Published on January 4, 2008
Slide2: This presentation was developed as a teaching aid with the support of the American Institute of Steel Construction. Its objective is to provide technical background and information for bolting and welding. The information provided is based on common design and construction practices for structures of twelve stories or less. The AISC Digital Library case study presentations document the construction of a steel frame for an office building. The case study includes photographs that were taken throughout the construction of the structural steel frame including detailing, fabrication, and erection. Project data including plans, schedules, specifications and other details are also included. The case study presentations are available in the Learning Opportunities section at www.aisc.org. This presentation provides technical information on bolting and welding, as well as the impacts of details and design choices on schedule, cost, sequence and overall project management. The information is presented with concerns of a construction manager or general contractor in mind. Introduction Slide3: General knowledge of structural steel An understanding of the different ways that structural steel is connected Insight into types of bolts and their installation An awareness of types of bolted joints used for structural steel Knowledge of welding terminology, weld types, and welding processes Familiarity with common weld inspection methods and considerations associated with field welding What Will You Gain From This Presentation? Slide4: Some benefits associated with use of structural steel for owners are: Steel allows for reduced frame construction time and the ability to construct in all seasons Steel makes large spans and bay sizes possible, providing more flexibility for owners Steel is easier to modify and reinforce if architectural changes are made to a facility over its life Steel is lightweight and can reduce foundation costs Steel is durable, long-lasting and recyclable (AISC 1999) Benefits of Structural Steel Slide5: Procurement and management of structural steel is similar to other materials, but there are some unique aspects to steel construction: Steel is fabricated off-site (above left) On-site erection is a rapid process (above right) This gives use of structural steel some scheduling advantages Coordination of all parties is essential for achieving potential advantages (AISC 1999) Unique Aspects of Steel Construction Slide6: Connecting Structural Steel The primary connection methods for structural steel are bolting and welding A structure’s strength depends on proper use of these connection methods Connections made in a fabrication shop are called shop connections Connections made in the field by the steel erector are called field connections Bolting and welding may be used for shop connections and field connections Slide7: A fabrication shop will have a desired fastening method suited to its equipment and fabrication methods Field connections are typically bolted Welding may be used for field connections where bolting is either impractical or undesirable Welding is better suited to the controlled environment of a fabrication shop Connecting Structural Steel Slide9: The Research Council on Structural Connections (RCSC) prepares specifications and documents related to structural connections RCSC’s Specification for Structural Joints Using ASTM A325 or A490 Bolts (2000) is a widely used specification which discusses joints, fasteners, limit states, installation, and inspections Structural Bolting Slide10: During hoisting, connectors will install a minimum of two bolts per connection The rest of the bolts are installed and tightened after the structure is plumbed A systematic pattern must be followed when tightening bolts so that a joint is drawn together and all fasteners are properly installed (SSTC 2001) Structural Bolting Slide11: Per the Occupational Safety & Health Administration Standard 1926.754(b)(2), “At no time shall there be more than four floors or 48 feet (14.6 m), whichever is less, of unfinished bolting or welding above the foundation or uppermost permanently secured floor, except where the structural integrity is maintained as a result of the design.” Structural Bolting Slide12: There are many bolt types, installation methods, and joint types used in structural steel construction When left exposed, bolts may be used to make an architectural expression (Green, Sputo, and Veltri) Structural Bolting (AISC & NISD 2000) Slide13: ASTM Bolt Types (AISC & NISD 2000) A307 – Low carbon steel Not commonly used Only used for secondary members A325 – High-strength medium carbon steel (above left) Most common bolts used in building construction A490 – High-strength heat treated steel (above right) Cost more than A325’s, but are stronger so fewer bolts may be necessary Note that the ASTM designation is indicated on the head of the bolts above Slide14: A325 and A490 bolts are available in diameters ranging from 1/2” to 1-1/2” The most common sizes are 3/4”, 7/8”, and 1” High-strength bolts are commonly available in incremental lengths up to 8” (AISC) Common Bolt Sizes Slide15: Hardened steel washers are used in many structural connections to spread pressure from the bolt tightening process over a larger area Washers may also be used to cover an oversized or slotted hole (RCSC 2000) Flat washers are most commonly used Tapered washers (above left) are used when the surface being bolted has a sloped surface, such as the flange of a channel or an S shape A325 bolts require a washer under the element (head or nut) being turned to tighten the bolt (shown under the nut, above right) A490 bolts require a washer under both the head and nut (AISC & NISD 2000) Washers Slide16: Grip is the distance from behind the bolt head to the back of the nut or washer It is the sum of the thicknesses of all the parts being joined exclusive of washers Thread length is the threaded portion of the bolt Bolt length is the distance from behind the bolt head to the end of the bolt (AISC & NISD 2000) Parts of the Bolt Assembly Head Shank Washer Nut Washer Face Grip Thread Length Slide17: There two basic bolted joint types: Bearing The load is transferred between members by bearing on the bolts Slip-critical The load is transferred between members by friction in the joint Bolted Joint Types Slide18: Bolts in bearing joints are designed to meet two limit states: Yielding, which is an inelastic deformation (above left) Fracture, which is a failure of the joint (above left) The material the bolt bears against is also subject to yielding or fracture if it is undersized for the load (above right) Tension connections act similarly to bearing connections Many times, connections in direct tension are reconfigured so that the bolts act in shear (AISC) Bolted Joint Failure Modes Slide19: In a bearing joint the connected elements are assumed to slip into bearing against the body of the bolt If the joint is designed as a bearing joint the load is transferred through bearing whether the bolt is installed snug-tight or pretensioned (AISC) Bearing Joints Slide20: The shear plane is the plane between two or more pieces under load where the pieces tend to move parallel from each other, but in opposite directions The threads of a bolt may either be included in the shear plane or excluded from the shear plane The capacity of a bolt is greater with the threads excluded from the shear plane The most commonly used bolt is an ASTM A325 3/4” bolt with the threads included in the shear plane (AISC & NISD 2000) Threads in the Shear Plane Threads Included In The Shear Plane Threads Excluded From The Shear Plane Slide21: In a slip-critical joint the bolts must be fully pretensioned to cause a clamping force between the connected elements This force develops frictional resistance between the connected elements The frictional resistance allows the joint to withstand loading without slipping into bearing against the body of the bolt, although the bolts must still be designed for bearing The faying surfaces in slip-critical joints require special preparation (AISC) Slip-Critical Joints Slide22: When to Use Slip-Critical Joints Joints that are subject to fatigue load with reversal of the loading direction (not applicable to wind bracing) Joints that utilize oversized holes Joints that utilize slotted holes, except those with applied load approximately perpendicular to the direction of the long dimension of the slot Joints in which slip at the faying surfaces would be detrimental to the performance of the structure Per the RCSC Specification (2000), Slip-critical joints are only required in the following applications involving shear or combined shear and tension: Slide23: Snug-tight is the tightness attained with a few hits of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the connected plies into firm contact (RCSC 2000) Snug-tight Installation Slide24: Installation beyond snug-tight is called pretensioning Turn-of-nut pretensioning involves several steps: The bolt is snug-tightened Matchmarks are placed on each nut, bolt, and steel surface in a straight line The part not turned by the wrench is prevented from turning The bolt is tightened with a prescribed rotation past the snug-tight condition The specified rotation varies by diameter and length (between 1/3 and 1 turn) (RCSC 2000, AISC) Turn-of-Nut Installation Slide25: Calibrated Wrench Installation Calibrated Wrench pretensioning uses an impact wrench (above left) to tighten the bolt to a specified tension A Skidmore-Wilhelm calibration device (above right) is used to calibrate the impact wrench to the torque level which will achieve the specified tension A sample of bolts representative of those to be used in the connections are tested to verify that the correct tension will be achieved (RCSC 2000, AISC) Slide26: F1852 bolts are twist-off-type tension-control bolts These bolts must be pretensioned with a twist-off-type tension-control bolt installation wrench that has two coaxial chucks The inner chuck engages the splined end of the bolt The outer chuck engages the nut The two chucks turn opposite to one another to tighten the bolt The splined end of the F1852 bolt shears off at a specified tension (AISC 2003) ASTM F1852 Installation (AISC) Slide27: Another way to try to ensure proper pretensioning of a bolt is through the use of direct tension indicators (DTIs) These washers have protrusions that must bear against the unturned element As the bolt is tightened the clamping force flattens the protrusions and reduces the gap The gap is measured with a feeler gage When the gap reaches the specified size the bolt is properly pretensioned (AISC & NISD 2000) ASTM F959 Direct Tension Indicators Feeler Gages DTI’s Slide28: It is essential that direct tension indicators be properly oriented in the assembly The bolt head is stationary while the nut is turned – DTI under bolt head The bolt head is stationary while the nut is turned – DTI under nut (washer required) The nut is stationary while the bolt head is turned – DTI under bolt head (washer required) The nut is stationary while the bolt head is turned – DTI under nut (RCSC 2000) Installation of DTIs (Adapted from Figure C-8.1 RCSC 2000) Slide29: Bolts are installed in one of four types of holes (see table above) Standard holes can be used anywhere Oversized holes may only be used in slip-critical connections Short-slotted holes are used with the slot perpendicular to the direction of stress Long-slotted holes are primarily used when connecting to existing structures Nominal Bolt Hole Dimensions (Table 3.1 RCSC 2000) Slide30: Common tools used by Ironworkers include spud wrenches, pins, and corrections bars of various sizes (above left) Impact wrenches will be needed for certain installations (above center) Electricity or compressed air is required depending on the impact wrench being used A generator as well as an air compressor may be needed (above right) Equipment Requirements Slide31: Per the RCSC Specification: Fastener components must be protected from dirt and moisture in closed containers on the jobsite Only fasteners anticipated to be installed during the work shift are to be taken from protected storage Protected storage is defined as the continuous protection of fastener components in closed containers in a protected shelter Any unused fasteners must be promptly returned to protected storage Storage of Components Slide32: The lubrication on fasteners is vital to their proper installation A water-soluble oil is used on most black bolts This oil is easily washed off when exposed to moisture Fasteners that accumulate rust or dirt must be cleaned and relubricated before they may be installed F1852 bolts (shown above) shall not be relubricated, except by the manufacturer (RCSC 2000, SSTC 2001) Storage of Components Slide33: Storage of Galvanized Fasteners Galvanized bolts and nuts (above) are provided by the supplier in a set and special storage requirements Each bolt/nut set is pretested by the supplier and shipped together and must be kept together as an assembly Poor thread fit may result if the bolt and nut are mismatched The lubrication on galvanized fasteners is generally more durable than that on black bolts, but protected storage is still recommended A490 bolts are not allowed to be galvanized (SSTC 2001) Slide34: Production lot traceability is required by many standards Even if not required, it is good practice to record the lot numbers and keep all fasteners separated by lot It is necessary to keep lots separate for proper pre-installation verification testing which is required for pretensioned and slip-critical joints Mixing bolts and nuts from different production lots is not permitted (SSTC 2001) Production Lots Slide35: Inspections In addition to the erector’s quality control program, tests and inspection are specified by the Engineer of Record and/or the local building authority A local building inspector may request that tests in addition to those specified by the Engineer of Record be performed Snug-tightened joints require visual inspection for firm contact and proper use of washers Pretensioned joints require pre-installation verification and routine observation of proper application Slip-critical joints require inspection of the faying surfaces in addition to the above inspections Slide36: Inspections for the Construction Manager There are several bolted connection inspections a construction manager can perform: Look at the bolt stick-out (above) Stick-out is the amount the bolt extends beyond the outside surface of the nut Positive or zero stick-out is acceptable Negative stick-out, where the end of the bolt is inside the nut, is not acceptable Slide37: Inspections for the Construction Manager Inspect the turn-of-nut matchmarks to ensure the bolts have been pretensioned If F1852 bolts are used, make sure the ends have been snapped off all bolts (above) In some cases, due to insufficient clearance for the installation wrench, F1852 bolts will be tightened by alternative methods so the ends will not be snapped off Slide38: The erector may prefer certain bolt and joint types over others due to equipment requirements, experience, and installation times Snug-tightened joints are normally the most economical bolted joints (Ruby 2003) For pretensioned joints, F1852’s and DTI’s are popular and can be economical Slip-critical joints are the most costly joints, and should only be specified when necessary (Ruby 2003) Bolting Cost Considerations The types of joints used in a structure are somewhat dependent on the overall design of the structure, but these are some points to consider: Slide40: Another common method for connecting structural steel is welding Welding can be performed in the shop or in the field Many fabrication shops prefer to weld rather than bolt Welding in the field is avoided if possible due to welding condition requirements There are several welding processes, types, and positions to be considered in building construction Structural Welding Slide41: The American Welding Society (AWS) is a nonprofit organization with a goal to advance the science, technology and application of welding and related joining disciplines AWS develops codes, recommended practices, and guides under strict American National Standards Institute (ANSI) procedures D1.1 Structural Welding Code – Steel, one of the most consulted codes in the world, is produced by AWS (AWS 2004a) Structural Welding Slide42: Welding is the process of fusing multiple pieces of metal together by heating the filler metal to a liquid state A properly welded joint is stronger than the base metal Structural Welding Slide43: Welds may be loaded in shear, tension, compression, or a combination of these Capacities for welds are given in the AISC Specification Section J2 (2005) The strength of a weld is dependent on multiple factors, including: base metal, filler metal, type of weld, throat and weld size Strength of Structural Welds (Part of Table J2.5 AISC 2005) Slide44: Tack Weld (above left) A temporary weld used to hold parts in place while more extensive, final welds are made Continuous Weld A weld which extends continuously from one end of a joint to the other Stitch Weld (above right) A series of welds of a specified length that are spaced a specified distance from each other Welding Terminology Slide45: Shown above are types of structural joints which are established by positions of the connected material relative to one another Lap, tee, and butt joints are most common (AISC) Welding Terminology Slide46: Weld types define the configuration of the weld and its underlying design approach Fillet welds and groove welds are most common Groove welds fall into two categories Full penetration – the entire member cross-section is welded Partial penetration – just part of the member cross-section is welded (AISC) Welding Terminology Slide47: The most commonly used weld is the fillet weld Fillet welds are theoretically triangular in cross-section Fillet welds join two surfaces at approximately right angles to each other in lap, tee, and corner joints (AISC & NISD 2000) Fillet Welds Slide48: Groove welds are specified when a fillet weld is not appropriate for the job The configuration of the pieces may not permit fillet welding A strength greater than that provided by a fillet weld is required Groove welds are made in the space or groove between the two pieces being welded (AISC & NISD 2000) Groove Welds Slide49: The bevel or “J” preparation extends over most of or the entire face of the material being joined Complete fusion takes place In some types of full penetration groove welds the material will be beveled from one side of the plate with a separate plate on the opposite side – called backing or a backing bar (AISC & NISD 2000) Full Penetration Groove Welds Slide50: Partial joint penetration welds are used when it is not necessary for the strength of the joint to develop the full cross section of the members being joined (AISC & NISD 2000) Partial Penetration Groove Welds Slide51: There are four recognized welding positions: Flat – The face of the weld is approximately horizontal and welding is performed from above the joint Horizontal – The axis of the weld is horizontal Vertical – The axis is approximately vertical or in the upright position Overhead – Welding is performed from below the joint The flat position is preferred because it is easier and more efficient to weld in this position (AISC & NISD 2000) Welding Positions Slide52: Weld symbols are used to communicate the specific details and requirements of each weld to the welder Weld symbols are included on fabrication and erection drawings Weld Symbols Leader Line Horizontal Weld Line Tail Basic Weld Symbol (Fillet weld symbol shown) Note (Indicating this is a typical weld) Length and Spacing of weld (In Inches) Size of weld (In Inches) Field Weld Symbol Slide53: Weld Size The size of a weld must match the size specified on the drawings Some welds may meet the required size after a single pass of the welder Larger weld sizes may require multiple passes to meet the size requirement Common single pass welds include fillet welds up to and including 5/16 inch and thin plate butt welds with no preparation Common multiple pass welds include single bevel full penetration groove welds, single bevel partial penetration groove welds, and fillet welds over 5/16 inch The weld in the above picture is a multiple pass fillet weld Slide54: Weld Accessibility Access holes are required for some welds, such as the welded flange connection shown to the right The top access hole allows for a continuous backing bar to be placed under the top flange The bottom access hole allows for complete access to weld the entire width of the bottom flange A detail of a weld access hole for a welded flange connection is shown below Column Seat Angle Weld Access Holes Backing Bar Extension Bar (Adapted from AISC 2001) (Adapted from AISC 2002a) Slide55: Shielded Metal Arc Welding (SMAW) is also known as manual, stick, or hand welding An electric arc is produced between the end of a coated metal electrode and the steel components to be welded The electrode is a filler metal covered with a coating The electrode’s coating has two purposes: It forms a gas shield to prevent impurities in the atmosphere from getting into the weld It contains a flux that purifies the molten metal (AISC & NISD 2000) SMAW Welding Slide56: Gas Metal Arc Welding (GMAW) is also known as MIG welding It is fast and economical A continuous wire is fed into the welding gun The wire melts and combines with the base metal to form the weld The molten metal is protected from the atmosphere by a gas shield which is fed through a conduit to the tip of the welding gun This process may be automated (AISC & NISD 2000) GMAW Welding Slide57: Flux Cored Arc Welding (FCAW) is similar to the GMAW process The difference is that the filler wire has a center core which contains flux With this process it is possible to weld with or without a shielding gas This makes it useful for exposed conditions where a shielding gas may be affected by the wind (AISC & NISD 2000) FCAW Welding Slide58: Submerged Arc Welding (SAW) is only performed by automatic or semiautomatic methods Uses a continuously fed filler metal electrode The weld pool is protected from the surrounding atmosphere by a blanket of granular flux fed at the welding gun Results in a deeper weld penetration than the other process Only flat or horizontal positions may be used (AISC & NISD 2000) SAW Welding Slide59: Equipment used for welding will vary depending on the welding process and whether the welding is being done in the shop or in the field A Flux Cored Arc Welding machine for shop welding is pictured above left A Shielded Metal Arc Welding machine for field welding is pictured above right Welding Equipment Slide60: Welding in the field is avoided if possible due to welding condition requirements Field welding is not to be performed while it is raining, snowing, or below 0° F In certain ambient temperatures preheating of the material to be welded is required AWS Code D1.1 (2004b) specifies minimum preheat and interpass temperatures, which are designed to prevent cracking Weather Impacts on Welding Slide61: It is important for both the welder and those working in the area around a welding process to be safety conscious The welding arc should never be looked at with the naked eye AWS publishes many safety and health fact sheets which are available for download at their web site: www.aws.org Welding Safety Slide62: A welder should wear the proper protective gear including: Welding Safety Helmet Face shield or goggles Gloves Boots Heavy fabric or leather shirt Cuffless pants Leather leggings Slide63: Welding to existing structures during retrofit projects requires careful consideration of numerous factors: Welding in Existing Structures Determine weldability – Identify the steel grade to establish a welding procedure Select and design the weld – Fillet welds are preferred and avoid over welding Surface preparation – Remove contaminants such as paint, oil, and grease Loads during retrofit – An engineer should determine the extent to which a member will be permitted to carry loads while heating, welding, or cutting Fire hazards – Follow all governing fire codes, regulations, and safety rules to avoid fires For complete details see the AISC Rehabilitation and Retrofit Guide (2002b) Slide64: Weld Inspections In addition to the erector’s quality control program, tests and inspections are specified by the Engineer of Record and/or the local building authority A local building inspector may request that tests in addition to those specified by the Engineer of Record be performed Some problems that can be found in welds include: Lack of fusion Porosity Cracks Insufficient penetration There are several weld tests and inspections that are commonly used Wrong size Poor workmanship Slide65: Visual inspection is the most frequently used inspection and is the only inspection required unless the specification calls for a more stringent inspection method Inspection is done by the welder before, during, and after welding When outside inspection is required it should also be done before, during, and after welding Minor problems can be identified and corrected before the weld is complete (AISC & NISD 2000) Visual Inspection Slide66: Dye penetrant testing locates minute surface cracks and porosity Dye types that may be used include: Color contrast dye - which shows up under ordinary light Fluorescent dye – which shows up under black light The dye is normally applied by spraying it directly on the weld (AISC & NISD 2000) Dye Penetrant Test Slide67: Magnetic particle inspection uses powdered magnetic particles to indicate defects in magnetic materials A magnetic field is induced in the part The magnetic powder is attracted to and outlines cracks within the material (AISC & NISD 2000) Magnetic Particle Inspection Slide68: Ultrasonic inspection can be used to detect flaws inside welds High frequency sound waves are directed into the metal with a probe held at a specific angle The flaws reflect some energy back to the probe Flaws show up as indications on a screen (above) and are subject to interpretation by an inspector (AISC & NISD 2000) Ultrasonic Inspection Slide69: Radiographic inspection, or X-ray, can also be used to detect flaws inside welds Invisible rays penetrate the metal and reveal flaws on an x-ray film or fluorescent screen (above) This is the most costly of the inspection methods (AISC & NISD 2000) Radiographic Inspection Slide70: Fillet weld is less expensive than groove weld No special preparation No backing required Less volume of weld Partial penetration groove weld is less expensive than full penetration groove weld Labor represents the majority of the cost associated with welding Welding Cost Considerations Slide71: Bolting is generally a faster operation than welding Bolting does not have the temperature and weather condition requirements that are associated with welding Unexpected weather changes may delay welding operations Bolting and Welding Scheduling Considerations Slide72: Structural Steel: The Material of Choice Slide73: References AISC. (n.d.). Steel Connections: Behavior and Practice [35mm Slide Show with Script]. American Institute of Steel Construction, Inc. Chicago, IL. AISC. (2001). LRFD Manual of Steel Construction, Third Edition. American Institute of Steel Construction, Inc. Chicago, IL. AISC. (2002a). Seismic Provisions for Structural Steel Buildings. American Institute of Steel Construction, Inc. Chicago, IL. AISC. (2002b). Design Guide 15 – AISC Rehabilitation and Retrofit Guide. American Institute of Steel Construction, Inc. Chicago, IL. AISC. (2003). High Strength Bolts: A Primer for Structural Engineers. American Institute of Steel Construction, Inc. Chicago, IL. AISC & NISD. (2000). Detailer Training Series [CD-ROM set]. American Institute of Steel Construction, Inc. and National Institute of Steel Detailing. Chicago, IL. (Available from AISC, One East Wacker Drive, Suite 3100, Chicago, IL 60601). American Welding Society, (AWS). (2004a). American Welding Society Web Site. Available at: http://www.aws.org/. Viewed August, 2004. American Welding Society, (AWS). (2004b). “Structural Welding Code.” ANSI/AWS D1.1-2004, Miami, FL. Green, P. S., Sputo, T., and Veltri, P. (n.d.). Connections Teaching Toolkit – A Teaching Guide for Structural Steel Connections. American Institute of Steel Construction, Inc. Chicago, IL. Research Council on Structural Connections, (RCSC). (2000). Specification for Structural Joints Using ASTM A325 or A490 Bolts. American Institute of Steel Construction, Inc. Chicago, IL. Ruby, D.I. (2003) . “All About Bolts.” AISC Modern Steel Construction, May. SSTC. (2001). Structural Bolting Handbook. Steel Structures Technology Center, Inc. Novi, MI.