Wood Structural Systems 3oc17

Wood Structural Systems R. Taher 2002

Wood Structural Systems Wood Frame Construction Ordinary Construction Heavy Timber Construction Prefabricated Structural Wood – Engineered Wood Products

1- Wood Frame Construction Used extensively in dwellings. Walls, partitions, floors and roof are totally or partially made of wood or other combustible material. Height is usually limited to 2 habitable floors, not including a finished attic or basement. Wood frame is not permitted within certain fire zones.

The structural frame is mostly lumber with a nominal thickness of 2 in. The spacing of the structural is normally a module of 48”, usually 16”, sometimes 12” or 24”. Most studs and plates are 2x4s The exterior wall framing is usually covered with sheathing, then a finished surface of siding or masonry veneer.

The roof framing is covered with a deck of sheathing or other material to receive roofing. The floor framing is covered with a subfloor on which a finish is laid. The wall and floor sheathing are factors in insulation and also provide some lateral rigidity.

Before the frame is started, termite shields should be installed on top of all foundation walls. Mud sills or plates 4 or 6 in. wide, are then placed on top of the foundation walls and anchored by bolts. If there are unexcavated spaces below the first floor, there should be a crawl space with a clearance of at least 18” between the joists and the ground, to provide for ventilation, and access for inspection and repair.

There are 2 types of wood frames commonly used in dwellings: ™The Platform Frame ™The Balloon Frame

The Platform Frame The framing and subfloor for the 1st floor can be completed to form a platform before the 1st story exterior walls and the partition framing is started. Subfloors are most frequently plywood sheets, the thickness depending on the spacing of the s (from ½ in. to 1 ¼ in.)

The face grain of the plywood is laid perpendicular to the ing joists. Bridging is provided at the joist midspan, or on rows not more than 8 ft. apart, in order to increase the strength of the floor and the resistance to lateral loads. There are 2 types of bridging: ™Cross bridging ™Solid blocking.

Cross-bridging or x-bracing is usually 1x2 and is nailed in at the top and bottom. Solid blocking consists of short pieces (same size as joists) fitted between the joists at right angles to them. The 1st story studs are erected, diagonally braced, fire blocked, and capped with double top plates. The frame is then ready to receive the 2nd floor joists, bridging and subfloor to complete the second platform and so on.

Joists are usually 2 in. wide and from 6 to 14 in. deep. For heavy loads, the joists may be 3 or 4 in. wide. The spacing is usually 12” or 16”. Double joists must be provided under partitions parallel to the joists.

The Balloon Frame The basic difference between the balloon frame and the platform frame is that the balloon frame exterior wall studs extend as one piece from the foundation sill ing through the 2 floors. The ends of the 2nd floor joists are ed on a flat 2x4 block resting on a 1x4 ledger board attached to the inside of the notched studs. The bridging, roof framing, double joists required for platform framing apply also to the balloon framing.

The Merits of Platform and Balloon Frame The main advantage of the platform framing is in its ease of construction. The main advantage of the balloon framing is its relatively smaller vertical shrinkage, resulting from the continuity of the wall studs.

2- Ordinary Construction It requires noncombustible walls, such as masonry or concrete, and a fire resistance rating of not less than 2 hours. The entire assemblies are entirely or partially of wood, with sizes smaller than those required for heavy timber construction. The interior is wood frame, however wood trusses and steel beams, girders and trusses without fireproofing are often used.

Ordinary construction could be used for most occupancies, when it is not prohibited by fire district regulations.

3- Heavy Timber Construction Also called slow-burning construction or mill construction. It requires exterior walls of noncombustible materials, such as masonry or concrete with a fire rating of at least 2 hours. The interior structural (columns, girders, beams) must be solid or laminated timber. Floors and roofs must be heavy plank or laminated wood or equivalent material.

The foundation walls are poured concrete walls on spread foundations. The min. code size for timber columns is 8x8 in. nominal. Beams and girders ing floors must be at least 6 in. wide and not less than 10 in. deep.

Prefabricated Structural WoodEngineered Wood Products Columns and Studs: ™Square timbers are usually used, rarely smaller than 4x4 in. and normally not larger than 12x12 in. ™Built-up column sections of small fastened timbers could be used. ™Glulam columns have the same advantage as glulam beams.

Solid Beams and Girders: ™In wood frame construction, the joists and rafters have usually a thickness of 2 in. ™Their common depth is : - 4 to 6 in. for rafters - 6 to 12 in. for joists.

Wood-I-Joists: ™These joists could be used for longer spans (up to 46 ft) compared to lumber. ™The section is I-shaped, with flanges made of lumber and a web made of plywood or OSB (Oriented Strand Board).

Glulam Beams and Girders: ™The usual thickness of laminations is 1 ½ in. or 2 in. nominal. ¾ in. thickness is used in curved beams. ™The min. number of lams is 2, the max. is 60. ™Standard widths are: 2 ¼, 3 1/8, 5 1/8, 6 ¾, 10 ¾, 12 ¼, 14 ¼.

™Dry-use adhesives are used for interior glulam . ™Wet-use adhesives (watertight glue) are used in exterior or exposed to extreme moisture. ™Glulam beams and girders are factory produced. ™Their spans could exceed 100 ft.

Other engineered wood products used for beams and columns include: ™Laminated Veneer Lumber (LVL) ™Parallel Strand Lumber (PSL)

Wood Trusses: ™Frequently column free floor areas. ™Factory produced with spans up to 150 ft or more. ™Glulam could be used in truss construction. ™Open-web wood joists are also commonly used in construction.

Rigid Frames: ™Glulam is extensively used in rigid frame construction. ™Frames are factory produced and transported to the site. ™Usually assembled on site. ™Glulam rigid frame are attractive in appearance and are used for buildings such as: gymnasiums, churches, auditoriums…

™They are used for unobstructed floor areas and to obtain a certain desired clearance. ™They span generally from 30 ft to 150 ft. ™The roof decks are often constructed of heavy matched planks spanning between frames, or between purlins ed by the frames.

Arches: ™Arches are usually fixed, two-hinged, or threehinged for longer spans. ™Glulam arches may be elevated and ed on reinforced concrete buttresses, or on walls or columns. ™The horizontal thrusts at the ends are usually carried by steel tie rods. ™Glulam arches could also be ed on piers

™An arch could be fabricated in 2 or more sections, to facilitate transportation. ™Spans exceeding 250 ft have been constructed. ™The decks are similar to those used in rigid frames. ™Glulam arched rafters spaced as close as 2 ft, are sometimes used for small buildings, such as barns. They are covered with a 1in. Sheathing.

Domes: ™Domes are used to cover circular areas. ™Glulam is usually used. ™Example 1: the structural of the walls and roof consist of 16 half three-hinged glulam timber rigid frames. The legs of the frames are ed on footings equally spaced around a complete circle. The crowns meet on a thrust block at the center of the roof. Equally spaced timber purlins span between frames and a timber roof deck.

Example 2: ™Heavy timber circular domed roof with arched radial ribs. ™The span is 300 ft, and the rise is 51 ft. ™There are 36 similar glulam ribs. ™The outward rib thrusts at the springing line are carried by steel base shoes anchored to reinforced concrete wall columns.

™36 straight segments of a built-up steel tension rings are welded to the steel base shoes. ™The inward rib thrusts at the crown are carried by a built-up steel compression ring ed radially by 18 steel struts framed together at the center. ™Equally spaced glulam straight purlins into the ribs. ™Light sub-purlins are attached to the purlins and normal to them.

™These 3 in. wood-fiber concrete roof deck s. ™The 36 sectors between ribs have diagonal crossbracing in steel. ™The 36 columns are connected by horizontal reinforced concrete girts for lateral . ™Brick walls are constructed between columns.