Base Plate and Anchor Rod Design

Transcription

1 Steel Design Guide Base late and nhor Rod Design Seond Edition JMES M. FISHER, h.d.,.e. Comuterized Strutural Design, S.C. Milwaukee, Wisonsin and LWRENCE. KLOIBER,.E. LeJuene Steel Comany Minneaolis, Minnesota MERICN INSTITUTE OF STEEL CONSTRUCTION, INC.

2 .0 INTRODUCTION Column base late onnetions are the ritial interae between the steel struture and the oundation. These onnetions are used in buildings to suort gravity loads and untion as art o lateral-load-resisting systems. In addition, they are used or mounting o equiment and in outdoor suort strutures, where they may be aeted by vibration and atigue due to wind loads. Base lates and anhor rods are oten the last strutural steel items to be designed but are the irst items required on the jobsite. The shedule demands along with the roblems that an our at the interae o strutural steel and reinored onrete make it essential that the design details take into aount not only strutural requirements, but also inlude onsideration o onstrutability issues, eseially anhor rod setting roedures and toleranes. The imortane o the aurate laement o anhor rods annot be over-emhasized. This is the one o the key omonents to saely ereting and aurately lumbing the building. The material in this Guide is intended to rovide guidelines or engineers and abriators to design, detail, and seiy olumn-base-late and anhor rod onnetions in a manner that avoids ommon abriation and eretion roblems. This Guide is based on the 005 ISC Seiiation or Strutural Steel Buildings (ISC, 005), and inludes guidane or designs made in aordane with load and resistane ator design or allowable stress design (SD). This Guide ollows the ormat o the 005 ISC Seiiation, develoing strength arameters or oundation system design in generi terms that ailitate either load and resistane ator design or allowable strength design (SD). Column bases and ortions o the anhorage design generally an be designed in a diret aroah based on either LRFD or SD load ombinations. The one area o anhorage design that is not easily designed by SD is the embedment o anhor rods into onrete. This is due to the ommon use o CI 38 endix D, whih is exlusively based on the strength aroah or the design o suh embedment. Other steel elements o the oundation system, inluding the olumn base late and the sizing o anhor diameters are equally roiient to evaluation using LRFD or SD load methods. In ases suh as anhors subjeted to neither tension nor shear, the anhorage develoment requirement may be a relatively insigniiant ator. The generi aroah in develoment o oundation design arameters taken in this Guide ermits the user a hoie to develo the loads based on either the LRFD or SD aroah. The derivations o oundation design arameters, as resented herein, are then either multilied by the resistane ator, φ, or divided by a saety ator, Ω, based on the aroriate load system utilized in the analysis; onsistent with the aroah used in the 005 Seiiation. Many o the equations shown herein are indeendent o the load aroah and thus are aliable to either design methodology. These are shown in singular ormat. Other derived equations are based on the artiular load aroah and are resented in a side-by-side ormat o omarable equations or LRFD or SD aliation. The tyial omonents o a olumn base are shown in Figure.. Material seletion and design details o base lates an signiiantly aet the ost o abriation and eretion o steel strutures, as well as the erormane under load. Relevant asets o eah o these subjets are disussed briely in the next setion. Not only is it imortant to design the olumn-base-late onnetion or strength requirements, it is also imortant to reognize that these onnetions aet the behavior o the struture. ssumtions are made in strutural analysis about the boundary onditions reresented by the onnetions. Models omrising beam or truss elements tyially idealize the olumn base onnetion as either a inned or ixed boundary ondition. Imroer haraterization an lead to error in the omuted drits, leading to unreognized seond-order moments i the stiness is overestimated, or exessive irst-loor olumn sizes i the stiness is underestimated. I more aurate analyses are desired, it may be neessary to inut the stiness o the olumn-base-late onnetion in the elasti and lasti ranges, and or seismi loading, ossibly even the yli ore-deormation relations. The ores and deormations rom the strutural analyses used to design the olumn-baselate onnetion are deendent on the hoie o the olumnbase-late onnetion details. Figure.. Column base onnetion omonents. DESIGN GUIDE, ND EDITION / BSE LTE ND NCHOR ROD DESIGN /

3 mentary to the ISC Seismi rovisions notes some signiiant dierenes:. Long anhor rods embedded in onrete will strain muh more than high-strength bolts or welds in beam-to-olumn onnetions.. Column base lates are bearing on grout and onrete, whih is more omressible than the olumn langes o the beam-to-olumn onnetions. 3. Column base onnetions have signiiantly more longitudinal load in the lane o the langes and less transverse load when omared to beam-to-olumn onnetions. 4. The shear mehanism between the olumn base and the grout or onrete is dierent rom the shear mehanism between the beam end late and the olumn lange. 5. ISC standard hole diameters or olumn base anhor rods are dierent than ISC standard holes or highstrength bolts. 6. Foundation roking and rotation may be an issue, eseially on isolated olumn ootings. s the Commentary to the ISC Seismi rovisions suggests, researh is laking regarding the erormane and design o base details or high seismi loading. However, the Commentary also aknowledges that these details are very imortant to the overall erormane o the SLRS. Thereore, areul onsideration must be given to the design o these details. 3.0 DESIGN OF COLUMN BSE LTE CONNECTIONS This setion o the Design Guide rovides the design requirements or tyial olumn base late onnetions in buildings, suh as the one shown in Figure.. Five dierent design load ases in olumn base late onnetions are disussed: Setion 3. Conentri Comressive xial Loads Setion 3. Tensile xial Loads Setion 3.3 Base lates with Small Moments Setion 3.4 Base lates Large Moments Setion 3.5 Design or Shear In olumn base onnetions, the design or shear and the design or moment are oten erormed indeendently. This assumes there is no signiiant interation between them. Several design examles are rovided in the ollowing setions or eah loading ase. The general behavior and distribution o ores or a olumn base late onnetion with anhor rods will be elasti until either a lasti hinge orms in the olumn, a lasti mehanism orms in the base late, the onrete in bearing rushes, the anhor rods yield in tension, or the onrete ullout strength o the anhor rod grou is reahed. I the onrete ullout strength o the anhor rod grou is larger than the lowest o the other aorementioned limit states, the behavior generally will be dutile. However, it is not always neessary or even ossible to design a oundation that revents onrete ailure. Figure.6. Tyial moment base detail. Figure.7. Embedded moment base detail. DESIGN GUIDE, ND EDITION / BSE LTE ND NCHOR ROD DESIGN / 3

4 For examle, in statially loaded strutures, i the strength is muh larger than the demand, the dutility is not neessary and it is aetable to design with the limit state o tensile or shear strength o the anhor rod grou governing the design. However, rames designed or seismi lateral load resistane are exeted to behave in a dutile manner and, in this ase, it may be neessary to design the oundation and the olumn-base-late onnetion so that the onrete limit states o tensile or shear strength o the anhor rod grou do not govern the design. See CI endix D, Setion D OSH Requirements The regulations o the Ouational Saety and Health dministration (OSH) Saety Standards or Steel Eretion (OSH, 00) require a minimum o our anhor rods in olumn-base-late onnetions. The requirements exlude ost-tye olumns that weigh less than 300 lb. Columns, base lates, and their oundations must have suiient moment strength to resist a minimum eentri gravity load o 300 lb loated 8 in. rom the extreme outer ae o the olumn in eah diretion. The OSH riteria an be met with even the smallest o anhor rods on a 4-in. 4-in. attern. I one onsiders only the moments rom the eentri loads (sine inluding the gravity loads results in no tensile ore in the anhor rods), and the resisting ore oule is taken as the design ore o the two bolts times a 4-in. lever arm, the design moment strength or w-in. anhor rods equals ()(9. kis)(4 in.) = 306 ki-in. For a 4-in.-dee olumn, the OSH required moment strength is only (.6)(0.300)(8 + 7) =.0 ki-in. 3.. Conentri Comressive xial Loads When a olumn base resists only omressive olumn axial loads, the base late must be large enough to resist the bearing ores transerred rom the base late (onrete bearing limit), and the base late must be o suiient thikness (base late yielding limit). 3.. Conrete Bearing Limit The design bearing strength on onrete is deined in CI 38-0, Setion 0.7, as φ(0.85 ) when the suorting surae is not larger than the base late. When the suorting surae is wider on all sides than the loaded area, the design bearing strength above is ermitted to be multilied by. The 005 ISC Seiiation, Setion J8, rovides the nominal bearing strength,, as ollows: Equation J8-: = 0.85 on the ull area o a onrete suort. Equation J8-: = ( 0 ).. 7 These equations are multilied by the resistane ator, φ, or LRFD or divided by the saety ator, Ω, or SD. Setion J8 stiulates the φ and Ω ators (in the absene o Code Regulations) or bearing on onrete as ollows: φ = 0.60 Ω =.50 (SD) lternatively, CI 38-0 stiulates a φ ator o 0.65 or bearing on onrete. This aarent onlit exists due to an oversight in the ISC Seiiation develoment roess. The authors reommend the use o the CI-seiied φ ator in designing olumn base lates. The nominal bearing strength an be onverted to a stress ormat by dividing out the area term equations suh that, On the ull area o a onrete suort: (max) = 0.85 When the onrete base is larger than the loaded area on all our sides: (max) = ( 0. ). 7 The onversion o the generi nominal ressure to an LRFD or SD available bearing stress is u(max) = φ (max) a(max) (max) = (SD) Ω The onrete bearing strength is a untion o the onrete omressive strength, and the ratio o geometrially similar onrete area to base late area, as indiated in Setion 0.7 o CI 38 (CI, 00), as ollows: (max) = φ( 0. ) where (max) = maximum onrete bearing stress, ksi φ = strength redution ator or bearing, 0.65 er Setion 9.3, CI 38-0 = seiied omressive strength o onrete, ksi 4 / DESIGN GUIDE, ND EDITION / BSE LTE ND NCHOR ROD DESIGN

5 = area o the base late, in. = maximum area o the ortion o the suorting surae that is geometrially similar to and onentri with the loaded area, in. The inrease o the onrete bearing aaity assoiated with the term aounts or the beneiial eets o the onrete oninement. Note that is the largest area that is geometrially similar to (having the same aset ratio as) the base late and an be insribed on the horizontal to surae o the onrete ooting, ier, or beam without going beyond the edges o the onrete. There is a limit to the beneiial eets o oninement, whih is releted by the limit on (to a maximum o our times ) or by the inequality limit. Thus, or a olumn base late bearing on a ooting ar rom edges or oenings, =. =. The bearing stress on the onrete must not be greater than (max) : a u (max) a (max) (SD) Many olumn base lates bear diretly on a layer o grout. Beause, the grout omressive strength is always seiied higher than the onrete strength the authors reommend that the grout strength be seiied as two times the onrete strength it is onservative to use the onrete omressive strength or in the above equations. The imortant dimensions o the olumn-base late onnetion are shown in Figure Base late Yielding Limit (W-Shaes) For axially loaded base lates, the bearing stress under the base late is assumed uniormly distributed and an be exressed as u a BN a = (SD) BN This bearing ressure auses bending in the base late at the assumed ritial setions shown in Figure 3..(b). This Thus, u(max) a = (SD) a(max) When =, the required minimum base late area an be determined as φ0. When 4, the required minimum base late area an be determined as u ( req ) = φ0. Ωa = (SD) 0. a = Ω (SD) 0. Figure 3... Design o base late with axial omressive load. DESIGN GUIDE, ND EDITION / BSE LTE ND NCHOR ROD DESIGN / 5

6 bearing ressure also auses bending in the base late in the area between the olumn langes (Thornton, 990; Drake and Elkin, 999). The ollowing roedure allows a single roedure to determine the base late thikness or both situations. The required strength o the base late an be determined as l M l = u Where the ritial base late antilever dimension, l, is the larger o m, n, and λn, N = base late length, in. B = base late width, in. b = olumn lange width, in. d = overall olumn deth, in. n = yield-line theory antilever distane rom olumn web or olumn lange, in. X λ = + X where M l m N = d B n = 0. 8 λn = λ db 4 b = a l (SD) db X = 4 d + b ( ) φ db X = 4 d + b ( ) u = the required axial omressive load, kis a = the required axial omressive load (SD), kis = a Ω (SD) It is onservative to take λ as.0. For the yielding limit state, the required minimum thikness o the base late an be alulated as ollows (Thornton, 990) (ISC, 005): where tmin = l φf BN φ = resistane ator or lexure, 0.90 Ω = ator o saety or SD,.67 F y = seiied minimum yield stress o base late, ksi Sine l is the maximum value o m, n, and λn, the thinnest base late an be ound by minimizing m, n, and λ. This is usually aomlished by roortioning the base late dimensions so that m and n are aroximately equal Base late Yielding Limit (HSS and ie) y a tmin = l Ω (SD) F BN y For HSS olumns, adjustments or m and n must be made (DeWol and Riker, 990). For retangular HSS, both m and n are alulated using yield lines at 0.95 times the deth and width o the HSS. For round HSS and ie, both m and n are alulated using yield lines at 0.8 times the diameter. The λ term is not used or HSS and ie General Design roedure Three general ases exist or the design o base lates subjet to axial omressive loads only: Case I: = Case II: 4 Case III: < < 4 The most diret aroah is to onservatively set equal to (Case I); however, this generally results in the largest base late lan dimensions. The smallest base late lan dimensions our when the ratio o the onrete to base late area is larger than or equal to 4, i.e., 4 (Case II). Base lates resting on iers oten meet the ase that is larger than but less than 4, whih leads to Case III. When a base late bears on a onrete edestal larger than the base late dimension, the required minimum base late area annot be diretly determined. This is beause both and are unknown. s mentioned beore, the most eonomial base lates usually our when m and n, shown in Figure 3..(b), are 6 / DESIGN GUIDE, ND EDITION / BSE LTE ND NCHOR ROD DESIGN

7 equal. This situation ours when the dierene between B and N is equal to the dierene between 0.95d and 0.8b. In seleting the base late size rom a strength viewoint, the designer must onsider the loation o the anhor rods within the late and the learanes required to tighten the bolts on the anhor rods. Stes or obtaining base lates sizes or these ases are suggested below. nhor rod design is overed in Setion 3.. Case I: = The largest base late is obtained when =.. Calulate the required axial omressive strength, u or a (SD).. Calulate the required base late area. 3. Otimize the base late dimensions, N and B. then N 0. 95d 0. 8 where = φ0. Ωa = (SD) 0. + Note that the base late holes are not deduted rom the base late area when determining the required base late area. s mentioned earlier in the Guide, rom a ratial view oint set N equal to B. 4. Calulate the required base late thikness. m N = d B n = 0. 8 b B b req = ( ) N N = base late length, in. B = base late width, in. b = olumn lange width, in. d = overall olumn deth, in. n = yield-line theory antilever distane rom olumn web or olumn lange, in. where X λ = + X Find l max (m, n, λn ) 5. Determine the anhor rod size and the loation o the anhor rods. nhor rods or gravity olumns are generally not required or the ermanent struture and need only to be sized or OSH requirements and ratial onsiderations. Case II: 4 db X = 4 d + b ( ) φ db X = 4 d + b ( ) The smallest base late is obtained when 4 or this ase.. Calulate the atored axial omressive load, u or a (SD). a Ω (SD) φ = φ0. 0. = (SD) Ω Ω tmin = l φf BN y a tmin = l Ω (SD) F BN y λn = λ db 4. Calulate the required base late area. φ0. DESIGN GUIDE, ND EDITION / BSE LTE ND NCHOR ROD DESIGN / 7

8 3. Otimize the base late dimensions, N and B. Use the same roedure as in Ste 3 rom Case I. 4. Chek i suiient area, exists or Case II aliability ( 4 ). Based on the ier or ooting size, it will oten be obvious i the ondition is satisied. I it is not obvious, alulate geometrially similar to. With new dimensions N and B, then equals (N )(B ). I 4, alulate the required thikness using the roedure shown in Ste 4 o Case I, exet that 5. Determine the anhor rod size and loation. Case III: < < 4. Calulate the atored axial omressive load, u or a (SD).. Calulate the aroximate base late area based on the assumtion o Case III. 3. Otimize the base late dimensions, N and B. Use the same roedure as in Ste 3 rom Case I. 4. Calulate, geometrially similar to. 5. Determine whether Ωa = 0. ( ) I the ondition is not satisied, revise N and B, and retry until riterion is satisied. (SD) φ = φ = (SD) Ω Ω φ0. Ωa = 0. ( ) (SD) u = φ φ0. 85 a = (SD) Ω Ω 6. Determine the base late thikness using Ste 4, as shown in Case I. 7. Determine the anhor rod size, and their loations. 3. Tensile xial Loads The design o anhor rods or tension onsists o our stes:. Determine the maximum net ulit or the olumn.. Selet the anhor rod material and the number and size o anhor rods required to resist ulit. 3. Determine the aroriate base late size, thikness, and welding to transer the ulit ores. 4. Determine the method or develoing the strength o the anhor rod in the onrete (i.e., transerring the tension ore rom the anhor rod to the onrete oundation). Ste The maximum net ulit or the olumn is obtained rom the strutural analysis o the building or the resribed building loads. When the ulit due to wind exeeds the dead load o a roo, the suorting olumns are subjeted to net ulit ores. In addition, olumns in rigid bents or braed bays may be subjeted to net ulit ores due to overturning. Ste nhor rods should be seiied to onorm to the material disussed in Setion.5. The number o anhor rods required is a untion o the maximum net ulit on the olumn and the strength er rod or the anhor rod material hosen. rying ores in anhor rods are tyially negleted. This is usually justiied when the base late thikness is alulated assuming antilever bending about the web and/or lange o the olumn setion (as desribed in Ste 3 below), and beause the length o the rods result in larger deletions than or steel to steel onnetions. The roedure to determine the required size o the anhor rods is disussed in Setion 3.. below. Ste 3 Base late thikness may be governed by bending assoiated with omressive or tensile loads. For tensile loads, a simle aroah is to assume the anhor rod loads generate bending moments in the base late onsistent with antilever ation about the web or langes o the olumn setion (one-way bending). See Figure 3... I the web is taking the anhor load rom the base late, the web and its attahment to the base late should be heked. lternatively, a more reined base late analysis or anhor rods ositioned inside the olumn langes an be used to onsider bending about both the web and the olumn langes (two-way bending). For the two-way bending aroah, the derived bending moments should be onsistent with om- 8 / DESIGN GUIDE, ND EDITION / BSE LTE ND NCHOR ROD DESIGN