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www PDHcenter com PDH Course S245 www PDHonline org. Steel Beam Reinforcement,Jonathon C Creviston P E SECB. 1 Introduction, A common problem in industrial facilities is reinforcement of existing beams either. due to corrosion damage increased loading or cutting of the beam in critical areas. Reinforcement may be for bending moment flanges or shear web The following. course presents analysis techniques suggested details and general guidelines of. reinforcement of steel beams for bending Although specific to steel the same. engineering principles are applicable to other materials. All examples assume that those taking this course are familiar with static analysis of. beams Any reference to the term manual refers to the AISC Manual of Steel. Construction Thirteenth Edition,2 Notations,A Area in2. Ai Initial beam cross sectional area in2,Af Built up beam cross sectional area in2. Ac Corroded beam cross sectional area in2,I Moment of inertia in4.
Ii Initial beam moment of inertia in4,If Built up beam moment of inertia in4. Ic Corroded beam moment of inertia in4,Si Initial beam section modulus in3. Sf Built up beam section modulus in3,Sc Corroded beam section modulus in3. y Distance to the centroid of a built up section component in. Y Distance to the neutral axis of a built up section in. y Distance from the centroid of a component to the neutral axis of the built up. M Moment ft k,q Shear Flow k in,Fy Yield strength ksi. r Radius of gyration in,d Depth of built up section in.
2010 Jonathon C Creviston Page 2 of 10, www PDHcenter com PDH Course S245 www PDHonline org. 3 Built up Sections and Shear Flow, The following section is a review of the procedures for analyzing built up sections. and calculating shear flow Both are necessary for the design of beam reinforcement. Itotal I1 I2 A1 Y y1 2 A2 Y y2 2,Scompression I Y,Stension I d Y. rx Itotal A 1 2,The same procedures are followed for the weak. Figure 1 Built up Section, Shear flow q is the horizontal shear at the interface of the different components of a.
built up section induced by bending moment In Figure 1 this would be at the. interface of the plate and wide flange Shear flow is used to determine connection. requirements between the components of a built up section The general form of the. equation is,q VAy Itotal,For the example given above the equation would be. q VA1 Y y1 Itotal,4 General Procedures, It is often unnecessary and uneconomical to reinforce a beam along its entire length. The first step is to determine the shear and moment diagrams in order to locate the. regions requiring reinforcement The following example uses a W12x26 twenty feet. long with a load of 2 0 klf The beam is continuously braced and grade A992. A check of the Table 3 6 ASD in the manual gives a maximum allowable shear. Vn n of 56k well above the shear on this particular beam See Figure 2 below. The same table gives an allowable bending moment Mp b of 92 ft k The capacity. can also be obtained by working backwards from the section modulus Referring to. Figure 3 below the bending moment is 100 ft k, The beam therefore requires reinforcement for bending moment The length of. reinforcement can be determined by drawing a horizontal line at the moment capacity. then vertical lines down at the points of intersection Referring to Figure 2 the beam. requires reinforcement roughly between 85 and 155 from the left end. Reinforcement should extend a minimum of 12 on either side of this area to allow. the abrupt change in cross section to occur outside the region of greater demand. 2010 Jonathon C Creviston Page 3 of 10, www PDHcenter com PDH Course S245 www PDHonline org. Beam Shear,Distance inches,Bending Moment,Moment ft k.
Distance inches, The second step is to determine the required section modulus working backward. from the bending moment At this point it is acceptable to assume the reinforcement. to be the same grade as the beam giving a required section modulus of 36 4 in3. Next a method of reinforcement is selected based upon field conditions experience. and the amount of reinforcement required Field conditions will generally govern the. selection due to the presence of conduit piping finishes stiffeners etc. 2010 Jonathon C Creviston Page 4 of 10, www PDHcenter com PDH Course S245 www PDHonline org. Figure 4 Examples of Beam Reinforcement, A plate will be added to the bottom flange of this beam to obtain the necessary. strength This is often a trial and error procedure an A36 3 8 x7 plate will the first. selection The width is selected wider than the beam flange to allow a downward. fillet weld when possible,Element y A yA I,Plate 0 19 2 63 0 49 0 03. W12x26 6 49 7 65 49 61 204 00,10 28 50 10 204 03,Itotal 281 5.
Scompression 41 9,Stension 57 7, The compression flange governs and exceeds the required section modulus of 36 4. The final check before designing the connections involves the difference in yield. strength between the plate and the W12 When the section modulus greatly exceeds. what is required it is generally acceptable to ignore this difference In this example. the plate is in the tension zone and is 150 greater than required To check convert. the bending moment into a force couple in the flanges. Tension Compression M d d 12 22 0 375 12 60,100 ft k 12 in ft 12 60 95 3k. Then convert the area of the reinforcement into an equivalent area of A992 material. and add it to the area of the W12 flange 2 47 in2 to obtain the total equivalent area of. the tensile flange,A Aplate Fy plate Fy W12 2 63 in2 36 50 1 89 in2. Atension 1 89 in2 2 47 in2 4 36 in2, The tensile stress can then be checked and compared to the allowable stress 0 60Fy. 30 ksi for A992 steel,ft T Atension 95 3 k 4 36 in2 21 9 ksi OK.
2010 Jonathon C Creviston Page 5 of 10, www PDHcenter com PDH Course S245 www PDHonline org. The last step is designing the connection between the reinforcement and the beam. using the shear flow equation As previously discussed. q VA1 Y y1 Itotal,20 k 2 63 in2 4 88 in 0 19 in 281 5 in4 0 9 k in. Since the connections will be through each side of the bottom flange this value can be. divided by two q 0 45 k in, The choice of a bolted or welded connection will depend on field conditions. Welding is generally preferred since it requires less labor than field drilling holes and. surface preparation To determine the spacing of bolts simply divide the capacity of a. slip critical bolt by the shear flow Using 3 4 A325 SC bolts 7 38 k shear capacity. results in,Bolt spacing 7 38 k 0 45 k in 16 4, Note that slip critical bolts are to be used Similarly weld requirements are. determined by calculating the shear flow per foot and designing the weld accordingly. q 0 45 k in 5 4 k ft, Field welds should be 5 16 or less whenever possible allowing the welder to place.
the beads in a single pass Fillet welds of 1 4 are preferred Using E70XX. electrodes, Shear strength of 1 4 fillet 21 ksi 0 707 0 25 in 4 6 k in. Required weld length foot 5 4 k ft 4 6 k in 1 2 of weld per foot. Finally the connection strength should be doubled at each end to account for higher. shear concentrations in these areas The final details are as shown in Figures 5 and 6. Figure 5 Bolted Reinforcement Detail,Figure 6 Welded Reinforcement Detail. 2010 Jonathon C Creviston Page 6 of 10, www PDHcenter com PDH Course S245 www PDHonline org. 5 Reinforcement for Notched Flanges, Reinforcement for damaged or notched must bridge across the damaged area to. transfer the tension or compression forces Notches in the tension flange can be. reinforced by simply adding an area of material greater than or equal to the flange. area adjusted for yield strength as previously discussed. Figure 7 Notched Tension Flange Reinforcement, Reinforcement for the compression flange is designed as a column with the effective.
length equal to the distance between connections Plate may be utilized for short gaps. but the low buckling strength generally prohibits their use for longer notches. Figure 8 Notched Compression Flange Reinforcement, As an example we will look at a W12x26 beam with a 6 notch in the compression. flange The bending moment is 92 ft k The beam is grade A992 and reinforcement. will be grade A36 As a starting point select reinforcement with an area 1 5 times. greater than the flange area multiplied by the ratio of beam to plate yield strengths. Ainitial 1 5Aflange Fy beam Fy reinforcement,1 5 2 47 in2 50 36 5 15 in2. For reinforcement will try two 2 3 4 x 1 x 2 6 bars as shown in Figure 9. Calculate the allowable compression stress,Area of one bar 2 75 in 1 in 2 75 in2. Ix 1 in 3 2 75 in 12 0 229 in4,rx I A 1 2 0 229 2 75 1 2 0 29 in. Use L notch width 1 7 in,KL r 1 0 7 in 0 29 in 24 1.
Figure 9 Reinforcement for,Notched Compression Flange. 2010 Jonathon C Creviston Page 7 of 10, www PDHcenter com PDH Course S245 www PDHonline org. Using Table 4 22 the critical stress Fcr c for A36 material is 20 9 ksi The total. allowable compressive load for the two plates is,Fc 2 2 75 in2 20 9 ksi 115 k. The compression force is determined by converting the bending moment into a force. T C 92 ft k 12 in ft 12 22 in 90 3k, The selected reinforcing plates have adequate capacity Had it not a larger plate. could be used or a grade with a higher yield stress used Connection of the. reinforcement to the beam follows the same procedures as previously discussed. Note however that the unbraced length of the reinforcement will be equal to the. maximum spacing between connectors,6 Corrosion, Steel exposed to harsh environments such as water or chemicals will corrode over.
time Corrosion results in a loss of cross sectional area that reduces the load carrying. capacity of the beam For a beam with near uniform corrosion the loss in moment of. inertia and section modulus will be nearly proportional to the loss in cross sectional. Ic Ac Ai Ii Sc Ac Ai Si, For beams with non uniform corrosion the section must be analyzed to determine the. properties Corrosion of the bottom flange only for instance will cause the centroid. to move upward and reduce the tension flange Modeling the beam with rectangular. elements will simplify calculations and yield conservative results Where a flange or. web is partially corroded using the least dimension or an average value will likewise. simplify calculations The reduced cross section shown in Figure 1 for example can. be modeled as three rectangular plate elements and the composite properties. determined,Figure 10 Cross Section of Corroded Beam. 2010 Jonathon C Creviston Page 8 of 10, www PDHcenter com PDH Course S245 www PDHonline org. Element y A yA I,Bottom Flange 0 13 1 63 0 20 0 03. Web 5 73 1 97 11 30 30 10,Top Flange 11 40 2 47 28 16 0 03.
6 07 39 67 30 16,Itotal 157,Scompression 31,Stension 24. Referring to the AISC Manual of Steel Construction I 204 in4 and S 33 4 in3 for a. W12x26 The tension flange has been significantly reduced and may require. reinforcement, Prior to reinforcement the steel must be cleaned of all loose materials generally with. the use of a grinder or needle gun Beams that are corroded to the point that little or. no material exists should be replaced or reinforced following the same procedures as. notched or cut beams,7 Other Considerations, The majority of beams or girders requiring reinforcement will be existing construction. with material properties that may vary from modern materials For instance. structural steel dated between the mid 1960s and late 1980s will generally have yield. strength of 36 ksi while later structural steel will generally be 50 ksi Structural steel. earlier than the 1960s may be 33 ksi or lower with carbon contents that may influence. welding As a further complication AISC increased the allowable stresses during. World War II to conserve steel AISC provides several valuable resources on this. Design Guide 15 AISC Rehabilitation and Retrofit Guide free to members for. Specification for Structural Steel Buildings 1923 2005 free to download for. Specification for Structural Steel Buildings 2005 free to download for the public. The student is required to read Appendix 5 Evaluation of Existing Structures of the. Specification for Structural Steel Buildings 2005 to provide a foundation of. knowledge prior to implementing any portion of this course. Economy should also be considered in every project Bolted reinforcement is labor. intensive which far outpaces material costs Use of A490 bolts may reduce the. numbers required and result in a net savings Likewise maximizing use of single. pass welds 5 16 or less elimination of special weld requirements and elimination. of awkward weld positions will reduce labor cost Finally where reinforcement is. required in multiple areas the use of repetition and possibly higher grade materials. may result in a savings,2010 Jonathon C Creviston Page 9 of 10. www PDHcenter com PDH Course S245 www PDHonline org. Finally increased loading conditions will likely result in increased reactions at the. beam supports Connection capacity at these supports should always be checked to. ensure adequacy, Beams and girders may require reinforcement due to a variety of reasons increased.
loading corrosion and cutting have been presented as examples Understanding the. Steel Beam Reinforcement Jonathon C Creviston P E SECB 1 Introduction A common problem in industrial facilities is reinforcement of existing beams either due to corrosion damage increased loading or cutting of the beam in critical areas Reinforcement may be for bending moment flanges or shear web The following course presents analysis techniques suggested details and general

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