STEEL CONNECTIONS RESEARCH IN BUILDING TECH 3
SUBMITTED TO: ARCH. MARJOIRE JOY QUE-MENDOZA
PREPARED & SUBMITTED BY: ANGELICA MARIE I. AZUCENA
STEEL CONNECTIONS
Bolting and welding are the two most common methods in use today for making steel connections. Riveting was once widely used but has been generally replaced with bolting because bolting is less expensive and does not take such a large crew of skilled workers to accomplish.
A. BOLTS
There are two types of bolted connections: bearing type and friction type. Bearing-type connections resist the shear load on the bolt through direct bearing of the steel being fastened on the sides of the bolt. As with wood connections, the bolt may either be in single shear or double shear. Friction-type connections are made when the bolt is tightened to such an extent that friction develops between the connecting members and loads are transferred through this friction rather than through the bolt itself. Bolts are further classified as to whether the bolt threads are Included or excluded from the shear plane. This affects the strength of the connection because there is less area to resist the load through the threaded portion. See Figure 8.6. Bearing-type connections have the lowest load-carrying capacity of bolted joints and are used in noncritical or secondary connections. Because the holes are slightly larger than the bolts there is usually some movement as load is applied. Where slippage is undesirable or the joint may be subject to vibration or repeated reversal of load, friction-type connections must be used. There are three basic types of bolts used in modem steel construction. Bolts designated with the American Society of Testing and Materials (ASTM) number A307 are called unfinished bolts and have the lowest load-carrying capacity. They are used only forbearing-type connections. Bolts designated A325 and A490 are high-strength bolts and may be used in bearing-type connections but roost be used in 1riction-type connections. In friction connections, the nuts are tightened to develop a high tensile stress in the bolt, thus causing the connected members to develop a high friction between them which resists the shear. Bolts range in diameter from 12 mm to 38 mm in 3 mm increments, but the roost typically used diameters are 20 mm and 22 mm. Bolts are installed with a was hereunder the head and nut. In addition to the ASTM designations, there are standard codes for the condition of use: o
F: friction-type connection
o
N: Bearing-type connection with threads included in the shear plane
o
X: bearing-type connection with threads excluded from t he shear plane
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S: bolt in single shear
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0: bolt in double shear
The American Institute of Steel Construction (AISC) Manual of Steel Construction gives the allowable loads for various types of connectors in both shear and bearing. For bearing connections, different values are given based on the minimum tensile strength of the base material of the connected part. For A36 steel, this value is 400 MPa. The maximum allowable bearing stress between the bolt and t he side of the hole is given by t he equation. Fp = 1.5 Fu There are several types of holes for bolted connections. Standard round holes are 1.5 mm larger than the diameter of the bolt. Other kinds of holes as listed below may be used with high-strength holes 16 mm in diameter and larger. Oversize holes may have nominal diameters up to 4.5 mm larger than bolts 22 mm and less in diameter, 6 mm larger than 25 mm bolts, and 7.5 mm larger than bolts 27 mm and greater in diameter. These holes may only be used in friction type connections. Short slotted holes are 1.5 mm wider than the bolt diameter and have a length that does not exceed the oversize hole dimensions by more than 1.5 mm. They may be used in either bearing or friction type connections, but if used in bearing, the slots have to be perpendicular to the direction of load. Long slotted holes are 1.5 mm wider than the bolt diameter and a length not exceeding 2 1/2 times the bolt diameter. They may be used in friction-type connections without regard to direction of load, but must be perpendicular to the load direction in bearing-type connections. Slotted holes are used where some amount of adjustment is needed. Long slotted holes can be used in one of the connected parts of a joint. The other part must use standard round holes or be welded. In addition to the load-carrying capacities of the bolts, the effect of reducing the crosssectional area of the members must be checked. Figure 8.7 shows a typical example of this. In this case, a beam is framed into a girder with an angle welded to the girder bolted to the beam. With a load applied to the beam, there is a tendency for the web of the beam to tear where the area of the flange has been reduced by the bolt holes. This area is known as the net area. As shown in the figure, there is both shear failure parallel to the load and tension failure perpendicular to the load. The AISC Specifications limit the allowable stress o n the net tension area to:
= 0.50Fu
The allowable stress on the net shear area is limited to: = 0.30Fu The total shearing force is the sum required to cause both forms of failure. The stress on the net tension area must be compared with the allowable stress on the gross section which is:
= 0.30 Fy
Example 8.3 A 1 0 mm A36 steel plate is suspended from a 12 mm plate with three 20 mm A325 boIts in standard holes spaced as shown in the drawing. The threads are excluded from the shear plane and the connection is bearing type. What is the maximum toad-carrying capacity of the 10 mm plate?
First, check the shear capacity of the bolts. From Table 8.2, one bolt can carry a load of 59.2 kN or three bolts can carry 3 x 59.2 kN or 177.5 kN. Next, check the bearing capacity. The thinner material governs, so use the 10 mm row in Table 8.3. From this row, read under the 20 mm diameter column and under the 75 mm spacing. The allowable load is 109.0 kN. Three bolts will then carry 3 x 109.0 kN, or 327.0 kN. Finally, determine the maximum stress on the net section through the holes. Once again, the thinner material is the most critical component. The allowable unit stress is: =
0.50F = 0.50 x 400 MPa = 200 MPa
The diameter of each hole is 1.6 mm larger than the bolt, or13116 inch, which is 20.6 mm. The net width of the 10 mm plate is: net width = 22.5 mm - (3 x 20.0 mm) = 166 mm The allowable stress on the net section is: =
(166 mm x 10 mm) x 200 MPa = 317.4 kN
From these three loads, the minimum governs, which is the shear capacity of the bolts, or 177.53 kN. There are many kinds of framed connections depending on the type of connector being used, the size and shape of the connected members, and the magnitude of the loads that must be transferred. Figure 8.8 illustrates some of the more typical kinds of steel connections. In most cases, the angle use to connect one piece with another is welded to one member in the shop and bolted to the other member during field erection. Slotted holes are sometimes used to allow for minor field adjustment. If the top flange of one beam needs to be flush with another, the web is coped as shown in Figure 6.8 {b). Simple beam to column connections are often made as illustrated in figure 8.8 (c~. The seat angle carries most of the gravity load, and the clip angle is used to provide stability from rotation. If a moment connection is required, a detail similar to Figure 8.6 {d) is used, although welding is more suitable for moment connections. For tubes and round columns, a single plate can be welded to the column and connected with beams as shown in Figure 8.8 (f). When the loads are heavy, some engineers prefer to slot the column and run the shear plate through, welding it at the front and back of the column. Since connecting beams to columns and other beams with angles and bolts is such a common method of steel framing, t he AISC Manual gives table of allowable loads for various types and diameters of bolts and lengths and thicknesses of angles. Two such tables are reproduced in Table 8.4 and 8.5. The first is for bearing type connections and the second is for friction type connections.
a.
Tabulated load values are based on double shear of bolts unless noted. See AISC Specification, Appendix E, for other surface co nditions.
b. Capacity shown is based on double shear of the bolts; however, for length L, net shear on the angle thickness specified is critical. See Table 11-C. c.
Capacity shown is based on bearing capacity of 32 mm (11/4j end distance {Specification Eq. (1.16-2)] on A36 angles of thickness specified; however, for length L, net shear on this angle is critical. See Table 11-C.
d. Capacity is governed by net shear on angles for lengths L and L '. S ee Table 11-C.
a.
Tabulated load values are based on double shear of bolts unless noted. See AISC Specification, Appendix E, for other surface conditions.
b. Capacity shown is based on double shear of the bolts; however, for length L. net shear on the angle thickness specified is critical. See Table 11 -C.
FIGURE 8.8 TYPICAL STEEL FRAMING CONNECTIONS
One of the more important considerations in bolted steel connections, just as in wood connections, is the spacing of bolts and the edge distance from the last bolt to the edge of the member. The AISC specifies minimum dimensions. The absolute minimum spacing is 2~213 times the diameter of the bolt being used with 3 times the diameter being the preferred dimension. Many times a dimension of 75 mm is used for all sizes of bolts up to 25 mm diameter. The required edge distance varies with the diameter of the bolt being used: at the edge of plates, shapes or bars the dimension is 25 mm for an 18 mm bolt and 31 for a 25 mm bolt. To simplify detailing, a dimension of 31 mm is often used for all bolts up to 25 mm diameter. B. WELDING Welded connections are quite frequently used in lieu of bolts for several reasons: o
The gross cross section of the members can be used instead of the net section.
o
Construction is often more efficient because there are no angles, bolts, or washers to deal with and no clearance problems with wrenches.
o
Welding is more practical for moment connections.
Since members must be held in place until welding is completed, welding is often used in combination with bolting. Connection angles and other pieces are welded to one member in the shop with the outreach leg punched or slotted for field connection with bolts. There are several types of welding processes, but the one most commonly used in building construction is the electric arc process. One electrode from the power source is attached to the steel members being joined, and the other electrode is the welding rod the welder holds in his or her hand. The intense heat generated by the electric arc formed when the welding rod is b rought close to the members causes some of the base metal and the end of the electrode to melt into the joint. So the material of the electrode and both pieces of the joint are fused together. Penetration refers to the depth from the surface of the base metal to the point where fusion stops. Two types of electrodes are in common use today: the E60 and the E70. The allowable shear stress for E60 electrodes is 124 MPa, and for E70 electrodes it is 145 MPa . There are many types of welds. W hich one to use depends on the configuration of the joint; the magnitude and direction of the load, the cost of preparing the joint and what the erection process will be. The three most common types of welded joints are the lap, the butt, and the tee. Some of the common welding conditions for these joints are shown in Figure 8.9 along with the standard welding symbol used on drawings. In addition to the welds shown, plug or slot welds are f requently used to join two pieces. In these welds, a hole is cut or punched in one of the members and the area filled with the weld.
The fillet weld is one of the most common types. In section, its form i s an isosceles triangle with the two equal legs of the triangle being the size of the weld. The perpendicular distance from the 90 degree comer to the hypotenuse of the triangle is called the throat. See Figure 8.10 (a). Because the angles are 45 degrees, the dimension of the throat is 0. 707 times the leg dimension. For a butt Joint, the throat dimension is the thickness of the material if both pieces are the same thickness, or the size of the thinner of two materials if they are unequal as shown in Figure 8.1 O(b) There are common symbols used for welding. These are listed in the AISC Manual of Steel Construction. A few are reproduced in Figure 8.11 (a). The full range of symbols gives information regarding the type, size location, finish, welding process, angle for grooves and other information. To indicate information about a weld, a horizontal line is connected to an arrowhead line which points to the weld. This is shown in Figure 8.11 (b). This type of weld is indicated with one of the standard symbols and placed below the fine if the weld is on the side near the arrow and above the line if it is on the side away from the arrow. lf the members are to be welded on both sides, the symbol is repeated above and below the line. Other data placed with weld symbol are the size, weld symbol, length of weld, and spacing, in that order, reading from left to right. Field welds are indicated with a flag placed at the Junction of the horizontal line and the arrowhead line and pointing toward the tail of the reference line. A circle at the same point indicates that the weld should be made all around. The perpendicular legs of the fillet, bevel, J, and flare bevel welds must be at the left.
Designing a welded joint requires that you know the load to be resisted and the allowable stress in the weld. For fillet welds, the stress is considered as shear on the throat regardless of the direction of the load. For butt welds, the allowable stress Is the same as for the base metal. As previously mentioned, the allowable stress for fillet welds of E60 electrodes is 124 MPa and for E70 electrodes it is 145 MPa. These stresses apply to A36 steel. For any size fillet weld you can multiply the size by 0.070 and by the allowable stress to get the allowable working strength per linear mm of weld, but these have been tabulated for quicker calculations. The allowable strengths are listed in Table 8.6.
In addition to knowing the allowable stresses, some AISC Code provisions apply to weld design. The following are some of the requirements. o
The maximum size of fillet weld is 1.5 mm less than the nominal thickness of the material being joined if it is 6 mm thick or more. If the material is less than 6 mm thick, the maximum size is the same as the material.
o
The minimum size of the fillet welds is shown in Table 8.7
o
The minimum length of fillet welds must not be less than 4 times the weld size plus 6 mm for starting and stopping the art.
o
For two or more welds parallel to each other, the length must be at least equal to the perpendicular distance between them
o
For intermittent welds, the length must be at least 36 mm
