OUTLINE DESIGN OF COLUMN BASE PLATES AND STEEL ANCHORAGE TO CONCRETE 12/21/ Introduction 2. Base plates. 3. Anchor Rods

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1 DESIGN OF COLUMN BSE PLTES ND STEEL NCHORGE TO CONCRETE OUTLINE 1. Introduction 2. Base plates a. Material b. Design using ISC Steel Design Guide Concentric axial load xial load plus moment xial load plus shear 3. nchor Rods a. Types and Materials b. Design using CI ppendix D Tension Shear 1

2 INTRODUCTION Base plates and anchor rods are oten the last structural steel items to be designed but the irst items required on the jobsite. Thereore the design o column base plate and connections are part o the critical path. Vast majority o column base plate connections are designed or axial compression with little or no uplit. INTRODUCTION (Cont d) Column base plate connections can also transmit uplit orces and shear orces through: nchor rods, Friction against the grout pad or concrete, Shear lugs under the base plate or embedding the column base can be used to resist large orces. Column base plate connections can also be used to resist wind and seismic loads: Development o orce couple between bearing on concrete and tension in some or all o the anchor rods. 2

3 INTRODUCTION (Cont d) nchor rods are needed or all base plates to prevent column rom overturning during construction and in some cases to resist uplit or large moments nchor rods are designed or pullout and breakout strength using CI 318 ppendix D Critical to provide well-deined, adequate load path when tension and shear loading will be transerred through anchor rods INTRODUCTION (Cont d) Grout is needed to serve as the connection between the steel base plate and the concrete oundation to transer compression loads. Grout should have design compressive strength at least twice the strength o oundation concrete. When base plates become larger than 24, it is recommended that one or two grout holes be provided to allow the grout to low easier. 3

4 BSE PLTE MTERILS Base plates should be STM 36 material unless other grade is available. Most base plates are designed as square to match the oundation shape and can be more accommodating or square anchor rod patterns. thicker base plate is more economical than a thinner base plate with additional stieners or other reinorcements. BSE PLTE DESIGN 4

5 DESIGN OF XILLY LODED BSE PLTES Required plate area is based on uniorm allowable bearing stress. For axially loaded base plates, the bearing stress under the base plate is uniorm ` 2 ` p max c 0.85 c 1. 7 c 1 2 = dimensions o concrete supporting oundation 1 = dimensions o base plate Most economical plate occurs when ratio o concrete to plate area is equal to or greater than 4 (Case 1) When the plate dimensions are known it is not possible to calculate bearing pressure directly and thereore dierent procedure is used (Case 2) DESIGN OF XILLY LODED BSE PLTES (Cont d) Case 1: 2 > Determine actored load P u 2. Calculate required plate area 1 based on maximum concrete bearing stress p =1.7`c (when 2 = 4 2 ) Pu 1( req) `.61.7 N 1 ( req ) B N 1(req) 0 c 3. Plate dimensions B & N should be determined so m & n are approximately equal: 0.95d 0.8b 2 5

6 DESIGN OF XILLY LODED BSE PLTES (Cont d) Case 1: 2 > Calculate required base plate thickness: N 0.95d B 0.8b m n 2 2 t min l 2Pu 0.90F BN y where l is maximum o m and n 5. Determine pedestal area, 2 : 2 4BN DESIGN OF XILLY LODED BSE PLTES (Cont d) Case 2: Pedestal dimensions known 1. Determine actored load P u 2. The area o the plate should be equal to larger o: 1 P u 1 ` or c 3. Same as Case 1 4. Same as Case P u ` c 6

7 DESIGN OF BSE PLTES WITH MOMENTS Equivalent eccentricity, e, is calculated equal to moment M divided by axial orce P. Moment and axial orce replaced by equivalent axial orce at a distance e rom center o column. Small eccentricities equivalent axial orce resisted by bearing only. Large eccentricities necessary to use an anchor bolt to resist equivalent axial orce. DESIGN OF BSE PLTES WITH MOMENTS (Cont d) I e < N/6 compressive bearing stress exist everywhere 1,2 P BN Mc I I e is between N/6 and N/2 bearing occurs only over a portion o the plate 1 2P B 7

8 DESIGN OF BSE PLTES WITH MOMENTS (Cont d) 1. Calculate actored load (P u ) and moment (M u ) 2. Determine maximum bearing pressure, p ` 2 ` p c 0.85 c 1.7 c 3. Pick a trial base plate size, B and N 4. Determine equivalent eccentricity, e, and maximum bearing stress rom load, 1. I 1 < p go to next step, i not pick dierent base plate size. 5. Determine plate thickness, t p : 4M plu t p M 0.90F plu is moment or 1 in wide strip y 1 DESIGN OF BSE PLTE WITH SHER Four principal ways o transerring shear rom column base plate into concrete: 1. Friction between base plate and the grout or concrete surace: V n mp 0.2 u ` c c The riction coeicient (m) is 0.55 or steel on grout and 0.7 or steel on concrete 2. Embedding column in oundation. 3. Use o shear lugs. 4. Shear in the anchor rods. 8

9 DESIGN OF SHER LUGS 1. Determine the portion o shear which will be resisted by shear lug, V lgu. 2. Determine required bearing area o shear lug: Vlgu lg ` 0.85 c 3. Determine shear lug width, W, and height, H. 4. Determine actored cantilevered end moment, M lgu. Vlgu H G M lgu W 2 5. Determine shear lug thickness: t lg 4M lg u 0.90F y NCHOR RODS Two categories: a) Post-installed anchors: set ater the concrete is hardened. b) Cast-in-place anchors: set beore the concrete is placed. 9

10 NCHOR RODS (Cont d) Materials: Preerred speciication is STM F1554: - Grade 36, 55, 105 ksi. STM F1554 allows anchor rods to be supplied straight (threaded with nut or anchorage), bent or headed. Wherever possible use ¾-in diameter STM F1554 Grade 36: - When more strength required, increase rod diameter to 2 in beore switching to higher grade. Minimum embedment is 12 times diameter o bolt. CST-IN-PLCE NCHOR RODS When rods with threads and nut are used, a more positive anchorage is ormed: Failure mechanism is the pull out o a cone o concrete radiating outward rom the head o the bolt or nut. Use o plate washer does not add any increased resistance to pull out. Hooked bars have a very limited pullout strength compared with that o headed rods or threaded rods with a nut o anchorage. 10

11 NCHOR ROD PLCEMENT Most common ield problem is placement o anchor rods. Important to provide as large as hole as possible to accommodate setting tolerances. Fewer problems i the structural steel detailer coordinates all anchor rod details with column base plate assembly. NCHOR ROD LYOUT Should use a symmetrical pattern in both directions wherever possible. Should provide suicient clearance distance or the washer rom the column. Edge distance plays important role or concrete breakout strength. Should be coordinated with reinorcing steel to ensure there are no intererences, more critical in concrete piers and walls. 11

12 DESIGN OF NCHOR RODS FOR TENSION When base plates are subject to uplit orce T u, embedment o anchor rods must be checked or tension. Steel strength o anchor in tension: N s se ut se = eective cross sectional area o anchor, ISC Steel Manual Table 7-18 ut = tensile strength o anchor, not greater than 1.9 y or 125 ksi Concrete breakout strength o single anchor in tension: N Ncb 2 3 No N b N k ` 1.5 c e h e = embedment k = 24 or cast-in place anchors, 17 or post-installed anchors 2, 3 = modiication actors b h DESIGN OF NCHOR RODS FOR TENSION (Cont d) no = projected area o the ailure surace o a single anchor remote rom edges N = approximated as the base o the rectilinear geometrical igure that results rom projecting the ailure surace outward 1.5h e rom the centerlines o the anchor. Example o calculation o N with edge distance (c 1 ) less than 1.5h e No 2 9h e N ( c 1 1.5h e )(21.5h e ) 12

13 DESIGN OF NCHOR RODS FOR TENSION (Cont d) Pullout strength o anchor: N pn ` 4 brg 8 c Nominal strength in tension N n = min(n s, N cb, N pn ) Compare uplit rom column, T u to N n. I T u less than N n ok! I T u greater than N n, must provide tension reinorcing around anchor rods or increase embedment o anchor rods. DESIGN OF NCHOR RODS FOR SHER (Cont d) When base plates are subject to shear orce, V u, and riction between base plate and concrete is inadequate to resist shear, anchor rods may take shear. Steel Strength o single anchor in shear: V s se Concrete breakout strength o single anchor in shear: 0.2 v l ` 1.5 Vcb 6 7Vb Vb 7 do c c1 d o vo ut 6, 7 = modiication actors d o = rod diameter, in l = load bearing length o anchor or shear not to exceed 8d o, in 13

14 DESIGN OF NCHOR RODS FOR SHER (Cont d) vo = projected area o the ailure surace o a single anchor remote rom edges in the direction perpendicular to the shear orce v = approximated as the base o a truncated hal pyramid projected on the side ace o the member. Example o calculation o v with edge distance (c 2 ) less than 1.5c 1 v 2 vo 4.5 c 1.5c1 (1.5c 1 c2) 1 DESIGN OF NCHOR RODS FOR SHER (Cont d) Pryout strength o anchor: V k N cp cp cb Nominal strength in shear V n = min(v s, V cb, V cp ) Compare shear rom column, V u to V n. I V u less than V n ok! I V u greater than V n must provide shear reinorcing around anchor rods or use shear lugs. 14

15 COMBINED TENSION ND SHER ccording to CI 318 ppendix D, anchor rods must be checked or interaction o tensile and shear orces: Tu N n Vu V n 1.2 REFERENCES merican Concrete Institute (CI) ISC Steel Design Guide, Column Base Plates, by John T. DeWol, ISC Steel Design Guide (2 nd Edition) Base Plate and nchor Rod Design. ISC Engineering Journal nchorage o Steel Building Components to Concrete, by M. Lee Marsh and Edwin G. Burdette, First Quarter

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