Practical ideas for design professionals
June 13, 2002
This article provides background on bolted connections and situations in which welds are added to mechanically fastened joints. It also offers information on AWS code provisions for mixing fasteners and welds.
In a variety of structural situations, an engineer may need to assess the strength of a connection made with welds and mechanical fasteners. Today mechanical fasteners typically are bolts, but older structures may include rivets.
Such situations may occur during retrofit, repair, or strengthening projects. For new construction, bolts and welds may be required to work together in connections in which the materials being joined first are secured with bolts and then are welded to obtain full connection strength.
However, determining the connection's total load capacity isn't as simple as adding together the sum of the individual components—the welds, bolts, and rivets. Such an assumption can lead to disastrous consequences.
Bolted joints are described in the American Institute of Steel Construction's (AISC's) Specification for Structural Joints Using ASTM A325 or A490 Bolts as snug-tightened, pretensioned, or slip-critical.
A snug-tightened joint is tightened by the force of an impact wrench or by an ironworker using an ordinary spud wrench to bring plies into firm contact. In a pretensioned joint, bolts are installed so they are under significant tensile load, with the plates under compressive load.
Under Section 8.2, four methods of making these joints are acceptable:
1. Turn-of-nut. The turn-of-nut method involves snugging down the bolt and then turning the nut an additional amount, which is a function of the bolt diameter and length.
2. Calibrated wrench. The calibrated wrench method measures torque, which is correlated to the tension applied to the bolt.
3. Twist-off-type tension-control bolts. Twist-off-type tension-control bolts have small studs on the end of the bolt, opposite the head. When the required torque is achieved, the stud twists off.
4. Direct-tension indicators. Direct-tension indicators are special washers with protrusions. The amount of compression on the protrusions indicates what level of tension has been applied to the bolt.
In this illustration, the connection has two separate components: a welded connection on the left and a bolted connection on the right. Each part of the overall connection behaves independently.
In layman's terms, the bolts act as pins in snug-tightened and pretensioned joints, similar to a brass brad that holds together a stack of hole-punched papers. Slip-critical joints work by friction: The pretension forces create clamping forces, and the friction between the contact surfaces works together to resist joint slippage. This is similar to a binder clip that holds together a stack of papers, not because holes are punched in the paper, but because the binder clip presses the sheets of paper together, and friction keeps the packet together.
ASTM A325 bolts have a minimum tensile strength of 150 to 120 kilopounds per square inch (KSI), depending on the bolt diameter, while A490 bolts must fall between 150- and 170-KSI tensile strength. Riveted joints behave more like snug-tightened joints, but the pins in this case are the rivets, which typically are about half the strength of A325 bolts.
When a mechanically fastened joint is loaded in shear—when one member tends to slide over the other because of the applied forces—one of two situations can occur. The bolts or rivets may bear against the sides of the holes, causing the bolt or rivet to shear at the same time. The second possibility is that friction, introduced by the clamping forces from the pretensioned fastener, may resist the shear loading. No slippage is expected in this joint, but the possibility exists.
Snug-tight joints can be acceptable for many applications, because minor slippage may not affect the connection performance negatively. For example, consider a hopper that is designed to contain some granular material. The first time it's loaded, minor slippage may occur. Once the slippage has happened, it won't recur because all subsequent loading would be of the same nature.
Some applications involve load reversal, such as when rotating members experience alternating tensile and compression loading. Bending members that are subject to complete load reversal is another example. When significant load reversal occurs, pretensioned joints may be required to eliminate cyclic slippage. Such slippage eventually leads to elongated holes and even greater slippage.
Some connections are subject to many loading cycles that may lead to fatigue. They include connections in presses, crane supports, and bridges. When joints are subject to fatigue loads with direction reversal, slip-critical joints are necessary. For these types of conditions, it's essential that the joint does not slip, hence the requirement for slip-critical joints.
An existing bolted connection may have been designed and built to any of these criteria. Riveted joints are considered the snug-tight type.
Welded connections are rigid. Welded connections are stiff. Unlike snug-tightened bolted joints that may slip as they are loaded, welds are not expected to stretch and distribute the applied load to any great extent. In most cases, welds and bearing-type mechanical fasteners won't deform equally.
When welds and mechanical fasteners are used together, load is transferred through the stiffer part; therefore, the weld can carry almost all the load, sharing little with the bolts. That's why caution needs to be taken when welds, bolts, and rivets are combined.Code Provisions. The issue of mixing mechanical fasteners and welds is addressed in the AWS D1. 1:2000 Structural Welding Code—Steel. Provision 2.6.3 states that for rivets or bolts used in bearing-type connections (that is, when the bolt or rivet acts as a pin), the mechanical fasteners shouldn't be considered as sharing the load in combination with welds. If welds are used, they should be provided to carry the entire load in the connection. However, connections that are welded to one member and riveted or bolted to another are permitted.
|Figure 2 (left) and 3 (right):|
This lap joint originally was connected with snug-tight bolts (left). Additional capacity has been added to the structure, and the connection and the attached members must be increased to provide twice the strength. The basic plan is to strengthen the members (right).
When the mechanical fasteners are of the bearing type and a weld is added, the capacity of the bolt essentially is ignored. The weld must be designed to transfer all the load, according to this provision.
Basically, this is the same as AISC LRFD-1999, provision J1.9. However, the Canadian standard, CAN/CSA-S16.1-M94, also permits the use of the capacity of the mechanical fastener or the bolt alone when it is higher than the capacity of the weld.
All three standards are in agreement on this issue: The capacities of the bearing-type mechanical fasteners and the welds can't be added together.
AWS D1.1, paragraph 2.6.3 also addresses a situation in which bolts and welds can be combined in a connection consisting of two separate components, as illustrated in Figure 1. A welded connection is on the left, and a bolted connection is on the right. Here the full capacity of the welds and bolts can be considered. Each part of the overall connection behaves independently. Therefore, the code provides an exception to the principles as contained in the first part of 2.6.3.
The provisions just discussed are applicable for new construction. For existing structures, D1.1, paragraph 8.3.7 states that when design calculations show rivets or bolts will be overstressed by the new total load, only the existing dead load should be assigned to them.
The same provision requires that if rivets or bolts are overstressed by dead load alone or are subject to cyclic (fatigue) loading, then sufficient base metal and welding should be added to support the total load.
Sharing loads between mechanical fasteners and welds is acceptable if the structure is preloaded; in other words, if slip between the connected members already has occurred. But only the dead load can be assigned to the mechanical fastener. Live loads, which may cause more slippage, must be resisted by the application of welds capable of carrying the whole load.
Welds must be used to take up the entire applied or live load. No sharing of loads is permitted when the mechanical fasteners already are overloaded. When cyclic loading is involved, no load sharing is permitted because the loading could cause ongoing slippage and overload the welds.
An Illustration. Consider a lap joint originally connected with snug-tight bolts (see Figure 2). Additional capacity is being added to the structure, and the connection and the attached members must be increased to provide twice the strength. Figure 3illustrates the basic plan to strengthen the members. What should be done to the connection?
Because the new steel is going to be joined to the old with fillet welds, the engineer decides to add some fillet welds to the connection. Since the bolts still are in place, the initial idea is to add only the welds required to transfer the additional capacity of the new steel, with 50 percent of the load expected to go through the bolts and 50 percent through the new welds. Will this be acceptable?
First assume that no dead load currently is applied to the connection. In this case, AWS D1.1, paragraph 2.6.3 applies.
In this bearing-type connection, the welds and bolts cannot be considered as sharing the load, so the specified weld size must be large enough to carry the entire dead and live load. The capacity of the bolts can't be considered in this example, because without the dead load, the connection would be in a relaxed state. When the full load is applied, the welds (designed to transfer half the load) initially would break. Then the bolts—also designed to transfer half the load—would attempt to transfer the load and would break.
Next assume a dead load is applied. Further assume that the existing connection is adequate to transfer the existing dead load. In this case, D1.1, paragraph 8.3.7 applies. The new welds are required to carry only the increased dead load and the total live load. The existing dead load can be assigned to the existing mechanical fasteners.
With the dead load, the connection is not relaxed. Instead, the bolts already are carrying their load. Any slip in the connection has occurred already. Therefore, welds can be applied, and they can transfer the live load.
Answering the question "Is this acceptable?" depends on loading conditions. In the first case in which no dead load was assumed, the answer is no. Under the specific conditions of the second scenario, the answer is yes.
It can't be concluded that the answer always will be yes just because dead load is applied. The level of dead load, the adequacy of the existing mechanical connection, and the nature of the final loading—whether it's static or cyclic—could change the answer.
Duane K. Miller, Sc.D., P.E., is the Weld Technology Center manager at The Lincoln Electric Company, 22801 Saint Clair Ave., Cleveland, OH 44117-1199, Web site www.lincolnelectric.com.The Lincoln Electric Company manufactures welding equipment and welding consumables worldwide. Its Weld Technology Center engineers and technicians assist customers with welding application questions.
American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126-5671, phone 305-443-9353, fax 305-443-7559, Web site www.aws.org.
ASTM Intl., 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, phone 610-832-9585, fax 610-832-9555, Web site www.astm.org.
American Institute of Steel Construction, One E. Wacker Drive, Suite 3100, Chicago, IL 60601-2001, phone 312-670-2400, fax 312-670-5403, Web site www.aisc.org.