Connections- Art, Science, And Information in the Quest for Economy and Safety

Please download to get full document.

View again

of 13
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Categories
Published
STEEL CONNECTION
  Connections: Art, Science, and Information in  the Quest for Economy  and  Safely DR. WILLIAM A. THORNTON INTRODUCTION Connections are an extremely important part of the final configuration of  a  steel structure. Many, if not most, collapses are caused by inadequate connections. The constructed cost of  a  steel frame is heavily dependent on the connections used, both the type of connection and how they are configured. Yet, connections are often an afterthought. Commercially available software will pretty much automatically design the members of the frame, but there is no commercially available software that will do the same for the connections. In fact, the frame design software chooses optimal members, usually least weight, with no regard whatsoever as to how these members will impact on the connections. The emphasis in engineering schools is similar to that of commercially available design software, i.e., it is on the design of members. Very little work is done on connections at the undergraduate level and probably also at the graduate level. Connections are considered by many professors as essentially trivial in a mathematical sense. Very sophisticated and mathematically elegant solutions can be prescribed for member and frame design; e.g., lateral torsional buckling of members, and the direct stiffness method for frames. Connec tions,  on the other  hand,  are thought  to  be designed  by no  more sophisticated analysis than counting bolts and determining weld lengths. This is not true except for the simplest connec tions.  While there are essentially only three types of members in a structural frame (beams, columns, and beam-columns), there is an almost infinite variety of connections depending on frame geometry. For this reason, connection design is actually more interesting than member design, because this great variety often requires the designer to rely on intuition and art as well as science. As mentioned above, connections are often an afterthought. In many engineering offices, once the frame is designed and on paper, the drawings are ready to be released for construction. Connections are handled by a series of typical details and general notes which refer to AISC manuals. Typical notes for shear connections might make reference to full depth connections or the Uniform Design Dr. William A. Thornton is chief engineer, Cives Steel Co. and president, Cives Engineering Corp., Roswell, GA. Load (UDL): For moment connections, the notes might say use stiffeners and doublers as required, and design for the strength of the beams. For bracing connections, a typical detail might be shown with a statement to design for all eccentricities. It is the primary purpose of this paper to show by anecdotal examples, i.e., war stories, that this approach to connections can be both uneconomical and unsafe. A secondary purpose is to give the motivation behind a new method of bracing design called the Uniform Force Method. Examples will be given of shear connections, bracing connec tions,  and moment connections. SHEAR CONNECTIONS Shear connections  are a  subject where information  is  the primary quantity lacking on most jobs. These connections have been heavily studied over the years, and other than questions regarding ductility and robustness, are well understood. Shear connections are the most common connection on all jobs.  Ideally, the engineer should give the shear for every beam end. While this may appear to be a lot of extra work, it is not as difficult as it first seems since the loads are known from sizing the beams. Why not put them on the drawing? (In addition to helping the fabricator, having the loads used in the srcinal design right on the drawing is very handy for future renovations.) If the loads are shown for every beam end, there is very little room for error, and the connections will be as economical as possible. However, instead of actual  loads,  most jobs these days have one or more of the statements regarding shear connections: ã Item  1.  All shear connections shall contain the maximum possible number of rows of bolts; ã Item 2. Design all shear connections for one-half UDL; ã Item  3.  Design all shear connections for the shear capacity of the beam; ã Item  4.  Minimum design loads for standard rolled shapes, unless noted otherwise: W8 C8 . . W10C10 W12C12 W14C15 W16 . . . W18 . . . 10 kips W21 65 kips 15 kips W24 75 kips 25 kips W27 90 kips 35 kips W30 125 kips 45 kips W33 140 kips 55 kips W36 175 kips 132 ENGINEERING JOURNAL / FOURTH QUARTER /1995  Let's consider each of these. Iteml Item  1  requires full depth connections. The fabricator assumes the engineer has reviewed his design and the capacity of these connections will exceed the actual loads in all cases. But in many cases, these will be uneconomical, as with long span beams. In other cases, they may be unsafe. Suppose a beam has a large cope, as when connecting a small beam to a large one (see Figure 1). This may greatly reduce the capacity of the full depth connection because of the reduced beam section. Has the engineer considered this, or has he reviewed his drawings by checking the actual load against the capacity of  a  full depth connection on an uncoped beam? It is very likely that he has done the latter. As a second example, consider steel at different elevations. Figure 2 shows a full depth connection for the upset W 18x35. The capacity of this full depth connection is 20k whereas a true full depth connection for the W 18x35 (Figure 3) is 49k. Will the engineer realize this if he specifies use the maximum possible number of rows ? Item 2 If in-fill beams frame near the ends of  a  main beam, the UDL method can be unsafe. If beams are short, it will be uneconomic. Figure 4 shows a floor framing plan. All beam shear connections are contractually required to be designed for one-half UDL. The three W10x22s framing between the W36xl70 and the W36x230 are 3-ft. long. The one-half UDL reaction is 61.8k Of course, this is ridiculous—but the fabricator is contractually obliged to supply it if the engineer insists, and we have done jobs where the engineer did just that. Figure 5 shows the resulting connection. Note that the shear capacity of a W10x22 is only 35.4 kips, so designing for 61.8 kips is doubly ridiculous and leads to a discussion of Item 3. Item 3 Item 3 requires the connection to develop the shear capacity of the beam, but this is impossible with the usual shear connections (single clips, double clips, shear end plate, shear tab) unless the beam is haunched or web doublers are used. Also, since most beams are coped, just what is the shear capacity of the beam? Is it the uncoped capacity (35.4 kips for the W 10x22 shown previously) or should the capacity of what is left be used? It's clear that Item  3  is ambiguous, which can lead to errors affecting safety as well as result in ridiculous designs. In Figure 6, the W 10x22 of Figure 4 has end connections good for 35.4 kips, which means the W10x22 is capable of supporting 35.4 tons Obviously, these W10x22s  are  just intended to reduce the unbraced length or provide decking support. If a real load of 35.4 tons must be carried, a short W 18x35 with five rows of bolts would be cheaper and safer. Item 4 While at first glance, Item 4 appears to be innocuous, try to develop 15k in the W 10x22 shown previously. Figure 7 results. Single Angles and UDL The uniform design load UDL is  a  great crutch of the engineer because it allows him to issue design drawings without putting the beam reactions on the drawings. Instead, often the fabricator is told to design the beam end connections for one-half UDL, or some other percentage to account for composite design, unless greater reactions are shown. Unless concentrated loads are located very near the beam ends, UDL reactions are generally very conservative. Because the reactions are too large, extremely strong connections, such as double framing angles, will often be required. Single angles, because the bolts are in single shear, will have about half the strength of double clips for the same number of rows of bolts. But if actual reactions are given, it CARRYING DEAM \ r IN-FILL BEAM -jr— 1J jjr#] Hr   I Hr   I nr   l COPE 7% X 7% CAPACITY = 20 KIPS W18 x  35 1- BOLTS A325N  >U  0 W30  x 173 Fig. 1. Usual beam to beam connection top of steel at common elevation. Fig. 2. Full depth   connection for upset beam. ENGINEERING JOURNAL/FOURTH QUARTER/ 1995 133  will almost always be found that a single angle connection will work, perhaps with a couple of extra rows of bolts. Figure 8 is part of an industrial building with dead load of 140  psf and live load of 250 psf. Beam 1, of Figure  8  is shown in Figure 9. The total load on Beam  1  is 82 kips and the actual reactions are thus 41 kips. The one-half UDL reaction is 45 kips, which is pretty close. Now look at the connections. The minimum double clip connection on this coped beam  has  four rows and is good for  81  kips, almost twice the actual reaction. Many designers routinely require full depth connections, i.e. six rows. The six row double dip connection is good for 166 kips, almost three times the actual reaction. However, a five row single angle is good for 52 kips, which is okay for the actual and the one-half UDL reactions. As this example illustrates, single angles will work even in heavy industrial applications, and they are much less expensive than double clips, especially for erection. In Figure 10, the connections for this W24x55 beam have the same strength and have a differential cost of  10  for fabrication. But, including erection, the single angle beam costs approximately $25 less than the double clip beam. For a 30-story building, 200 ft.x200 ft. with 25-ft. bays and  200 beams  per floor with single angles, there is a savings of 200 x 30 x 25 = $150,000. Returning to Figure 8, suppose Beam  1  is subjected to the same load of 82 kips total, but 32 of the 82 is a concentrated load located at mid-span, such as from a vessel. Figure 11 shows the actual reaction of the beam, now a W24x76, is still 41 kips, while the one-half UDL reaction is 56 kips—which is 37 percent greater than the actual reaction. This means while a five row single angle connection  is  okay for the actual reaction, a six row connection with a capacity of 66 kips would be required for the one-half UDL reaction. Figure 12 shows the disparity between actual and one-half UDL reactions for Beam 2. Again, single angles are sufficient. BRACING CONNECTIONS Bracing connections are subject where the art and science of connection design can be used to achieve a safe and efficient design. They are also an area where missing or misleading information can lead to drastically unsafe connections or connections which are grossly over-designed. COPE l 3 '« X 7 3 '4 r W18 x 35 CAPACITY = 49 KIPS / W30 x 173 BOLTS A325N 3 '4 0 COPE 2 x  6 l U COPE  2\  x 8''i Fig. 3. Normal full depth connection. Fig. 5. Section AA of Figure 4 W 10x22 to carry V2  UDL  =  61.8 kips reaction each end. ENGINEERS NOTE: DESIGN BEAM END CONNECTIONS FOR AXIAL FORCES SHOWN ON PLANS 24-6 ^, 24-6 COPE 2 x 6 1 ' COPE 2% x 8''i W12«96  1*8$*)   i ' p \ L £3«3>' 2 (*3S K )  TYP THIS BAT  D WT6 I8&75 K UN0)TYPTHIS BAY Fig. 4. Ambiguous forces for connection design. 1 \'3C> x 170 -] r* r* =U li= W10 x 22 —/\ A ^ / \ 'J  PL  x ã 1 ã 4 ã 4 ã 4 ã -j [ DOLT W36 r 1 1 .vu I I p Fig. 6. Section AA of Figure 4 W10x22 to carry maximum uncoped shear capacity of 35.4 kips as reactions at each end. 134 ENGINEERING JOURNAL / FOURTH QUARTER / 1995  Art and Science Figure  13  shows  a  bracing connection design method which satisfies all of the requirements  for  equilibrium for  the  gusset, the beam,  and the  column.  It  includes consideration  of all eccentricities and  it  is simple  to  use because all forces acting between  the  gusset  and the  beam  and  column  are  known before the size  of  the gusset is known.  It  has been referred  to as  the  KISS method  by  a  detailer who  was  impressing upon his people the necessity  of  getting the shop drawings out the door. Thus,  the  sarcastic comment  to   Keep  It  Simple, Stupid ,  or use the  KISS method. Unfortunately, while this method  is a  boon  to the  detailer,  it  is a  bane  to  the fabricator and  owner.  It results in large and expensive  connections.  Also, the engineer  and  owner  do not  like  it  when,  if  there  are  four gussets  in a  building panel, they almost meet  at  the  center. Also,  the  load paths through this gusset, beam,  and  column are very unnatural  and  inefficient as will  be  shown. Beginning about 15 years ago, AISC began to address this problem with a research program  at the  University of Arizona. This program resulted  in  published work  by  Richard (1986) which contained figures similar  to  Figure 14.  In  this Figure, the resultant forces  on the  gusset edges  for  a  wide variety  of gusset edge support conditions  are  seen  to  fall within  the envelopes shown. The edge resultants appear  to  intersect with the line  of  action  of  the brace  at a  point  on  this line  on the other side  of  the working point (WP) from  the  gusset. Note that  no  couples were required  in  Figure  14.  This data from Richard  is  the  genesis  of  the  author's development  of  what has come  to be  called  the  Uniform Force Method (Thornton 1991,  1992  and  AISC  1992,  1994).  The  method  is  shown  in Figure  15A.  Figure  15B  shows  a  force distribution which captures  the  essence  of  the  distributions given fuzzily  in Figure  14.  In  other words,  a  force structure  is  imposed  on Richard's data.  In  order  to  test  the  efficacy  of  this structure, the data of six full scale tests were filtered through  it.  The tests were performed by Chakrabarti and Bjorhovde, (1983,1985) and Gross  and  Cheok (1988, 1990). Typical test specimens are shown  in  Figures  16 and  17.  The  limit states considered in  the  filtering process  are  given  in  Tables  1 and 2.  Table  3 shows the results. For the Chakrabarti/Bjorhovde tests, excellent agreement  is  achieved.  The  ratio  of  test capacity  to predicted capacity  is  close  to but  slightly larger than unity  as COPE 2 x  6   U COPE  2\ x B l 't A W36 x 170 fe- ã*-  BOLTS A32SN> 4 0 *\ W36 x 230 3*0 4 ,— 3.25 Wft W21x68  v  / ~r  i i  .i i r V 25-0 BEAM 1 SeoiON UZ)*66 Lc*o» IWoftM ^FT 3Z3 COMC. hA o -&TAL fclFS 82 RlACTlCNi ACTUAL bps 41 fcUDL k-e5 i5 C.o**cncki5 DOUBLE. CLIPS MIKI ROMS Of (tori) Rows CAP i  |  81  | 6 | M6 SINGLE  CUP CAP ãb  |  S2 BOLTS  7 8 0 A  325N,  CUPS  4 X 3» 2  X 3 8 Fig.  7.  Section AA of Figure  4  W10x22  to carry 15 kip reaction each end. Fig.  9.  Comparisons for beam 1 of Figure 8 — uniform  load. (EH W33X1I9 JL±. W33xitg g-4 i g-4 SINGLE CLIPS DOUBLE CLIPS U24>55 T  rvows  5AME. STKENGTM  4 ^ows FABRICATION  - $io  PER BEAM LESS FOR SINGLE CUPS ERECTION  -  $ 15 PER BEAM LESS FOR SINGLE CUPS TOTAL COST REDUCTION  $25 PER  BEAM USING SINGLE CLIPS Fig.  8.  Partial plan of industrial building floor. Fig. 10. Cost comparison same strength single and double clips. ENGINEERING JOURNAL  /  FOURTH QUARTER  /  1995  135
Similar documents
View more...
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks