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After the foundation is in place, you are ready to start constructing the framework for the floor system.

1-1. Types of Sills

Sills are the horizontal timbers of a building which either rest up the masonry foundations or, in the absence of such, form the foundations. The sill is the foundation that supports all of the building above it. It is the first part of the building to be set in place and rest directly on the foundation, posts, or the ground. Sills are joined at the corners and spliced when necessary. The type of sill used depends on the type of construction used in the frame.

Box Sills. Figure 1-1 shows box sills. Box sills are often used with the common style of platform framing (either with or without a sill plate). With this type of ill, the part that lies on the foundation wall or ground is called the sill plate. The sill is laid edgewise on the outside edge of the sill plate.

Figure 1-1. Box sills

 T-Sills. There are two types of T-sill construction sills commonly used in dry, warm climates (see Figure 1-2 ) and sills used in colder climates (see Figure 1-3 ). Although these T-sill constructions are similar, notice that in Figure 1-2 the joists are nailed to the studs and sole plates. In Figure 1-3 the joists are nailed to the studs and sills and headers are used between the floor joists.

Figure 1-2. Dry-climate T-sill


Figure 1-3. Cold-climate T-sill

Built-Up Sills. Joints are stagger where built-up sills are used. Notice in Figure 1-4 how the built-up sill corner joints are made. Heavier sills are used if posts are used in the foundation. Sills are single heavy timbers or built-up of two or more pieces of timber (see Figure 1-5 ). Where heavy timbers are used, the joints should be placed over the post (see Figure 1-6 ).

Figure 1-4. Built up sills


Figure 1-5. Braced framing sill


Figure 1-6. Heavy timber sill

1-2. Types of Girders

A girder is a large horizontal member used to support joists or beams. A girder is made of several beams nailed together with 16d (sixteen penny) common nails, solid wood, steel, reinforced concrete, or a combination of these materials. Girders carry a very large proportion of the weight of a building. They must be well-designed, rigid, and properly supported at the foundation walls and on the columns. Girders must be installed so that they support the joists properly. The ends of the wood girders should be at least 4 inches on the posts.

Built-up Girder. The built-up girder is commonly used in house construction. It is generally made of three boards nailed together with 16d common nails. Figure 1-7 shows a built-up girder, walls, joists, and columns.

Figure 1-7. Built-up girder

A shows two outside masonry walls.

B shows the built-up girder.

C shows the floor joists.

D shows the support columns that support the girder.

 Girder with Ledger Board. Use a girder with a ledger board when vertical space is limited and where more headroom is needed (see Figure 1-8 ).

Figure 1-8. Girder with ledger board

Joist Hangers. A girder with joist hangers is used where there is little headroom or where the joists must carry an extremely heavy load (see Figure 1-9 ).

Figure 1-9. Joist hangers

1-3. Girder Size Requirement

A girder should be large enough to support the load. The carpenter should understand the effect of length, width, and depth of the wood girder. The principles which govern the size of a girder are:

  1. The distance between girder posts.
  2. The girder load area.
  3. The total floor load per square foot on the girder.
  4. The load per linear foot on the girder.
  5. The total load on the girder.
  6. The material to be used.

1-4. Depth

When the depth of a girder is doubled, the safe load is increased four times. For example, a girder that is 3 inches wide and 12 inches deep will carry four times as much weight as a girder 3 inches wide and 6 inches deep. To obtain greater carrying capacity, it is better to increase the depth than to increase the width of the girder. The sizes of built-up wood girders for various loads and spans may be determined by using Table 1-1 .

Table 1-1. Sizes of built-up wood girders

1-5. Load Area

Both the foundation walls and the girder carry the load area of a building. Because the ends of each joist rest on the girder, there is more weight on the girder than on either of the wall.

Example 1. Before considering the load on the girder, consider the weight of a single joist. Suppose that a 10-ft board weighing 5 pounds per foot is led by two men. If the men are at opposite ends of the plank, they would each be supporting 25 pounds (see Figure 1-10 ).

Figure 1-10. Example of weight on a single joist

Example 2. Now assume that one of these men lifts the end of another 10-ft board with the same weight as the first one, and a third man lifts the opposite end. The two men on the outside are each supporting half the weight of one plank, or 25 pounds apiece, but the man in the center is supporting one half of each of the two boards, or a total of 50 pounds (see Figure 1-11 ).

Figure 1-11. Example of weight on a girder

The two men on the outside represent the foundation walls. The center man represents the girder. The girder carries half of the weigh and the other half is equally divided between the outside walls. However the girder may not always be located halfway between the outer walls.

Example 3. Imagine the same three men lifting two planks that weigh 5 pounds per foot. One of the planks is 8 feet long and the other is 12 feet long. The total length of these two planks is the same as before. The weight per foot is the same, so the total weight in both cases is 100 pounds.

One of the outside men is supporting half of the 8-foot plank, or 20 pounds. The man on the opposite outside end is supporting half of the 12-foot plank, or 30 pounds. The man in the center is supporting one half of each plank, or a total of 50 pounds. This is the same total weight he was lifting before. It is important to remember that a girder carries the weight of the floor on each side to the midpoint of the joists which rest upon it.

1-6. Floor Load

After the girder load area is known, the total floor load per square foot must be determined for safety purposes. Both dead and live loads must be considered.

Dead Load. A buildings structure weight is called the dead load. The dead load per square foot of floor area is carried directly or indirectly to the girder by bearing partitions. Dead load varies according to the method of construction and the building height. The structural parts included in the dead bad are:

  • Floor joists for all floor levels.
  • Flooring materials, including the attic if it is floored.
  • Bearing partitions.
  • Attic partitions.
  • Attic joists for the top floor.
  • Ceiling lath and plaster, including the basement ceiling if it is plastered.

Total Dead Load. For a building of light fame construction similar to an ordinary frame house, the dead-load allowance per square foot of all structural parts must be added together to determine the total dead load. The allowance for an average subfloor, finished floor, and joist without basement plaster should be 10 pounds per square foot. If the basement ceiling is plastered, an additional 10 pounds per square foot should be allowed. If the attic is unfloored, a load allowance of 20 pounds must be made for ceiling plaster and joists when girders or bearing partitions support the first-floor partition. If the attic is floored and used for storage, an additional 10 pounds per square foot should be allowed.

Live Load. The weight of furniture, persons, and other movable loads, not actually a par of the building but still carried by the girder, is called the live load. The live load per square foot will vary according to the use of the building and local weather conditions. Snow on the roof is considered part of the live load. The allowance for the live load on the floors used for living purposes is usually 30 pounds per square foot. If the attic is floored and used for light storage, an additional 20 pounds per square foot should be allowed. The allowance per square foot for live loads is usually governed by local building specifications and regulations.

Load Per Linear Foot. When the total load per square foot of floor area is known, the load per linear foot on the girder can easily be figured. Assume that the girder load area of the building shown in Figure 1-12 is sliced into 1-foot lengths across the girder. Each slice represents the weight supported by 1 foot of the girder. If the slice is divided into 1-foot units, each unit will represent 1 square foot of the total floor area. The load per linear foot of a girder is determined by multiplying the number of units, 12, by the total load per square foot, 70 pounds. This gives you 840 pounds per linear foot on the girder (12 x 70 = 840 pounds). Now you can take the 840 pounds per load per linear foot of girder and use Table 1-1 to determine the girder size. If your number is not on the table, round up.

Figure 1-12. Girder load per linear foot

Total Floor Load. Note in Figure 1-12 that the girder is off center. Remember that half of the load is supported by the girder and half is supported by the foundation walls. Therefore, the joist length to be supported on one side of the girder is 7 feet (half of 14 feet), and the other side is 5 feet (half of 10 feet) for a total distance of 12 feet across the load area. Since each slice is 1 foot wide, it has a total floor area of 12 square feet. Assume that the total floor load for each square foot is 70 pounds. Multiply the length times the width (7 feet x 12 feet) to get the total square feet supported by the girder (7 feet x 12 feet = 84 square feet).

1-7. Girder Material

Wooden girders are more common than steel girders in small frame buildings. Solid timbers may be used, or girders may be built up by using two or more 2-inch planks. Built-up girders warp less easily than solid wooden girders and are less likely to decay in the center.

Choice of Material. Regardless of whether the girder is built-up or solid, it should be of well-seasoned material. For a specific total girder load and span, the size of the girder will vary according to the kinds of wood used, since some woods are stronger than others.

Use of Nails. When built-up girders are used, the pieces should be securely nailed together to prevent individual bucking. A two-piece girder of 2-inch lumber should be nailed on both sides with 16d common nails. The nails should be located near the bottom, spaced approximately 2 feet apart near the ends and 1 foot apart in the center. A three-piece girder should be nailed in the same way. The nailing pattern should be square across the end of the board (1 1/2 inches from each end) and then diagonal every 16 inches.

1-8. Girder Splices

To make a built-up girder, select straight lumber free from knots and other defects. The stock should be long enough so that no more than one joint will occur over the span between footings. The joints in the beam should be staggered, taking care to square the planks at each joint and butt them tightly together.

Half-Lap Joint Sometimes a half-lap joint is used to join solid beams. In this case, place the beam on one edge so the annual rings run from top to bottom. The lines for the half-lap joint are then laid out (see Figure 1-13 ). Cuts are made along these lines, then checked with a steel square to assure a matching joint. Repeat this process on the other beam.

Figure 1-13. Girder splices

Temporary Strap. Tack a temporary strap across the joint to hold it tightly together. Drill a hole the joint with a bit about 1/16 inch larger than the bolt to be used, and fasten the joint with a bolt, a washer, and a nut.

Strapped Joint. When a strapped butt joint is used to join solid beams, the ends of the beams should be cut square. The straps, which are generally 18 inches long, are bolted to each side of the beams.

1-9. Girder Supports

When small houses are built without an architect, the carpenter must know the principles that determine the proper size of girder supports.

Columns. A vertical member, designed to carry the live and dead loads placed upon it is called a column or a post. It can be made of wood, metal, or masonry. Wooden columns may be solid timbers or several wooden members nailed together with 16d or 20d common nails. Metal columns are made of heavy pipe, large steel angles, or I-beams.

Column Spacing. A good arrangement of the girder and supporting columns for a 24-foot by 40-foot building is shown in Figure 1-14 . Column B will support one half of the girder load existing in the part the building lying between wall A and column C. Column C will support half of the girder load between columns B and D. Likewise, column D will share the girder loads equally with column C and wall E.

Figure 1-14. Girder and column spacing


When locating columns which must support girders, avoid spans of more than 10 feet between columns. The farther apart columns are spaced, the heavier the girder must be to carry the joist over the span between the columns.

Bearing Plates and Footings. Regardless of the material used in a column, it must have some form of a bearing plate at the top and bottom. These plates distribute the load evenly across the column. Basement posts that support girders should be set on masonry footings. Columns should be securely fastened at the top to the load-bearing member and at the bottom to the footing on which they rest.

Column Fastening. Figure 1-15 shows a solid wooden column with a metal bearing cap drilled so that it can be fastened to the column and to the girder. The bottom of this type of column may be fastened to the masonry footings by a metal dowel. The dowel should be inserted in a hole drilled in the bottom of the column and in the masonry footing. The base is coated with asphalt at the drilling point to prevent rust or rot.

Figure 1-15. Girder and column fastening

1-10. Floor Joists

Joists are wooden members, usually 2 or 3 inches thick, that make up the body of the floor frame. The flooring or subflooring is nailed to them.

Joist Loads. Joists usually carry a uniform load of materials and personnel. These are live loads. The weight of joists and floor is a dead load. Joists are spaced 16 or 24 inches on the center. Sometimes the spacing is 12 inches, but where such spacing is made necessary by the load, heavier joists should be used. In certain parts of the floor frame, to support heavily concentrated loads or a partition wall, it may be necessary to double the joists or to place two joists together (see Figure 1-16 ).

Figure 1-16. Reinforced joists

Joists and Sills. When joining joists to sills, be sure that the connection can hold the load that the jolt will carry. A joist resting on the sill and girder is shown in Figure 1-17 . This connection method is most commonly used because it provides the strongest possible joint. The method shown in Figure 1-18 a joist with ledger plates is used when it is not desirable to use joists on top of the sill. The ledger plate should be securely nailed to the sill and girder. If the joist must be notched, it should be securely nailed to the sill and girder. If the joist must be notched, it should not be notched over one third of its depth (to prevent splitting). Joists must be level when framed to girders. If the joist is not the same height as the girder, the joist must be notched (see Figure 1-19 ).

Figure 1-17. Joist resting on sill


Figure 1-18. Joist with ledger plates


Figure 1-19. Joist connected to a girder

Joist Hangers. When it is desirable to have the joists and girders flush, the ends of the joists can be supported by joist hangers (see Figure 1-20 ). Joist hangers support joists at the girders. When joists are hung using joist hangers, the maximum headroom is obtained below the girder.

Figure 1-20. Joist hangers

1-11. Bridging

When the joists are used over a long span, they tend to sway from side to side. Therefore, bridging is installed. Floor frames are bridged for stiffening and to prevent unequal deflection of the joists. Bridging enables an overloaded joist to receive some help from the joist on either side of it. A pattern for the bridging stock is obtained by placing a piece of material between the joist, then marking and sawing it. There are three types of bridging: solid, cross, and compression.

Solid. To provide maximum rigidity to the joist, use solid bridging. The bridging is offset to permit end nailing where posible (see Figure 1-21 ).

Figure 1-21. Solid bridging

Cross. Wood-cross bridging is used most often. It is cut to ft diagonally between joists (see Figure 1-22 ). Each piece is nailed to the top of each joist before the subfloor is placed. The bottoms are left free until the subfloor is laid. This permits the joists to adjust themselves to their final positions and keeps the bridging from pushing up the joists and causing an uneven floor.

Figure 1-22. Cross bridging

Compression. Use hammer blows to install compression bridging. Where the bridging is drilled, it is nailed in place (see Figure 1-23 ).

Figure 1-23. Installation of cross bridging

David L. Heiserman, Editor

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Revised: June 06, 2015