AISC Live Webinars. AISC Live Webinars. High-Strength Bolts: The Basics. AISC Live Webinars. There s always a solution in steel.

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1 AISC Live Webinars Thank yo for joining or live webinar today. We will begin shortly. Please standby. Thank yo. Need Help? Call ReadyTalk Spport: AISC Live Webinars Today s adio will be broadcast throgh the internet. Alternatively, to hear the adio throgh the phone, dial International callers, dial For additional spport, please press *0 and yo will be connected to a live operator. There s always a soltion in steel. There s always a soltion in steel. AISC Live Webinars Today s live webinar will begin shortly. Please standby. As a reminder, all lines have been mted. Please type any qestions or comments throgh the Chat featre on the left portion of yor screen. High-Strength Bolts: The Basics Presented by Geoff Klak, Ph.D. Professor Emerits at the University of Alberta Today s adio will be broadcast throgh the internet. Alternatively, to hear the adio throgh the phone, dial International callers, dial For additional spport, please press *0 and yo will be connected to a live operator. There s always a soltion in steel. There s always a soltion in steel. There's always a soltion in steel. 1

2 High-Strength Bolts: The Basics Fndamentals and Behavior Specification Reqirements (AISC 2010) Role of the Strctral Engineer Selection of sitable bolt types and grades Design of the fasteners Responsibility for installation Responsibility for inspection 5 6 ASTM A307 Bolts often a good choice when loads are static strength level inferior to high- strength bolts (60( ksi tensile lt.) pretension indeterminate ASTM A325 Bolts Type 1 or Type 3 (weathering steel) ASTM Spec. RCSC Spec. Minimm tensile strength: 120 ksi Pretension can be indced if desired 7 8 There's always a soltion in steel. 2

3 ASTM A490 Bolts Types 1 or Type 3 (weathering steel) Minimm tensile strength: 150 ksi, (maximm 170 ksi) ASTM Spec. RCSC Spec. Pretension can be indced if desired bolt tension kips Comparison of Bolts: Direct Tension /8 in. dia. A490 bolt 7/8 in. dia. A325 bolt 7/8 in. dia. A307 bolt elongation (inches) 10 Comments Note: we qote the ltimate tensile strength of the bolt benchmark for strength statements (e.g. shear strength is some fraction of ltimate tensile strength) What abot yield strength? What is proof load comments cont d Nts: ASTM A563 Washers: if needed, ASTM F436 Bolt nt washer sets implied so far, bt other configrations available Bolt notation: Grop A (A325, F1852) and Grop B (A490, F2280) There's always a soltion in steel. 3

4 Loading of Bolts Shear load transfer by shear in bolt and bearing in connected material OR load transfer by friction (followed by shear and bearing) Tension Combined Tension and Shear Shear Loading Trss Joint Bolts Loaded in Tension Bolts in Tension prying Prying force Bolt force bolts in tension these bolts in shear Applied force High-strength bolts in tension can be a sorce of problems There's always a soltion in steel. 4

5 Bolts in combined tension and shear Consider a simple joint bolts in shear bolts in combined shear and tension P P P Free body of bolt P { and associated shear stress P P τ = P A { a bearing force Finally... P/2 P P/2 this force is eqal and opposite to the bearing force shown previosly d t There's always a soltion in steel. 5

6 In the example, we identified force in the bolt (a shear force) force that the bolt imposed on the plate (a bearing force) force in the plate itself (a tensile force) force transfer cold also be by friction: not inclded in this illstration AISC Standard 2010 Parallel LRFD and ASD rles LRFD ses a resistance factor, Ø ASD ses a safety factor, Ω Loads as appropriate: factored loads for LRFD non-factored loads for ASD AISC Specification cont d LRFD: req d strength LRFD φ R n ASD: req d strength ASD R n / Ω Installation Sng-tight only Pretensioned Calibrated wrench Trn-of of-nt Other means: Tension control bolts Load-indicator washers There's always a soltion in steel. 6

7 Behavior of a large joint (shear splice) Bolts in Shear: Isses average bolt shear MPa deformation over α, mm 25 Shear strength of bolt (single shear or doble shear, threads in shear plane?) Bearing capacity of bolt (never governs) Bearing capacity of plate Tensile (comp.) capacity of plate 26 Slip in bolted joints Can be as mch as two hole clearances Some bolts will already be in bearing at start of loading Both laboratory tests and field measrements indicate that slip is more like 1/2 hole clearance Bolts in shear-type connections: Specifications inclde information for: bearing type connections slip-critical connections There's always a soltion in steel. 7

8 Bearing-type connections: Bolts in bearing-type connections Isses bolt shear strength bearing capacity of connected material member strength load Region of bearing- type behavior Shear strength of bolts is not dependent on presence or absence of pretension. (How come?) deformation Bolt Shear Strength Individal bolt in shear Bolt shear strength 62% of bolt ltimate tensile strength (theory( + tests) Design rle takes 90% of this vale Threads in shear plane? Long joint effect: another discont applied There's always a soltion in steel. 8

9 Physical test Uneven loading of bolts (End for bolts of 13) Bolt Pretension v. Shear The bolt pretension is attained as a reslt of small axial elongations introdced as nt is trned on These small elongations are relieved as shear deformations and shear yielding take place Confirmed by both bolt tension measrements and shear strength tests Back to bolt in shear Shear strength of single bolt (tests) τ = 0.62 σ bolt So, bolt shear strength NOT dependent on pretension in the bolt. Shear deformation There's always a soltion in steel. 9

10 Bolts in Shear AISC φ R = φ F n nv A φ R n = design shear F nv = nominal shear strength, ksi b strength 37 nominal shear strength F = 90% (0.625 F ) = F nv e.g. A325 bolt, no threadsin shear plane, Grop A : see tablatedvalein Table J3.2 (0.563 ksi F = x 120 ksi = 68 ksi) 38 and For threads inclded,, the tablated vales are 80% of the above. Comments The discont for length (se of 90%) is conservative If joint length > 38 in., a frther redction, to 83% The ø vale sed for this case (0.75) is conservative There's always a soltion in steel. 10

11 Let s s retrn now to slip-critical connections Slip-Critical Connection Clamping force from bolts (bolt pretension) Load at which slip takes place will be a fnction of? 42 Bolts in slip-critical connections load deformation region of slip-critical joint behavior 43 Slip-critical joints specified when Load is repetitive and changes from tension to compression (fatige by fretting cold occr.) Change in geometry of strctre wold affect its performance. Certain other cases. Comment: : for bildings, slip- critical joints shold be the exception, not the rle (bt, see also seismic rles) 44 There's always a soltion in steel. 11

12 First principles, slip resistance is P = k n Σ s T i k s = slip coefficient (µ) n = nmber of slip planes (sally 1 or 2) T i = clamping force (i.e., bolt pretension) Design slip resistance, AISC R n = μ D slip coefficient no. slip planes clamping force terms h f and D need to be defined and a vale inserted for Ø h f T b n s and the modifiers h f = modifier re fills : either 1.0 or 0.85 φ = 1.0 for std. holes and for short slots = 0.85 for oversize and short slots parallel = 0.70 for long slotted holes D = 1.13, ratio of installed bolt tension to specified minimm bolt tension μ = 0.30 clean mill scale, hot dipped galvanized and roghened,etc.(class A srfaces) μ = 0.50 npainted and blast cleaned, etc. (Class B srfaces) A note for advanced readers!! D is a statistical parameter that reslts in a probability of slip of 5% at the service load level when the joint is designed sing factored loads. The resistance factor reflects the conseqence of exceeding the slip limit state. As the conseqence of slip gets more severe, the resistance factor is decreased There's always a soltion in steel. 12

13 Bolts in Tension Capacity of a bolt in tension: prodct of the ltimate tensile strength of the bolt and the tensile stress area of the bolt (i.e. F A st ) Specifications directly reflect this calclated capacity ( to( come) Force in bolt mst reflect any prying action effect Bolts in Tension some comments Preference: avoid joints that pt bolts into tension, especially if fatige is an isse Use A325 bolts rather than A490 bolts Minimize the prying action Qestion pretensioned bolt in a connection apply external tension force to the connection do the bolt pretension and the external tension add? Bolt tension + external tension 1. Pretension the bolt tension in the bolt, compression in the plates 2. Add external tension force on connection Bolt tension increases Compression between plates decreases Examine eqilibrim and compatibility There's always a soltion in steel. 13

14 And the reslt is AISC rle, bolts in tension The bolt force does increase, bt not by very mch ( 7%) This increase is accommodated within the design rle. φ R n = φ F nt A b φ R n = design bolt area for nominal diameter nominal tensile strength tensile strength What is nominal tensile strength, F nt? Plt = F Ast = or, P = lt F 0.75 F { (0.75A A Call this F nt b b ) Adjsted area So, the AISC rle for bolts in tension φ R = φ F n n t A b where F nt = 0.75 F as tablated in the Specification As we now know, the 0.75 really has nothing to do with F There's always a soltion in steel. 14

15 Retrning to shear splice joints, we still have to deal with the bearing capacity of the connected material. P/2 P P/2 d t 57 Bearing capacity (of connected material) Shear-ot of a block of material or yielding 58 Bearing stresses at bolt holes s L e Shear-ot rle t 1 d L c t 2 Needed: 1. shear-ot rle 2. yield rle (deformation) Shear - ot is or, R n 2 ( τ lt = 2 ( 0.75 σ L L c c t) t) and AISC rle is: R n = 1.5 F L c t There's always a soltion in steel. 15

16 Plate bearing from tests: σ σ b = pl L e d..after some arithmetic R n = σ d t = σ b pl Le d t d Plate bearing Making the sbstittion and sing F σ pl R n = 3 d t F valid for Le 3 d Finally, the AISC rle for plate bearing capacity is Rn = 1.5 F Lct 3.0 d t F (with a φ-vale still to be inserted) Frther note re bearing When deformation a consideration, se R n = 1.2 F L c t 2.4 d Why this difference, and when do we se the latter? (vale of φ still to be applied) t F There's always a soltion in steel. 16

17 Block shear rptre Failre (lt. load) is by tensile fractre at location shown, regardless of geometric proportions. Shear yield along vertical planes. Failre is controlled by dctility not strength Basics T r where and + V r = φ A A A nt gv nt F φ A = net area in tension gv = gross area in shear F y An example of shear + tension failre in a coped beam tension fractre shear yield and some other reqirements, inclding specific case of coped beams, limit on shear There's always a soltion in steel. 17

18 Back to installation Bearing-Type Connections Installation of Bolts Bolts can be installed to sng-tight condition ordinary effort of worker sing a spd wrench. (Pretension nknown, bt sally small) Installation 1. Calibrated Wrench Installation Reliable relationship between torqe and resltant bolt tension? NO! (and forbidden by RCSC) bring parts together, contine trning nt, bolt elongates, tension develops in bolt, and clamped parts compress Establish relationship by calibration of the installing wrench There's always a soltion in steel. 18

19 Hydralic calibrator Calibrated wrench, cont d Adjst wrench to stall or ct ot at desired level of bolt pretension Target vale of pretension (RCSC) is 1.05 times specified min. vale Calibrate sing at least three bolts Calibration is niqe to bolt lot, length, diameter, grade of bolt Washers mst be sed Trn-of of-nt Installation Rn nt down, bring parts into close contact Work from stiffer regions to edges Establish sng-tight condition (first impact of impact wrench or fll effort of worker sing a spd wrench) Apply additional one-half trn (or other vale, depending on bolt length) Does this definition of sng-tight seem a little vage? How inflential is sng-tight? There's always a soltion in steel. 19

20 60 Bolt Tension by Trning the Nt 60 Bolt Tension by Trning the Nt bolt tension (kips) 40 specified minimm tension bolt tension (kips) 40 specified minimm tension range of bolt elongations at sng bolt elongation (in.) bolt elongation (in.) bolt elongation at one-half trn bolt elongation at one-half trn Inspection of Installation Principles: Determination of the bolt pretension after installation is not practical Understand the reqirements e.g., are pretensioned bolts reqired? Monitor the installation on the site Proper storage of bolts is reqired Inspection of Installation Is bolt tension reqired? if not, why inspect for it! Know what calibration process is reqired and monitor it on the job site Observe the work in progress on a reglar basis There's always a soltion in steel. 20

21 Inspection of installation: Consider the following AISC cases 1. Bolts need be sng-tight only 2. Bolts are pretensioned (bt not a slip- critical joint) 3. Slip-critical joint Sng tight only req d. Bearing-type connections Bolts in tension (A325 only) only when no fatige or vibration (bolt cold loosen) Inspection sng tight Bolts, nts, and washers (if any) mst meet the reqirements of the specifications Hole types (e.g., slotted, oversize) mst meet specified reqirements Contact srfaces are reasonably clean Parts are in close contact after bolts sngged All material within bolt grip mst be steel Inspection: if pretensioned bolts reqired All of reqirements for sng-tight case Observe the pre-installation verification process trn of nt, or; calibrated wrench, or; other (direct tension washers, tension-control bolts) Calibration process done minimm once per day Calibration process done any time conditions change There's always a soltion in steel. 21

22 Inspection: for slip-critical joints An inspected joint (trn-of of-nt) All of the above, pls Condition of faying srfaces, holes, etc. In addition to observing the calibration process, the inspection mst ensre that the same process is applied to the field joints and some other comments Pretension vales greater than those specified are not case for rejection. Rotation tests are sefl for short-grip bolts or coated fasteners (reqirement is in ASTM A325 spec. and is for galvanized bolts) There's always a soltion in steel. 22

23 Actal pretensions, cont d For A325 bolts, trn-of-nt: Average tensile strength exceeds spec. min. tensile by abot 1.18 Average pretension force is 80% of actal tensile Reslt is that actal bolt tension is abot 35% greater than specified bolt tension Actal pretensions, cont d A325, ½ trn-of-nt: 35% increase A490, ½ trn-of-nt: 26% increase A325 and A490, calibrated wrench: 13% increase etc. for other cases Note: these increased pretensions are embodied in the specification rles Some other options for bolts Tension Control Bolts region of constant torqe ASTM F1852, F2280 groove at which shear will take place There's always a soltion in steel. 23

24 Tension control bolts. NOTE: evidence that tips have sheared off is not in itself evidence that desired pretension is present Consider limits: Friction conditions are very high Friction conditions are very low Hence, calibration is essential! Tension-Control Bolts Advantages Installation is from one side Electric wrench is sed Installation is qiet Disadvantages More expensive Pre-installation calibration reqired Direct tension indicators Direct Tension Indicators Protrsions formed in special washer Protrsions compress as force in bolt is developed Use feeler gage to measre gap (or refsal) User mst verify the process (like calibrated wrench) ASTM There's always a soltion in steel. 24

25 Reliability of these... Calibration reqired Reliability same as calibrated wrench Tension-control bolt is torqe-dependent Load-indicating washer is elongation- dependent Some additional topics Details, other topics washers (bt not today!) slotted or oversize holes (bt not today!) seismic design Seismic design of connections Analyze strctre in order to compte the forces Use FEMA 350 and/or AISC Seismic Design Spec. With forces now known, design connectors Advisable to se pre-qalified configrations 99 Pre-qalified bolted connections Note: some details not shown, e.g., continity plates 100 There's always a soltion in steel. 25

26 American Institte of Steel Constrction bolted joints, seismic design All bolts pretensioned Faying srfaces as per slip-critical Use bearing vales for bolts AllAll-bolted connection moderate qakes: no slip major qakes: slip will occr and bolts go into bearing Normal holes or short slotted only (perpendiclar) No bolts + welds in same faying srface 101 Seismic design, cont d 102 It all started with rivets. Non-dctile limit state in either member or connection mst not govern. Calclate bolt shear strength as per bearing type bt se 2.4 d t F bearing rle Mst se expected yield and ltimate strengths, not the specified vales e.g. A36 plate : se 1.3 σ y spec. 103 There's always a soltion in steel

27 Design example: gsset plate connection 105 Determine ltimate load for this gsset plate (which is one that was tested) F y = 39.9 ksi F = 69.0 ksi 7/8 A325 bolts (holes 15/16 in.) P test =164 kips (compression) @2.76= t = 0.26 in Set ot the isses Contining Brace force in tension slip load of bolts (no slip at service load) shear load of bolts bearing capacity of plate block shear Brace force in compression slip capacity of bolts (already checked for load in tension) shear capacity of bolts (already checked for load in tension) bearing capacity of plate (already checked) block shear (doesn t t apply) capacity of gsset plate in compression (New) There's always a soltion in steel. 27

28 Slip load (calclate at factored load level) T A F n R n = = π d μ D / 4 h = 2 slip planes f T b n μ = 0.30 (clean mill scale) b b s = 2 = 0.60 in. = 120 ksi (A325 bolts) spec. min. bolt pretension 2 s (per bolt) h = 1.0 (7/8 in.dia.) f (no fills)) = (0.75 A )(F )70% 2 = in. 120 ksi 70% = kips b 109 Slip load calclation cont d. R n = μ D kips / bolt h f T b N s (per = kip 2 slip planes = bolt) (std.holes) : or, for 8 bolts, ( φ = 1.0); R n = 205 kips 110 Shear resistance of bolts φ R n = φ F v A b Use ø =1.0 so that we can compare this load with the test load, assme threads in shear plane, no joint length effect F v = 90% [ ksi] = 68 ksi Bearing resistance (se φ = 1.0) R n = 1.5 F L c t 3.0 d 3 d t F = 3 7 /8 in in ksi = 47.1 k/bolt t F φ R n = ksi 0.60 in. = 41.0 kips (per bolt) or, for 8 bolts, 2 shear planes, threads in shear plane = ( )kips 0.80 = 525 kips Lc t F = in in ksi = 41.2 k 112 There's always a soltion in steel. 28

29 Bearing resistance the governing vale is 41.2 kips/bolt and, for 8 bolts Block shear [email protected]= Bearing resistance is 330 kips nt = (0.26)( /16) 0.45 in. A = 2 gv = ( ) in. A = T r + V r = φ A nt F φ A gv F y Block shear, cont d Brace force in compression: 2 Tr = 0.45 in ksi = 31.0 kips 2 Vr = in ksi = kips and the total block shear resistance (nfactored) is ( ) =159 kips isse is sway bckling in this region There's always a soltion in steel. 29

30 Checking the bckling Whitmore method (checks yield) Thornton method (checks bckling) Modified Thornton method (checks bckling) Whitmore method. Use beam formlae to check perceived critical sections Use 30 o, as shown to check yielding at location shown. Does not predict ltimate capacity very well, sally conservative bt sometimes non- conservative Thornton method Thornton method, modified Use longest (or average) of L 1, L 2, L 3 to compte a bckling load on a nit width colmn, then apply this to the total width. Use k = 0.65 in the colmn formlae L 1 L 2 30 As per Thornton method bt spread load ot at 45 o L 1 L 2 45 o L 3 L There's always a soltion in steel. 30

31 Yam & Cheng gsset plate tests (U of A, 13 tests) P P W P P T P P T' mean Calclations for bckling capacity: L 1 L 2 Using scale dwg. L 2 = 9.65 in. Width of the 45 o base is 19.2 in. std. dev we ll se this method 121 L 3 φ P F c cr n = φ A F (seφ = 1.0) c = (0.658 g cr F / F y e ) F se k = 0.65 y c 122 Consider a 1 in. wide strip that is 9.65 in. long length =9.65 width = 1 t = 0.26 r = I A = = in. and then completing the calclations, P n = 6.91 kips (on a 1 in. wide strip) 123 And applying this to the total width P = (6.91 k/in.) (19.2 in.) = 132 kips and the test ltimate load on this particlar specimen was 164 kips so, P / P T = 1.23 (The corresponding ratios for Whitmore and Thornton for this specimen were 1.31 and 1.80) 124 There's always a soltion in steel. 31

32 Smmary of or calclations Brace Force slip load bolt shear plate bearing block shear bckling test load Tension Compress Some references Load and Resistance Factor Design Specification for Strctral Joints Using ASTM A325 or A490 Bolts, Research Concil on Strctral Connections, 2004 (RCSC) (free download available at boltconcil.org) References, cont d. G.L. Klak, J.W. Fisher, and J.A.H. Strik, Gide to Design Criteria for Bolted and Riveted Joints,, Second Edition, John Wiley, New York, 1987 (free download at RCSC website) Bickford, John H., "An Introdction to the Design and Behavior of Bolted Joints," Second Edition, Marcel Dekker Inc., New York, 1990 G.L. Klak, A Bolting Primer for Strctral Engineers,, AISC Design Gide 17, Chicago, 2002 Larry Kloiber and Larry Mir, The 2010 AISC Specification: Changes in Design of Connections, Modern Steel Constrction, Sept Thank Yo! Please give s yor feedback! There's always a soltion in steel. 32

33 AISC Seminars 2011 Spring Schedle 6 seminar topics coming to 26 cities Upcoming Cities for April: Detroit Hoston Denver Upcoming Cities for May: St. Lois Boston - Miami Philadelphia Sacramento AISC Seminars 2 nd Thrsday of the month No webinar in May Jne 9, 2011: Extended Shear Plate Connections Larry Mir Jly 14, 2011: Design For Stability Lo Geschwindner AISC elearning AISC SteelCamp Over 60 hors of presentations available anytime, online. CEUs/PDHs are available. 2 day, 4 topics, 15 hors of Contining Edcation, One low price. New York City Jne San Francisco Jly There's always a soltion in steel. 33