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Mixing Welds and Bolts

Introduction
There are a variety of circumstances in which the engineer may need to assess the strength of a connection that is composed of both welds and mechanical fasteners. Today, mechanical fasteners are typically bolts, but older structures may include rivets. Such situations may be encountered during the course of rehabilitation, repair or strengthening projects. For new construction, bolts and welds may be combined in connections where the materials being joined are initially secured with bolts, and then welded to gain the full connection strength. As will be seen, calculating the total capacity of the connection is not as simple as totaling the arithmetical sum of the individual components (welds, bolts, and rivets). Such an assumption is un-conservative, and the consequences could be disastrous. Part 1 of this two-part edition of “Design File” will deal with snug-tightened and pre-tensioned mechanical fasteners combined with welds. Part 2 will address combining welds with slip-critical, high-strength bolted connections.

Some Background on Bolted Connections
Bolted joints are described in the AISC Specification for Structural Joints Using ASTM A325 or A490 Bolts(June 23, 2000) as either snug-tightened, pretensioned, or slipcritical (p. 23). A snug-tightened joint has the condition of “tightness that is attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the plies into firm contact.”(p. xi). A pretensioned joint is one in which the bolts have been installed in a manner so that the bolts are under significant tensile load with the plates under compressive load (p. x). Four acceptable methods are listed in Section 8.2: turn-ofnut, calibrated wrench, twist-off-type tension-control bolts, and direct-tension-indicators. Slip-critical joints have bolts installed just as they would be in a pretensioned joint, but also have “faying surfaces that have been prepared to provide a calculable resistance against slip.” (p. xi).

In simple terms, in snug-tightened joints and pretensioned joints, the bolts act as pins. Slip-critical joints work by friction: the pretension forces create clamping forces and the friction between the faying surfaces work together to resist slippage of the joint.

ASTM A325 bolts have a minimum tensile strength of 105–120 ksi (725–830 MPa) depending upon the bolt diameter, while A490 bolts must fall between 150 and 170 ksi (1035–1175 MPa) tensile strength. Riveted joints behave more like snugtightened joints, but the “pins” in this case are the rivets, and are typically about half the strength of A325 bolts. When a mechanically fastened joint is loaded in shear, one of two types of behavior is possible. The joint may have the bolts or rivets bear against the sides of the holes in the connected material, concurrently putting the bolt or rivet into shear. The second possible behavior is that friction, introduced by the clamping forces provided by the pretensioned fastener, resists the shear loading. No slippage is expected in this joint, but the possibility exists nonetheless.

Snug-tight joints are acceptable for many applications since minor slippage may not adversely affect the performance of the connection. When there is significant load reversal, pretensioned joints may be required. When joints are subject to fatigue loads with reversal of direction, slipcritical joints are required. Thus, an existing bolted connection may have been designed and built to any of these criteria. Riveted joints would be considered the snug-tight type.

Adding Welds to Mechanically Fastened Joints
While a weld may be composed of metal that is capable of demonstrating an elongation of 20% or more in an all-weldmetal
tensile specimen, the same metal in a restrained joint may be incapable of delivering any significant deformation prior to fracture, due to the interaction of triaxial stresses. In other words, 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 will not deform equally. The load is transferred through the stiffer part, and therefore the weld will carry virtually all the load, sharing little with the bolts. And that’s why caution needs to be taken when welds and bolts and rivets are combined.

by Duane K. Miller

Mixing Welds and Bolts

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