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1、Base Plate and Anchor Rod Design1.Concentric Compressive Axial Loads2.Tensile Axial Loads3.Base Plates with Small Moments4.Base Plates Large Moments5.Design for shearFor Typical Column base Plate Connections in Buildings,Five Different Design Load Cases in Column Base Plate Connections.1.Concentric
2、Compressive Axial Loads1.1Concrete Bearing LimitOn the full area of a concrete supportWhen the concrete base is larger than the loaded area on all four sides萬鼎工程Base plate lengthBase plate widthColumn flange widthOverall column depth1.2 Required minimum thickness of the base plateConservative to tak
3、e as 1.0Resistance factor for flexure,0.90Factor of safety for ASD,1.67NOTE:萬鼎工程2.1 Anchor Rod TensionTensile stress areaMajor diameterNumber of threads per in2005 AISC Specification2.2 Concrete Anchorage for Tensile Forces(锚栓拔出强度)if the anchor is located where the concrete is not cracked at service
4、 loadotherwisethe bearing area of the anchor rod head or nutdesign tensile strength=allowable tensile strength=2.Tensile Axial Loads萬鼎工程萬鼎工程萬鼎工程5.Determine the required minimum base plate thickness6.Determine the anchor rod sizeNOTE:公制与英制使用公式的系数相同萬鼎工程Design Procedure：1.Determine the axial load and m
5、oment2.Pick a trial base plate size，3.Determine the equivalent eccentricity，and the critical eccentricity4.Determine the bearing length，and tensile force in the anchor rod，5.Determine the required minimum base plate thickness6.Determine the anchor rod size4.Column Base Plates with Large Moments萬鼎工程萬
6、鼎工程5.1 Friction between the base plate and the grout or concrete surface0.55 for steel on grout0.7 for steel on concrete5.2 Bearing of column and base plate,and/or shear lug,against a concrete surfaceFor shear lugsFor bearing on a column or the side of a base plate5.Design for Shear萬鼎工程5.3 shear in
7、the anchor rodsThe edge distance(in)in the direction of loadConcrete compressive strength,ksiThe embedment depth,inThe rod diameter,in(All anchors at same load)a modifier to reflect the capacity reduction when side cover limits the size of the breakout coneThe total breakout shear area for a single
8、anchor萬鼎工程Example 1：Base Plate for Axial Compressive LoadW12X96 column24X24 in pedestalLRFD:萬鼎工程TryTryDetermine if the following inequality is metOK!萬鼎工程Calculate required base plate thicknessUSE萬鼎工程Determine the anchor rod size:Since no anchor rod force exist,the anchor rod size can be determined b
9、ased on the OSHA requirements,and practical considerations.USE 4 in diameter rods,ASTM F1554,Grades 36,Rod length=12in萬鼎工程ASD:TryTry萬鼎工程Determine if the following inequality is metOK!Calculate required base plate thickness萬鼎工程USEDetermine the anchor rod size:Since no anchor rod force exist,the ancho
10、r rod size can be determined based on the OSHA requirements,and practical considerations.USE 4 in diameter rods,ASTM F1554,Grades 36,Rod length=12in萬鼎工程Example 2：Column Anchorage for Tensile LoadsDesign a base plate and anchorage for a W10X45 column subjected to a net uplift,as a result of nominal l
11、oads shown 萬鼎工程Procedure:1.Determine the required strength due to uplift on the column2.Select the type and number of anchor rods3.Determine the appropriate base plate thickness and welding to transfer the uplift forces from the column to the anchor rods4.Determine method for developing the anchor r
12、ods in the concrete in the spread footing5.Reevaluate the anchorage if the column is on a 20-in 20-in pier萬鼎工程LRFD:Uplift=1.6WL-0.9DL =1.6(56)-0.9(22)=69.8 kipsT/rod=69.8/4=17.5 kipsUsing an ASTM F1554 Grade 36 material,select a 7/8 in diameter rodThe design strength of the rodOK萬鼎工程The rods are pos
13、itioned inside the column profile with a 4-in.square pattern.Prying forces are negligible.To simplify the analysis,conservatively assume the tensile loads in the anchor rods generate one-way bending in the base plate about the web of the column.If the column web strength controls the design,then con
14、sider distributing the forces to the flanges as well as the web.If the bolts are placed outside of the flanges,the 45 degree load distribution can be used to distribute the forces to the flanges.萬鼎工程Use 1 in thick plate (Fy=36ksi)Maximum weld load=Maximum weld for 0.35in column web=3/16 inNominal we
15、ld strength per in for a 3/16 in fillet weld with E70 electrode(using the 50%directional increase)萬鼎工程3/16in fillet weld on each side of the column web is O.K.Check web:Web stress=Force per in/Area of web per in萬鼎工程ASD:Uplift=WL-0.6DL =56-0.6(22)=42.8 kipsT/rod=42.8/4=10.7 kipsUsing an ASTM F1554 Gr
16、ade 36 material,select a 7/8 in diameter rodThe allowable strength of the rodOK萬鼎工程The rods are positioned inside the column profile with a 4-in.square pattern.Prying forces are negligible.To simplify the analysis,conservatively assume the tensile loads in the anchor rods generate one-way bending in
17、 the base plate about the web of the column.If the column web strength controls the design,then consider distributing the forces to the flanges as well as the web.If the bolts are placed outside of the flanges,the 45 degree load distribution can be used to distribute the forces to the flanges.萬鼎工程Us
18、e 1 in thick plate (Fy=36ksi)Maximum weld load=Maximum weld for 0.35in column web=3/16 inNominal weld strength per in for a 3/16 in fillet weld with E70 electrode(using the 50%directional increase)萬鼎工程3/16in fillet weld on each side of the column web is O.K.Check web:Web stress=Force per in/Area of
19、web per in萬鼎工程Example 3：Small Moment Base Plate DesignDesign a base plate for axial dead and live loads equal to 100 and 160 kips,respectively,and moments from the dead and live loads equal to 250 and 400 kip-in,respectively.Bending is about the strong axis for wide flange column W12X96 with d=12.7i
20、n and bf=12.2in.The ratio of the concrete to base plate area is unity;Fy of the base plate is 36 ksi and fc of the concrete 4 ksi萬鼎工程LRFD:Try N=19in and B=19in萬鼎工程Determine andTherefore,and the design meets the crteria for the case of a base plate with small moment萬鼎工程Determine bearing length,YY=N-2
21、e=19-(2)(2.50)=14inVerify bearing pressure:OKDetermine minimum plate thickness萬鼎工程Check the thickness using the value of ncontrolsUsing a base plate 1 “X19”X1-7”Determine the anchor rod sizeSince no anchor rod forces exist,the anchor rod size can be determined based on the OSHA requirements and prat
22、ical considerations Using four in diameter rods,ASTM F1554,Grade 36;rod length=12in萬鼎工程ASD:Try N=19in and B=19in萬鼎工程Determine andTherefore,and the design meets the crteria for the case of a base plate with small moment萬鼎工程Determine bearing length,YY=N-2e=19-(2)(2.50)=14inVerify bearing pressure:OKDe
23、termine minimum plate thickness萬鼎工程Check the thickness using the value of ncontrolsUsing a base plate 1 “X19”X1-7”Determine the anchor rod sizeSince no anchor rod forces exist,the anchor rod size can be determined based on the OSHA requirements and pratical considerations Using four in diameter rods
24、,ASTM F1554,Grade 36;rod length=12in萬鼎工程Example 4：Large Moment Base Plate DesignDesign a base plate for axial load and live loads equal to 100 and 160 kips,respectively,and moments from the dead and live loads equal to 1000 and 1500 kip-in.,respectively.Bending is about the strong axis for a W12X96
25、wide flange column with d=12.7 in.and bf=12.2 in.Conservatively,consider the ratio of the concrete to base plate area is unity.Fy of the base plate is 36ksi and fc of concrete is 4ksi.萬鼎工程LRFD:Try N=19in and B=19in萬鼎工程Determine andTherefore,this is the case of base plate with large moment.萬鼎工程Assume
26、 that the anchor rod edge distance is 1.5 in.Therefore,Since 315306,a larger plate dimension is required.萬鼎工程As the second iteration,try a 20X20 plate324306,a larger plate dimension is required.萬鼎工程As the second iteration,try a 20X20 plate324342,therefore a real solution for Y exists萬鼎工程Determine be
27、aring length,Y,and anchor rod tension,or萬鼎工程Determine minimum plate thicknessBecause At tension interface:萬鼎工程Check the thickness using the value of nBearing interface governs the design of base plate thickness.Use 2 in.plate.萬鼎工程Example 5：Shear Transfer Using BearingCalculate the minimum embedment
28、depth of a shallowly embedded W12X50 in 6000 grout for a factored shear load of 100 kips.The base plate is 15in.X15in.and is 1.5 in.thick.The projected area of the plate =(1.5)(15)=22.5in2.The design shear strength in bearing on the base plate edge per ACI 318-02 is萬鼎工程The remaining 31.2kips must be
29、 taken by bearing of the flange of the W12X50 against the concrete.The width of the flange is 8.08in.The required bearing area is:Thus,the required flange embedment depth isUse a total embedment of 4in.for the flange and base plate.萬鼎工程Example 6：Anchor Rod Resisting Combined Tension and shearDetermi
30、ne the required size of four anchor rods for the W10X45 column,using the anchor rods to resist wind shear萬鼎工程Wind shear forceLRFD:Try 4 1-1/8 in F1554 Grade 36 anchoruplift=69.8 kipsAISC Provision:where萬鼎工程Shear stress:Tensile stress:Tension from bendingAxial tensionThe bending moment in each rod eq
31、uals the shear times the half distance from the center of the plate washer to the top of the grout Use in plate washer and 1 in base plate萬鼎工程The axial stress equalsThe tensile stress萬鼎工程NGcheck萬鼎工程Try four 1 in rods Shear stress：The axial stress equalsThe tensile stress萬鼎工程Use four 1 in rods OKchec
32、k萬鼎工程Wind shear forceASD:Try 4 1-1/8 in F1554 Grade 36 anchoruplift=46.5 kipsAISC Provision:where萬鼎工程Shear stress:Tensile stress:Tension from bendingAxial tensionThe bending moment in each rod equals the shear times the half distance from the center of the plate washer to the top of the grout Use in plate washer and 1 in base plate萬鼎工程The axial stress equalsThe tensile stress萬鼎工程NGcheck萬鼎工程Try four 1 in rods Shear stress：The axial stress equalsThe tensile stress萬鼎工程OKUse four 1 in rods check