1、Connections TeachingToolkitA Teaching Guide for Structural Steel ConnectionsPerry S. Green, Ph.D.Thomas Sputo, Ph.D., P.E.Patrick VeltriConnections Teaching Toolkit iThis connection design tool kit for students is based on theoriginal steel sculpture designed by Duane S. Ellifritt, P.E.,Ph.D., Profe
2、ssor Emeritus of Civil Engineering at the Uni-versity of Florida. The tool kit includes this teaching guide,a 3D CAD file of the steel sculpture, and a shear connectioncalculator tool. The teaching guide contains drawings andphotographs of each connection depicted on the steel sculp-ture, the CAD fi
3、le is a 3D AutoCAD model of the steelsculpture with complete dimensions and details, and the cal-culator tool is a series of MathCAD worksheets thatenables the user to perform a comprehensive check of allrequired limit states.The tool kit is intended as a supplement to, not a replace-ment for, the i
4、nformation and data presented in the Ameri-can Institute of Steel Constructions Manual of SteelConstruction, Load it is a function ofthe gross shear area of the element. The failure path associ-ated with shear yielding is linear in the direction of loadfrom the top edge of the element to the bottom
5、edge andthrough the thickness of the ply.Shear rupture is an ultimate limit state; it is a function ofthe net shear area of the element. The failure path associ-ated with shear rupture is also linear, in the direction of loadfrom the top edge of the element to the bottom edge andthrough the thicknes
6、s of the ply. If both flanges of the sup-ported member are coped, then a potential shear failure pathon the beam is present and shear yielding and shear rupturemust be investigated for this member.The tension yielding limit state is a function of the grosscross-sectional area of the member subjected
7、 to tensionload. The tension rupture mode is a limit state that is a func-tion of the effective net area. The net area is the reducedgross area due to bolt holes or notches. This net area is fur-ther reduced to account for the effects of shear lag. Shearlag occurs when the tension force is not evenl
8、y distributedthrough the cross sectional area of a member. Certain geo-metric areas of a section may have higher localized stresses.Shear lag often occurs in angle members when they are usedas struts. The fastening (bolting or welding) is generallyFigure 2-11. Prying Action Limit State(Photo by J.A.
9、 Swanson and R. Leon, courtesy of Georgia Institute of Technology)Figure 2-12. Shear Yielding Limit State(Astaneh, A. and Nader, M.N. 1989)Figure 2-13. Shear Rupture Limit State(Astaneh, A. and Nader, M.N., 1989)TENSION YIELDINGAND TENSION RUPTURETY TR2-6 Connections Teaching Toolkitmade along only
10、one leg of the angle. When the angle isloaded in tension the leg that is fastened has a disproportion-ate share of the tension load. This unbalance causes a shearforce to lag across the section.WELD SHEARWeld shear is applicable to each welded ply of a connection.The failure mode for fillet welds is
11、 always assumed to be ashear failure on the effective throat of the weld. In a simi-lar fashion as bolt shear, if the load path does not passthrough the center of gravity of a weld group, then the loadis considered eccentric. Eccentrically loaded weld groupsare subject to a moment that tends to indu
12、ce either addi-tional shear (for in-plane loads) or combined shear and ten-sion (for out-of-plane loads). WHITMORE SECTION YIELDING / BUCKLINGWhitmore section yielding or buckling is a limit state thatapplies to bolted and welded gusset plates and similar fit-tings that are much wider than the patte
13、rn of bolts or weldswithin them. The stress distribution through the ends ofmembers that are attached to the gusset is complex. Thislimit state involves either the yielding or buckling of platematerial near the ends of the attached members. The Whit-more method of analysis assumes the member force i
14、s uni-formly distributed over an effective area. This effective areais determined by multiplying the gusset plate thickness byan effective length that is defined from the projection of 30-degree lines on each side of the “strut” member that is con-nected to the gusset plate. The projection is assume
15、d tooriginate at either the first row of bolts on the plate or theorigin of the weld. The projection is assumed to terminate atthe plane that passes through the last row of bolts or at theend of the welds. The 30-degree projection lines form atrapezoid, and the effective length is assumed as the bas
16、edimension of this trapezoid.Figure 2-14. Tension Fracture Limit State(Photo by J.A. Swanson and R. Leon, courtesy of Georgia Institute of Technology)Figure 2-15. Weld Shear Limit State(Astaneh, A. and Nader, M.N., 1989)WWSConnections Teaching Toolkit 2-7Figure 2-16. Whitmore Section Yielding/Buckli
17、ng Limit State(Beedle, L.S. and Christopher, R., 1964)BSR BB BS WLB FY SY SR W BSR BB BS Figure 2-17. Shear Connection; Potential Limit StatesFLB SY SR BSR BB BS BB BS W WC WLY WCB Figure 2-18. Moment Connection; Potential Limit StatesTY PA TR BT W Figure 2-19. Tension Connection (Hanger Connection
18、Potential Limit StatesWS TY TR BSR BB BS W Figure 2-20. Tension Connection (Gusset Plate); Potential Limit StatesConnections Teaching Toolkit 3-1In current construction practice, steel members are joinedby either bolting or welding. When fabricating steel forerection, most connections have the conne
19、cting materialattached to one member in the fabrication shop and the othermember(s) attached in the field during erection. This helpssimplify shipping and makes erection faster. Welding thatmay be required on a connection is preferably performed inthe more-easily controlled environment of the fabric
20、ationshop. If a connection is bolted on one side and welded onthe other, the welded side will usually be the shop connec-tion and the bolted connection will be the field connection.The use of either bolting or welding has certain advan-tages and disadvantages. Bolting requires either the punch-ing o
21、r drilling of holes in all the plies of material that are tobe joined. These holes may be a standard size, oversized,short-slotted, or long-slotted depending on the type of con-nection. It is not unusual to have one ply of material pre-pared with a standard hole while another ply of theconnection is
22、 prepared with a slotted hole. This practice iscommon in buildings having all bolted connections since itallows for easier and faster erection of the structural fram-ing.Welding will eliminate the need for punching or drillingthe plies of material that will make up the connection, how-ever the labor
23、 associated with welding requires a greaterlevel of skill than installing the bolts. Welding requires ahighly skilled tradesman who is trained and qualified tomake the particular welds called for in a given connectionconfiguration. He or she needs to be trained to make thevarying degrees of surface
24、preparation required dependingon the type of weld specified, the position that is needed toproperly make the weld, the material thickness of the partsto be joined, the preheat temperature of the parts (if neces-sary), and many other variables.STRUCTURAL BOLTINGStructural bolting was the logical engi
25、neering evolutionfrom riveting. Riveting became obsolete as the cost ofinstalled high-strength structural bolts became competitivewith the cost associated with the four or five skilled trades-men needed for a riveting crew. The Specification forStructural Joints Using ASTM A325 or A490 Bolts, pub-li
26、shed by the Research Council on Structural Connections(RCSC, 2000) has been incorporated by reference into theAISC Load and Resistance Factor Design Specification forStructural Steel Buildings. Many of the bolting standardsare based on work reported by in the Guide to Design Cri-teria for Bolted and
27、 Riveted Joints, (Kulak, Fisher andStruik, 1987).High strength bolts can be either snug tightened or pre-tensioned. When bolts are installed in a snug-tightened con-dition the joint is said to be in bearing as the plies of joinedmaterial bear directly on the bolts. This assumes that theshank of the
28、bolt provides load transfer from one ply to thenext through direct contact. Bearing connections can bespecified with either the threads included (N) or excluded(X) from the shear plane. Allowing threads to be includedin the shear planes results in a shear strength about 25% lessthan if the threads a
29、re specified as excluded from the shearplane(s). However, appropriate care must be taken to spec-ify bolt lengths such that the threads are excluded in the as-built condition if the bolts are indeed specified as threadsexcluded.In pretensioned connections, the bolts act like clampsholding the plies
30、of material together. The clamping forceis due to the pretension in the bolts created by properlytightening of the nuts on the bolts. However, the load trans-fer is still in bearing like for snug-tightened joints.The initial load transfer is achieved by friction betweenthe faying or contact surfaces
31、 of the plies of material beingjoined, due to the clamping force of the bolts being normalto the direction of the load. For slip-critical joints, the boltsare pretensioned and the faying surfaces are prepared toachieve a minimum slip resistance. The reliance on frictionbetween the plies for load tra
32、nsfer means that the surfacecondition of the parts has an impact on the initial strength ofslip-critical connections. The strength of slip-critical con-nections is directly proportional to the mean slip coefficient.Coatings such as paint and galvanizing tend to reduce themean slip coefficient. The t
33、wo most common grades of bolts available for struc-tural steel connections are designated ASTM A325 andASTM A490. The use of A307 bolts is no longer that com-mon except for the -in. diameter size where they are stillsometimes used in connections not requiring a pretensionedinstallation or for low le
34、vels of load. A307 bolts have a 60ksi minimum tensile strength. A325 and A490 bolts are des-ignated high-strength bolts. A325 bolts have a 120 ksi min-imum tensile strength and are permitted to be galvanized,while A490 bolts have a 150 ksi minimum tensile strength,but are not permitted to be galvani
35、zed due to hydrogenembrittlement concerns. High strength bolts are available insizes from - to 1-in. diameters in 1/8 in. increments andcan be ordered in lengths from 1 to 8 inches in in. incre-ments.CHAPTER 3Joining Steel Members3-2 Connections Teaching ToolkitWhen a pretensioned installation is re
36、quired, four instal-lation methods are available: turn-of-the-nut, calibratedwrench, twist off bolt, and direct tension indicator methods.The turn-of-the-nut method involves first tightening the nutto the snug tight condition, then subsequently turning thenut a specific amount based on the size and
37、grade of the boltto develop the required pretension. The calibrated wrenchmethod involves using a torque applied to the bolt to obtainthe required level pretension. A torque wrench is calibratedto stall at the required tension for the bolt. Twist-off boltshave a splined end that twists off when the
38、torque corre-sponding to the proper pretension is achieved. ASTMF1852 is the equivalent specification for A325 “twist-off”bolts. Currently, there is no ASTM specification equivalentfor A490 tension control bolts. Direct tension indicators(DTIs) are special washers with raised divots on one face.When
39、 the bolt is installed, the divots compress to a certainlevel. The amount of compression must then be checkedwith a feeler gage.WELDINGWelding is the process of fusing multiple pieces of metaltogether by heating the metal to a liquid state. Welding canoften simplify an otherwise complicated joint, w
40、hen com-pared to bolting. However, welds are subject to size andlength limitations depending on the thickness of the materi-als and the geometry of the pieces being joined. Further-more, welding should be preferably performed on baremetal. Paint and galvanizing should be absent from the areaon the m
41、etal that is to be welded.Guidelines for welded construction are published by theAmerican Welding Society (AWS) in AWS D1.1 StructuralWelding Code-Steel. These provisions have been adoptedby the AISC in the Load and Resistance Factor DesignSpecification for Structural Steel Buildings.Several welding
42、 processes are available for joining struc-tural steel. The selection of a process is due largely to suit-ability and economic issues rather than strength. The mostcommon weld processes are Shielded Metal Arc Welding(SMAW), Gas Metal Arc Welding (GMAW), Flux Core ArcWelding (FCAW), and Submerged Arc
43、 Welding (SAW). SMAW uses an electrode coated with a material thatvaporizes and shields the weld metal to prevent oxidation.The coated electrode is consumable and can be deposited inany position. SMAW is commonly referred to as stickwelding.GMAW and FCAW are similar weld processes that use awire ele
44、ctrode that is fed by a coil to a gun-shaped electrodeholder. The main difference between the processes is in themethod of weld shielding. GMAW uses an externally sup-plied gas mixture while FCAW has a hollow electrode withflux material in the core that generates a gas shield or a fluxshield when th
45、e weld is made. GMAW and FCAW can bedeposited in all positions and have a relatively fast depositrate compared to other processes.Figure 3-1. Structural Fastener - Bolt, Nut and WasherFigure 3-2. Direct Tension Indicators and Feeler GagesFigure 3-3. Structural Fastener - Twist-off BoltConnections Te
46、aching Toolkit 3-3In SAW welding, a consumable electrode is submergedbelow a blanket of granular flux. The flux protects andenhances the resulting weld. SAW tends to produce highquality welds that are strong and ductile. The major limita-tion of this process is that weld can only be deposited in the
47、flat position due to the granular flux used. This process isfrequently used for the web-to-flange connections of plategirders. The SAW process is most often found automated inthe better-controlled conditions of shop welding operations.For engineers, it is important to realize that the effectivethroa
48、t dimension for the SAW process is calculated differ-ently than for the other processes. Since the SAW processproduces higher quality welds with deeper penetration, theeffective throat is permitted to be equal to the weld throatsize if the weld is less than 3/8 in. For larger welds the effec-tive th
49、roat for SAW welds is the minimum distance from theroot to face of the weld plus 0.11 in. For the other processesthe effective throat is taken as the minimum distance fromthe root to the face of the weld (for equal legs: 0.707 leglength). There are four types of welds: fillet, groove, plug, andslot. Fillet and groove welds make up the majority of allstructural welds, therefore only those types will be dis-cussed here. There are five types of structural joints that canbe made using either fillet or groove welds. These arecalled butt, lap, tee, corner, or edge. The wel