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Module 1: Design of Batten Plates and Column Splices

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Video 1
In this lecture I am going to discuss about batten plates. Batten plates are used when the builtup sections are carrying axially compression load. So batten plates unlike lacing, lacing platesare placed in inclined way where the batten plates are placed perpendicular to the axis of thecolumn that means if columns are vertical then batten plates should be horizontally placed.Now batten plates are generally placed in both the side with equal spacing and in both theside at same position we provide batten plates and minimum number of batten plates alongthe length of the column will be atleast three. So in the codal provision it is told, right and as Itold that batten plate will be generally used when the purely axial compressions are acting onthe built-up column. So for that case we generally use batten plates.But when eccentricity comes into picture in the compression member then this does not workefficiently, so in that case we have to go for lacing system. So in this lecture our discussionswill be on batten members. In case of batten member we have to know certain things like oneis what will be the length of batten member, what will be the depth of batten member andwhat will be the thickness these three things we have to know. Then also we have to know thespacing, spacing between two batten members, then the connection between batten memberand the and the main member.So connections may be of two type bolt connections or weld connections. So what are theforces coming on the joint that we have to calculate and then we have to find out what will bethe number of bolts, how bolts will be placed so all the details can be made, right.
So let us come to some general requirements which are given in code IS: 800-2007 in clause7.7, which stated that compression members composed of two main components battenedshould be preferably have their two main components of the same cross section andsymmetrically disposed about their major axis, right. Where ever practicable the compressionmember should have a radius of gyration about the axis perpendicular to the plane of battennot less than the radius of gyration about the axis in the plane of batten.Let us provide two channel section back to back. Now this sections has to be provided in sucha way that ryy and rxx will be becoming mostly same or ryy should be little higher. So becausewe cannot change the value of rxx, rxx will be same for all the cases but if we increase thespacing between these two ryy value will be increased because Iyy value is going to increase.So ryy has to be increased in such a way that the strength in both the axis become more or lesssimilar.
Now in next clause that is in 7.7.1.3 it is told that the batten shall be placed opposite to eachother at each end of the member and points where the member is stayed in its length and asfar as practicable, be spaced and proportioned uniformly throughout and number of battensshall be such that the member is divided into not less than three bays within its actual lengthfrom center to center of connection.Another consider has to be remember that is the effective slenderness ratio, in case of battenmember the effective slenderness ratio of the column will be increased by 10 percent, youhave remember that what will be the increase of effective slenderness ratio for lacingmember. So in case of batten member it will be 10 percent, so these are some codalprovisions which we need to keep in mind.
Now in clause 7.7.2 we can see that the transverse shear to the batten is considered as 2.5percent of the axial force, right so that also has to be consider. So similar to the lacing systemthe transverse shear on batten member will come as 2.5 percent of the axial compressiveforce. Next the longitudinal shear on the batten member say Vl, which can calculate asLongitudinal shear Vl=V CNS along the column axisAnd moment M=V C2N at each connection,Where,C = Center to center distance of battens longitudinallyN = No of parallel planes of battensS = minimum transverse distance between the centroid of the bolt group/weldingconnecting the batten to the main memberV = The transverse shear forceNow few things we have to remember while using batten members, one is batten plates arehas to be provided symmetrically as far as possible. And at both ends we should providebatten, right batten plates should be provided both ends of the member or tie plate we canprovide.
And another thing again I am repeating that the number of battens should be such that themember is divided into not less than three bays. So say suppose I am providing one battenhere, another batten here so atleast three bays I have to provide, right and they should bespaced and proportioned uniformly throughout means this spacing should be uniformed, rightand minimum three battens has to be provided.
Now coming to the dimension of batten, so how to find out the batten thickness, batten lengthand batten depth. So thickness when I am coming which is mentioned in clause 7.7.2.3 battenminimum thickness tmin can be considered astmin>ai50Where, ai is the distance between the inner most connecting lines of rivet or bolt or weldperpendicular to the main member.Then we have to find out the batten depth of batten, so the effective depth should be• d > 0.75a for intermediate battens• d > a for end battens• d > 2b for any battensa = centroid distance of members
b = width of member in the plane of battenTherefore, the batten depth should be higher at the end batten. However in any case d can begreater than 2b, where b is the width of the member in the plane of batten.
Now let us come to the spacing of the batten, criteria for spacing between two batten is givenin clause 7.7.3, where it has beem mentioned that this spacing should be uniformedthroughout its length as far as possible.The spacing of the battens (C) be such that the slenderness ratio of the lesser main componentover the distance is not greater than 50 or 0.7 times the slenderness ratio of the main memberas a whole, about the axis parallel to the batten.Crminc<50∨0.7 λThen the end condition end conditions are described in clause 7.7.4, right. Design the endconnections to resist the longitudinal shear force V1 and the moment M as calculated in earlierstep.Similarly for bolt I will find out what is the forces or stresses coming in the bolt due tomoment and due to longitudinal shear and then we have to find out whether the number ofbolts we are going to provide is sufficient to carry that much load or not, right.
And in this clause it is told that for weld connections lap should be greater than 4t and thetotal length of weld at edge of batten should be greater than D/2, right. Then length of weld ateach edge of batten should be less than one third of total length of weld required and returnweld along transverse axis of column should be less than 4t.
This is what I was discussingTotal length of weld at edge of batten >D/2Length of weld at each edge of batten < 1/3 total length of weld required.Return weld along transverse axis of column < 4t.
Video 2
Now to summarize the things whatever I have discussed I will show few slides next, nowwhile going for design of batten member we will see what are the steps used to be carried outsystematically so that the batten members can be designed. So in fact when we are going todesign a batten member or we are going to do some workout example for designing of battenmember then we have to follow certain steps otherwise we will not be able to do the designproperly and we will be writing a computer program also step by step we have to go through.Step 1:Find the transverse shear Vt= 2.5100×PCalculate longitudinal shear along the column axis as V l=Vt CNSandCalculate moment at each connection as M=Vt C2N,Step 2:Calculate effective slenderness ratio (λe) as 1.1 × λStep 3: -For a given shape, find out gauge distance g on each side & find the distance a between thebolt
Step 4: -Calculate spacing of the batten plates (C) from the following conditions:Crminc<50∨0.7 λMinimum 3 nos of batten should be provided along column length.Step 5: - Size of end battens:Effective depth (d )=s+2×C yy and d >2b,[b is width of member in the plane of batten]Overall depth (D) = d+2e, e being edge distanceLength of batten ¿S+2bThickness of batten = a/50 where a is the distance between inner bolt/rivet/weld
Step 6: - Size of intermediate battens:Effective depths+2×Cyy(d )= 34¿ ) and d >2b,[b is width of member in the plane of batten]Overall depth (D) = d+2e, e being edge distanceLength of batten ¿S+2bThickness of batten = a/50 where a is the distance between inner bolt/rivet/weldStep 7: -Design the end connections for batten system to resist calculated V l and M.So these are the steps which will be used to design a batten system, right. So in this lecturewhat in summary if we discuss what we have learned that certain specifications in terms ofthe dimension of the batten and its spacing has been calculated as per the codal provisionsand code has dictated certain guidelines as per the guidelines we have provided the steps andalso we have seen how to calculate the transverse shear, then longitudinal shear and momenton the batten plate and then we need to design the end connections the connection using weldor bolt to resist this moment and longitudinal shear, right.And in next class we will go through one example through which it will be clear how todesign a batten, thank you.