ARCHITECTURE EXAM REVIEW VOLUME I: STRUCTURAL TOPICS FIFTH EDITION

Steven E. O’Hara, PE David Kent Ballast, AIA

PROFESSIONAL PUBLICATIONS, INC. Belmont, CA

7

CONNECTIONS

Nomenclature Fc Fg Fn Fp Ft Fu Fv Fy θ

unit stress in compression perpendicular to the grain design value for end grain in bearing parallel to grain unit compressive stress at inclination θ with the direction of grain allowable bearing in stress allowable tensile stress minimum tensile strength of steel or fastener allowable shear stress specified minimum yield stress of steel angle between the direction of grain and direction of load normal to face considered

psi psi

A. Species of Wood

psi ksi ksi

The species and density of wood affects the holding power of connectors. Species are classified into four groups. There is one grouping for timber connectors, such as split ring connectors and shear plates, and another grouping for lag screws, nails, spikes, wood screws and metal plate connector loads. The four groups for timber connectors are designated Groups A, B, C, and D, while the grouping for other connectors are designated Groups I, II, III, and IV. Tables that give the allowable loads for connectors have separate columns for each group. Design values for connectors in a particular species apply to all grades of that species unless otherwise noted in the tables.

ksi ksi ksi

The majority of structural failures occur in the connections of members, not in the members themselves. Either the incorrect types of connectors are used, or they are undersized, too few in number, or improperly installed. It is therefore important for ARE test candidates to have a good understanding of the various types of connectors and how they are used. 1

Additional design considerations are the critical net section, the type of shear the joint is subjected to, the spacing of the connectors, and the end and edge distances to connectors.

WOOD CONNECTIONS

There are several variables that affect the design of wood connections. The first, of course, is the loadcarrying capacity of the connector itself. Nails and screws, for example, carry relatively light loads, while timber connectors can carry large loads. Other variables that apply to all connections include the species of wood, the type of load, the condition of the wood, the service conditions, whether or not the wood is fireretardant treated, and the angle of the load to the grain.

B. Type of Load The design values for connectors can be adjusted for the duration of loading just as wood members can be (see Chapter 9). This is because wood can carry greater maximum loads for short durations than for long durations. The tables of allowable connector loads are for normal duration of ten years. For other conditions, the allowable values can be multiplied by the following factors. • 0.90 for permanent loading over 10 years • 1.15 for 2 months’ duration (snow loading, for example) • 1.25 for 7 days’ duration • 1.33 for wind or earthquake loads • 2.00 for impact loads

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7-2 C. Condition of Wood

G. Critical Net Section

Tabulated design values found in building codes and elsewhere are for fastenings in wood seasoned to a moisture content of 19 percent or less. This is adequate for most use, but partially seasoned or wet wood (either at the time of fabrication or in service) reduces the holding power of the connector.

When a wood member is drilled for one of the many types of connectors (except for nails and screws), there is a decrease in area of wood to carry the imposed load. The section where the most wood has been removed is called the critical net section. Once the size of the drilled area is known, the member must be checked for load-carrying capacity at this section. It may be necessary to increase the size of the member just to compensate for this decrease in area. See Figure 7.1(a).

D. Service Conditions Service conditions refer to the environment in which the wood joint will be used. These conditions can either be dry, wet, exposed to weather, or subject to wetting and drying. Any service conditions other than dry or continuously wet reduce the holding power of the connector. E. Fire-Retardant Treatment Wood that has been fire-retardant treated does not hold connectors as well as wood that has not been treated. The UBC and NDS both specify that allowable design values for treated wood be obtained from the manufacturer.

;;

F. Angle of Load One of the most important variables affecting allowable loads carried by connectors is the angle of the load to the grain, which is defined as the angle between the direction of load acting on the member and the longitudinal axis of the member. Wood connectors can carry more load parallel to the grain than perpendicular to it, so tables of design values include both. If the load is acting other than parallel or perpendicular to the grain, it must be calculated using the Hankinson formula or by using one of the graphs that gives the same results.

single shear

multiple shear

end distance for vertical member

end distance for horizontal member unloaded edge distance

7.1 loaded edge distance for horizontal member

load

A 2

× 6

truss member bears on a 4

× 6

member at an angle of 40 degrees. Both pieces of lumber are select structural Douglas fir (Fg = 1400 psi and Fc = 625 psi). What is the allowable unit compressive stress for the connection?

(c) spacing and edge conditions Figure 7.1 Wood Connector Design Variables

H. Type of Shear

Using the Hankinson formula,

=

spacing

spacing

F g Fc Fg sin θ + Fc cos2 θ 2

Example 7.1

Fn =

double shear

(b) types of shear conditions

The Hankinson formula gives the unit compressive stress at angle θ. Fn =

;

(a) critical net area

(1400)(625) 1400 sin2 40 + 625 cos2 40 875,000 = 926 psi (1400)(0.413) + (625)(0.587)

Connectors such as bolts and lag screws can be in single shear, double shear, or multiple shear as shown in Figure 7.1(b). The type of shear condition and the relative thickness of each piece to the others are especially important when designing bolted connections.

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7-8

ARCHITECTURE EXAM REVIEW Table 7.2 Allowable Loads in Shear for Connectors

c American Institute of Steel Construction, Inc. Copyright  Reprinted with permission. All rights reserved.

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CONNECTIONS SAMPLE QUESTIONS 1. Which of the following are the most important variables in designing a bolted wood connection? I.

the angle of the load to the grain

II.

the thickness of the members through which the bolt is placed

7-19

5. In designing a composite section, what device would most likely be used?

6. Two 1/4

×6

A36 steel bars are welded, as shown in the figure, with E70 electrodes. What is the maximum allowable tensile load that this joint can resist?

III. the species of wood IV. the type of washers used under the head and nut V.

the area of the net section at the bolt holes A. I, III, and V B. I, II, III, and V C. I, II, IV, and V D. I, III, IV, and V

A. 30.8 kips B. 32.4 kips C. 33.6 kips D. 44.4 kips

A0 A1 A2 A3 A4 A5 A6 A7 A8 A9

bevel common bolt dowel edge distance fillet Hankinson formula headed anchor stud high strength bolt lag screw oversize hole

B0 B1 B2 B3 B4 B5

penetration plug shear plate split ring connector vee weld plate

7. Which of the welding symbols would indicate that the weld shown should be made at the job site?

;

The answers to questions 2 through 5 can be found on the following key list. Select only one answer for each question.

A.

B.

2. What connector would be best for a wood truss covering a temporary building with a long span? C. 3. What is used to account for wood members that are loaded at an angle to each other?

4. What type of weld would most likely be used to connect two overlapping steel plates in compression?

D.

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