Adjustment of Design Values for Connections

Nominal design values for laterally loaded fasteners Z, withdrawal of fasteners W, load parallel to grain P, and load perpendicular to grain Q should be multiplied by applicable adjustment factors to determine adjusted design values Z, W, P, and Q’, respectively. Table 10.28 summarizes the adjustment factors that should be applied to the design values Z, W, P, and Q for connections made with commonly used types of fasteners. The load applied to a connection should not exceed the adjusted design value.

Load Duration Factor

Nominal design values should be multiplied by the load duration factor CD specified in Art. 10.5.1, except that CD may not exceed 1.6 or when the capacity of the connection is controlled by the strength of metal. The impact load duration factor shall not apply to connections.

Wet-Service Factor

Nominal design values apply to wood that will be used under dry-service conditions;
that is, where moisture content of the wood will be a maximum of 19% of the oven-dry weight, as would be the case in most covered structures. For connections in wood that is unseasoned or partly seasoned, or when connections will be exposed to wet-service conditions in use, nominal design values should be multiplied by the appropriate wet-service factor CM in Table 10.29.

Temperature Factor

Nominal design values should be multiplied by the appropriate temperature factor Ct listed in Table 10.30 for connections that will experience sustained exposure to elevated temperatures up to 150F.

Group-Action Factor

Nominal design values for split-ring connectors, shear-plate connectors, bolts with diameter D up to 1 in, and lag screws in a row should be multiplied by the groupaction factor Cg given in Table 10.31. The NDS contains design criteria for determination of Cg for additional configurations.
For determination of Cg, a row of fasteners is defined as any of the following:

1. Two or more split-ring or shear-plate connectors aligned with the direction of the load.
2. Two or more bolts with the same diameter, loaded in shear, and aligned with the direction of the load.
3. Two or more lag screws of the same type and size loaded in single shear and aligned with the direction of the load.
When fasteners in adjacent rows are staggered but close together, they may have to be treated as a single row in determination of Cg. This occurs when the distance between adjacent rows is less than one-fourth of the spacing between the closest fasteners in adjacent rows.
The group-action factor is necessary because of the following characteristics of a joint with more than two fasteners in a row: The two end fasteners carry a larger load than the interior fasteners. With six or more fasteners in a row, the two end fasteners may carry more than 50% of the load. With bolts, a small redistribution of load from the end bolts to the interior bolts occurs due to crushing of the wood at the end bolts. If failure is in shear, a partial failure occurs before substantial redistribution of load takes place.

Geometry Factor

The NDS specifies minimum edge distance, end distance, and spacing required for full design value for bolts, lag screws, and split-ring and shear-plate connectors.
The NDS also tabulates nominal design values for these fasteners based on the minimum distances. When the end distance or the spacing is less than the minimum required for full design value but larger than the minimum required for reduced design value, nominal design values should be multiplied by the smallest applicable geometry factor C determined from the end distance and spacing requirements for the type of connector specified. The smallest geometry factor for any connector in a group should be applied to all in the group. For multiple shear connections or for asymmetric three-member connections, the smallest geometry factor for any shear plane should be applied to all fasteners in the connection. The NDS also outlines geometry factors for timber rivet connections loaded perpendicular to grain.

Penetration-Depth Factor

Nominal lateral design values for lag screws, wood screws, nails, and spikes are based on a specific penetration into the main member, as established by the shank  diameter. The NDS also set a minimum penetration into the main member for a reduced design value for each fastener. When the penetration is larger than the minimum but less than that assumed in establishment of the full lateral design value, linear interpolation should be used in determination of the penetration-depth factor Cd. In no case should Cd exceed unity. Table 10.32 lists minimum penetration and assumed penetration for full design value as well as Cd for each type of fastener.

End-Grain Factor

Woods screws, lag screws, nails, and spikes are used in two types of connections, withdrawal and lateral load. In withdrawal connections, the load is applied parallel  to the length of the fastener. In laterally loaded connections, the load is applied perpendicular to the length of the fastener. Either type of connection is weaker when fasteners are inserted in the ends of a member, parallel to the grain, rather than in the side grain.
Withdrawal Design Value. Wood screws, nails, and spikes should not be loaded in withdrawal from end grain. Tests show that splitting of the wood member causes erratic results relative to those for withdrawal from side grain.
When lag screws are loaded in withdrawal from end grain, the nominal withdrawal design value should be multiplied by the end-grain factor Ccg  0.75.
Lateral Design Value. When lag screws, wood screws, nails, or spikes are inserted, with the axis in the direction of the wood grain, into the end grain of a main member, the nominal design value for lateral loading should be multiplied by the end-grain factor Ccg = 0.67.

Metal-Side-Plate Factor

For timber rivets and 4 shear plates, larger design values are permitted when metal side plates are used in lieu of wood plates. For 4-in shear-plate connectors, the nominal design value for load parallel to grain P should be multiplied by the appropriate Cst given in Table 10.33(a). The values depend on the species of wood used in the connection, such as group A, B, C, or D listed in the NDS. Table 10.33(b) outlines metal side plate factors for timber rivet connections.

Diaphragm Factor

A diaphragm is a large, thin structural element that is loaded in its plane. When nails or spikes are used in a diaphragm connection, the nominal lateral design value should be multiplied by the diaphragm factor Cdi = 1.1.

Toenail Factor

For such connections as stud to plate, beam to plate, and blocking to plate, toenailing is generally used. The NDS recommends that toenails be driven at an angle of about 30 with the face of the stud, beam, or blocking and started about onethird the length of the nail from the end of the member. For toenailed connections, the nominal lateral design values for connections with nails driven into side grain should be multiplied by the toenail factor Ctn  0.83.

Adjustments for Fire-Retardant Treatment

For connections made with lumber or other wood products pressure-treated with fire-retardant chemicals, design values should be obtained from the company providing the treatment and redrying service. The load-duration factor for impact does  not apply to such connections.

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