Welding is used to fasten together components of a built-up member, such as a plate girder, and to make connections between members. This technique, which uses fusion is a controlled atmosphere, requires more highly skilled labor than does bolting. However, because of cost advantages, welding is widely used in steel construction, especially in fabricating shops where conditions are more favorable to closely controlled procedures. When field welding is specified, the availability of skilled welders and inspection technicians and the use of more stringent qualitycontrol criteria should be considered. Any of several welding processes may be used: manual shielded metal arc, submerged arc, flux cored arc, gas metal arc, electrogas, and electroslag. They are not all interchangeable, however; each has its advantageous applications. Many building codes accept the recommendations of the American Welding Society Structural Welding Code (AWS D1.1) (Table 7.1). The AISC specification incorporates many of this codes salient requirements. Weld Types. Practically all welds used for connecting structural steel are of either of two types: fillet or groove. Figure 7.3a and b illustrates a typical fillet weld. As stated in Art. 7.27, all stresses on fillet welds are resolved as shear on the effective throat. The normal throat dimension, as indicated in Fig. 7.3a and b, is the effective throat for all welding processes, except the submerged-arc method. The deep penetration characteristic of the latter process is recognized by increasing the effective throat dimension, as shown in Fig. 7.3c. Groove welds (Fig. 7.3d, e, and ) are classified in accordance with depth of solid weld metal as either complete or partial penetration. Most groove welds, such as those in Fig. 7.3d and e, are made complete-penetration welds by the workmanship requirements: use backup strips or remove slag inclusions and imperfections (step called back-gouging) on the unshielded side of the root weld. The partialpenetration groove weld shown in Fig. 7.3 is typical of the type of weld used for box-type members and column splices. Effective throat depends on the welding process, welding position, and the chamfer angle
. The indicated effective throat (Fig. 7.3) is proper for the shielded-metal-arc processes and for all welding positions. (See also Art. 7.27.) Welding Electrodes. Specifications for all welding electrodes, promulgated by the American Welding Society (AWS), are identified as A5.1, A5.5, A5.17, etc., depending on the welding process. Electrodes for manual arc welding, often called stick electrodes, are designated by the letter E followed by four of five digits. The first two or three digits designate the strength level; thus, E70XX means electrodes having a minimum tensile strength of 70.0 ksi. Allowable shear stress on the depositied weld metal is taken as 0.30 times the electrode strength classification; thus, 0.30 times 70 to an E70 results in an allowable stress of 21.0 ksi. The remaining digits provide information on the intended usage, such as the particular welding positions and types of electrode coating. FIGURE 7.3 Effective throats of fillet and groove welds. Welding Procedures. The variables that affect the quality of a weld are controlled by welding procedures that must be approved by the structural engineer. Specification AWS D1.1 contains several prequalified welding procedures, the use of which permits fabricators and erectors to avoid the need for obtaining approvals for specific routine work. Where unusual conditions exist, the specification requires that formal documentation be submitted for review and approval. Base-Metal Temperatures. An important requirement in production of quality welds is the temperature of base metal. Minimum preheat and interpass temperature as specified by the AWS and AISC standards must be obtained within 3 inches of the welded joint before welding starts and then maintained until completion. Table 7.7 gives the temperature requirements based on thickness (thickest part of joint) and welding process for several structural steels. When base metal temperature is below 32F, it must be preheated to at least 70 and maintained at that temperature during welding. No welding is permitted when ambient temperature is below 0F. TABLE 7.7 Minimum Preheat and Interpass Temperatures for Base Metal to Be Welded

Bowling alleys, alleys only Broadcasting studios Catwalks Corridors: Areas of public assembly, firstfloor lobbies Other floors same as occupancy served, except as indicated elsewhere in this table Fire escapes: Single-family dwellings only Others Garages: Passenger cars Trucks and buses Hospitals: Operating rooms, laboratories, service areas Patients rooms, wards, personnel areas Corridors above first floor Kitchens other than domestic Laboratories, scientific Libraries: Corridors above first floor Reading rooms Stack rooms, books and shelving at 65 lb / ft3, but at least Manufacturing and repair areas: Heavy Light Marques Morgue Office buildings: Corridors above first floor Files Offices Penal institutions: Cell blocks Corridors Residential: Dormitories Nonpartitioned Partitioned Dwellings, multifamily: Apartments Corridors Hotels: Guest rooms, private cooridors Public corridors Housing, one- and two-family: First floor Storage attics Uninhabitable attics Upper floors, habitable attics

To be acceptable as a means of egress, a ramp inside a building more than three stories high or a building of noncombustible or fire-resistant construction is required to be of noncombustible construction. The ramp also should be protected by separation from other parts of the building in the same way as other means of egress. There should be no enclosed usable space, such as closets, under the ramp, nor should the open space under the ramp be used for any purpose. (Other enclosed ramps, however, are permitted to be located under the ramp.) For all inside ramps, guardsvertical protective barriersshould be provided along the edges of ramps and along the edges of floor openings over ramps, to prevent falls over the open edges. Requirements for type of construction and minimum height for such barriers are the same as those for stairs (see Art. 16.3). Handrails are required only for Class B ramps. Outside Ramps. A ramp permanently installed on the outside of a building is acceptable as a means of egress if the life-safety requirements for inside egress ramps are met. For outside ramps more than three stories high, however, guards along ramp edges should be at least 4 ft high. Also, for such ramps, provision should be made to prevent accumulations of snow or ice. Powered Ramps. In some buildings, such as air terminals, in which pedestrians have to be moved speedily over long distances, traffic may be transported on a moving walk, a type of passenger-carrying powered device on which passengers stand or walk. In the moving walk, the treadway, guards, and handrails are continuous and travel parallel to the direction of motion, which may be horizontal or on a slope up to 15
. (For greater slopes, an escalator should be used. See Art. 16.4.) Although moving walks can transport passengers at speeds up to 180 ft /min, speeds are generally between 90 and 120 ft /min. Inclined moving walks are classified as powered ramps. Such ramps are acceptable as a means of egress if they meet the egress requirements of stationary ramps. (Moving walks are acceptable if they meet the requirements for exits. See Art. 3.10.5.) Powered ramps, however, must be incapable of operation in the direction opposite to normal exit travel. The installation of moving walks and powered ramps should meet the requirements of the American National Standard Safety Code for Elevators, Dumbwaiters, Escalators and Moving Walks, ANSI A17.1. Basically, moving walks and powered ramps consist of a grooved treadway moved by a driving machine; a handrail on each side of the treadway that moves at the same speed as the treadway; balustrades, or guards, that enclose the treadway on each side and support the handrails; brakes; control devices; and threshold plates at the entrance to and the exit from the treadway. The purpose of the threshold plates is to facilitate smooth passage of passengers between treadway and landing. The plates are equipped with a comb, or teeth, that mesh with and are set into grooves in the treadway in the direction of travel. Their purpose is to provide firm footing and to prevent things from becoming trapped between the treadway and the