SIMPLE WELDED CONNECTIONS General Welded connections are used frequently because of their simplicity They have fewer parts and weigh less than other connections particularly when the welding is performed in the shop Combining shop welded and field bolted elements is usually the most economical method The shop welded parts of a connection frequently reduce required bolting clearances for field erection A properly designed and executed weld can be stronger than the base metal The weld s design is as important as its execution in achieving a good connection Improperly made welds though they may appear to be good can be worthless When designing a weld it is important to specify the type number and size of only the welds needed to obtain the necessary strength Welding in excess of what is needed increases assembly costs and may reduce the ductility of the connection AISC Specification Sec J1 and Sec J2 provide the requirements for welded connections AISC Manual Parts 8 10 11 and 12 contain many tables that can help in designing and analyzing connections TYPES OF WELDS Figure below shows some of the most common weld types and the standard symbols used to indicate them on drawings The fillet weld is the most common and economical weld used in structural steel Fillet welds are often used for lightly loaded connections This type of weld needs little or no preparation of the material to be joined Groove welds are often used for heavier loads because they can be designed to develop the full strength of the elements being joined Groove welds are further classified by the type of joint preparation used to receive the weld Types of groove welds include square bevel V J U flare bevel and flare V Plug welds and slot welds are less commonly used than fillet and groove welds They are used primarily to transmit shear in lapped joints and to prevent buckling of elements in built up members WELDED CONNECTIONS Structural welding is a process whereby the parts to be connected are heated and fused with supplementary molten metal added to the joint For example the tension member lap joint shown in Figure below can be constructed by welding across the ends of both connected parts A relatively small depth of material will become molten and upon cooling the structural steel and the weld metal will act as one continuous part where they are joined The additional metal sometimes referred to as filler metal is deposited from a special electrode which is part of an electrical circuit that includes the connected part or base metal In the Shielded Metal Arc Welding SMAW process shown schematically in Figure below current arcs across a gap between the electrode and base metal heating the connected parts and depositing part of the electrode into the molten base metal A special coating on the electrode vaporizes and forms a protective gaseous shield preventing the molten weld metal from oxidizing before it solidifies The electrode is moved across the joint and a weld bead is deposited its size depending on the rate of travel of the electrode As the weld cools impurities rise to the surface forming a coating called slag that must be removed before the member is painted or another pass is made with the electrode For shop welding an automatic or semiautomatic process is usually used Foremost among these processes is submerged arc welding SAW In this process the end of the electrode and the arc are submerged in a granular flux that melts and forms a gaseous shield There is more penetration into the base metal than with shielded metal arc welding and higher strength results Other commonly used processes for shop welding include gas shielded metal arc flux cored arc and electro slag welding Submerged Arc Welding SAW gas metal arc welding molten slag base metal welding wire Acetylene oxygen electrode Types of Welding The two most common types of welds are the fillet weld and the groove weld The lap joint illustrated before is made with fillet welds which are defined as those placed in a corner formed by two parts in contact Fillet welds can also be used in a tee joint Groove welds are those deposited in a gap or groove between two parts to be connected They are most frequently used for butt tee and corner joints In most cases one or both of the connected parts will have beveled edges called prepared edges as shown in Figure FILLET WELDS The design and analysis of fillet welds is based on the assumption that the cross section of the weld is a 45 right triangle as shown in Figure Any reinforcement buildup outside the hypotenuse of the triangle or penetration is neglected The size of a fillet weld is denoted w and is the length of one of the two equal sides of this idealized cross section Standard weld sizes are specified in increments of 1 16 inch Although a length of weld can be loaded in any direction in shear compression or tension a fillet weld is weakest in shear and is always assumed to fail in this mode Specifically failure is assumed to occur in shear on a plane through the throat of the weld For fillet welds made with the shielded metal arc process the throat is the perpendicular distance from the corner or root of the weld to the hypotenuse and is equal to 0 707 times the size of the weld Here we conservatively assume that the shielded metal arc welding process is used 0 707 For a given length of weld L subjected to a load of P the critical shearing stress is Where w is the weld size If the weld ultimate shearing stress Fnw is used in this equation the nominal load capacity of the weld can be written as Rn 0 707wLFnw The strength of a fillet weld depends on the weld metal used that is it is a function of the type of electrode The strength of the electrode is defined as its ultimate tensile strength with strengths of 60 70 80 90 100 110 and 120 kips per square inch available for the shielded metal arc welding process The standard notation for specifying an electrode is the letter E followed by two or three digits indicating the tensile strength in kips per square inch and two digits specifying the type of coating As strength is the property of primary concern to the design engineer the last two digits are usually represented by XX and a typical designation would be E70XX or just E70 indicating an electrode with an ultimate tensile strength of 70 ksi Electrodes should be selected to match the base metal For the commonly used grades of steel only two electrodes need be considered Use E70XX electrodes with steels that