes a method -of sill construction known as box sill construction.

The box sill consists of a 2 x 6 plate (also called a sill or mudsill) and a header that is the same size as the floor joists.


Balloon FramingBalloon Framing

Balloon framing was once used extensively, but in recent years has diminished in importance.

Its distinguishing feature is that the wall studs rest directly on the sill plate.



Two types of sill construction are used: The solid or standard sill. The T-sill.

The studs are nailed directly to the sill and joists in solid sill construction.

No header is used. Joists are supported by a ribbon

and nailed to the studs on the second floor level.



A firestop must be provided between the studs.



In T-sill construction, a header is used, which also serves as a firestop.

The studs Rest on the wall plate. Are nailed to the header as well as the sill

plate. The sill may be very wide, to provide

a good base for the joists to rest on. Solid sill is used more extensively in

two story homes.


Balloon Framing AdvantagesBalloon Framing Advantages

Small potential shrinkage. Vertical Stability. Can be used for two-story homes

with brick veneer or stucco exterior wall finishes.


Balloon Framing DisadvantagesBalloon Framing Disadvantages

A “less than desirable” surface to work on during construction.

Firestop blocks must be used.


Joist and BeamsJoist and Beams

Joists provide support for the floor. Usually made from a common soft

wood such as southern yellow pine fir larch hemlock or spruce.

The size of floor joists ranges from a nominal size of 2 x 6 to 2 x 12.

Spacing from 12" to 24".



The size joist required for a given situation will depend on the length of space load to be supported specie and grade of wood and distance the joists are spaced apart.

Spacing of floor joists may be 12, 16, or 24" o.c. (on center).

A spacing of 16" o.c. is most common.



Span data for floor joists is presented in tables.

The span data presented assumes a maximum deflection of 1/360th of the span with a normal live load.

This is the amount which most codes require.

The normal live load is 40 pounds per square foot.


Floor SystemsFloor Systems

A floor system may also be constructed using girders or trusses usually 4 x 6, 4 x 8, or 4 x 10 depending on

the span) in the place of floor joists. The purpose of this approach is to use

fewer support members (joists). The typical spacing of girders or

trusses in this system is 48" o.c. with 1 1/8" thick tongue-and-groove plywood as the floor decking.



The distance which joists must span is usually so great that a beam or load-bearing wall is needed to reduce the span.

The beam may be a solid timber a built-up beam from dimension lumber a metal S-beam.

Load-bearing walls may be concrete block, cast concrete, or frame construction.


Floor Systems Floor Systems



Partition walls that are supported by the floor joists require added support.

It is good practice to double the joists under parallel partition walls.

If space between the joists is used as a cold air duct, solid blocking is used between the joists.



Openings in the floor for stairs and chimneys require double joist framing.



Cross bridging is commonly used to stiffen the floor and spread the load over a broader area.

Bridging boards are ordinarily 1 " x 3" in size with the ends cut at an angle so they fit snugly against the joist.

They are nailed securely in place midway between the beam and wall.

Metal bridging is also available.


Floor TrussesFloor Trusses

Engineered wood floor trusses for light-frame construction are widely used for residential structures. minimum of depth lightweight assembly easy to handle.

The open web construction reduces transmission of sound through floor/ceiling assemblies.

Makes installation of plumbing, heating, and electrical systems easy.



Computers are used to design modern engineered floor trusses to assure load capabilities for a given design.

Each truss has a built-in camber so that the floor/ceiling will be level once a load is applied.

Stress-graded lumber is used in their construction so that a minimum amount of material is required.


Floor TrussesFloor Trusses Some trusses are fabricated with

lumber chords and patented galvanized steel webs, instead of the typical wood webs.

The webs have metal teeth which are pressed into the sides of the chords.

A reinforcing rib withstands both tension and compression forces.

Engineered wood floor trusses are usually fabricated from 2 x 4 or 2 x 6 lumber and generally spaced 24" o.c.


SubfloorSubfloor

Plywood, tongue-and-groove boards, common boards, and other panel products are used for subfloors.

Plywood is advantageous. large size of plywood short time required to nail the sheets in

place


Other Subfloor MaterialsOther Subfloor Materials One-half inch thick plywood Composite board Waferboard Oriented strand broad Structural particleboard,

These be used when joists are spaced 16" o.c.

Some builders prefer 5/8 in. stock.


Other Subfloor MaterialsOther Subfloor Materials

When these products are used, the joist spacing must be is very accurate.

All edges of the panels must be supported.


Other Subfloor MaterialsOther Subfloor Materials


Subfloor MaterialsSubfloor Materials

Some localities combine the subfloor and underlayment (usually 5/8"particleboard) into a single thickness that is generally 1 1/8" thick.

The sheets have tongue-and-groove edges and require no blocking between the joists.

A single thickness sheet of 3/4" tongue-and-groove plywood may also be used for some applications.



Plywood should be installed with the grain direction of the outer plies at right angles to the joists. It is stronger when positioned in this

manner. Panel products should also be

staggered so that end joints in adjacent panels break at different joists.

A slight space must be allowed between sheets for expansion.



Subfloor panels may also be glued and nailed to the joists. Structural tests have shown that stiffness is

increased by 25 percent with 2 x 6 joists and 5/8" plywood.

Gluing advantage squeak-free structure eliminates nail-popping reduces labor costs.


Cantilevered JoistsCantilevered Joists

Home designs where a section of the floor projects beyond a lower level are called cantilevered joists.

These joists are parallel to the overhanging area



Rule of thumb for necessary length of cantilevered joists:

Extend the joists inside at least twice the distance they overhang outside. If the inside distance is too short the floor

may sag along the outside wall. If a ledger strip is used, locate it

along the top of the inside double header joist since the force will be up rather than down.


Framing Under Slate or TileFraming Under Slate or Tile

Areas which have ceramic tile, slate, or stone floors require a substantial base.

If a concrete base is provided, the floor framing must be lowered to provide for the concrete.



Several techniques are used: A smaller size joist may be used and the

space between joists reduced to provide adequate support.

This is a common solution to the problem. Another technique is to use one or more

beams under the section to support the added weight.

The dead weight may be as much as 40 or 50 pounds per square foot in a bathroom with a tile floor and heavy fixtures.



The concrete base for the tile or stone should be reinforced with wire mesh cast on a plywood subfloor covered with

building paper. A special type of concrete is

generally used. a mixture of 1 part portland cement and 6 parts sand, known as a cement mortar mix.


Post and Beam ConstructionPost and Beam Construction

Large framing members (posts, beams, planks) are spaced farther apart than conventional framing members



Post and beam construction provides a greater freedom of design than conventional framing techniques.

The system is basically simple, but presents problems related to larger structural sizes framing connectors methods of joinery.



Most of the weight is carried by the posts.

The walls are called curtain walls. Curtain walls provide for wide

expanses of glass without the need for headers.



Wide overhangs are also possible by extending the large beams to the desired length.

Spacing of the posts is determined by the design of the building and the load to be supported.



The foundation for a post and beam structure may be a continuous wall a series of piers where each post is to be

located. Size of footings will be determined

by the weight to be supported soil bearing capacity local building codes.



The size of posts required will be at least 4" x 4".

If the floor is also to be supported by the posts, they should be at least 6" x 6".

Vertical height of the posts will be a factor in determining the size.

Again check local codes.



Beams may be solid, laminated, reinforced with steel, or plywood box beams.

Spacing and span of the beams will be determined by the size and kind of materials and load to be supported.

In most normal situations, a span of 7'-O" may be used when 2" thick tongue-and-groove subfloor or roof decking is applied to the beams.



Thicker beams must be used if a span greater than 7'-O" is required.

Two systems of beam placement are possible with post and beam construction.



First system: the longitudinal method. Beams are placed at right angles to the

roof slope. Roof decking is laid, therefore from the

ridge pole to the eaves line.



Second system: the transverse method. The beams follow the roof slope and

decking runs parallel to the roof ridge. A post and beam structure has a

limited number of joints. Fastening small members by

nailing does not provide a satisfactory connection.



Metal plates or connectors are used. These are fastened with lag screws or

bolts. Decking planks for the roof and

floor range in thickness from 2 to 4 inches.

The planks are usually tongue-and-grooved along the edges and they may be tongue-and-grooved on the ends as well.



It is customary to leave the underside of the planked roof exposed.

If added insulation is required, it may be placed above the decking and under the roofing material.

Rigid type insulation should be used.

EDT 300 - Sill and Floor Construction1 Weekend Cabin Retreat Project Sill and Floor Construction...

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