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Hello, in last lecture, I have discussed the design methodology of ordinary black bolt. Nowtoday I will discuss the design principle of high strength friction grip bolt. Now in case ofhigh strength friction grip bolt, the friction will be coming into picture for calculating thedesign strength of the bolt. Now as I told earlier that high strength friction grip bolt is usedwhen the external force is quite high. To accommodate the bolt in a shorter length of the joint,we may have to reduce the number of bolt. So in that case, generally we go for high strengthfriction grip bolt with laser number of friction grip bolt. Now I will come to the designphilosophy and first we will see how to calculate the shear strength of high strength frictiongrip bolt.
Shear strength of HSFG Bolts:Now as per the codal provision the formula is given that
WhereVnsf = nominal shear capacity of boltµf = coefficient of friction (Ref. Clause 10.4.3, Table 20)Kh = 1.0 for fasteners in clearance holes= 0.85 for fasteners in oversized and short slotted holes and long slotted holes loadedperpendicular to the slots= 0.7 for fasteners in long slotted holes loaded parallel to the slotsne = number of effective interfaces offering frictional resistance to slipF0 = proof load = Anbf0f0 = proof stress = 0.7 fubAnb = net area of bolts at threadsfub = Ultimate tensile stress in bolt
Now the design shear force,
Vdsf = design shear forceγmf = partial safety factor= 1.1 if slip resistance is designed at service load= 1.25 if slip resistance is designed at ultimate loadReduction Factor for Long Joints:If length of joint lj > 15d, where d = nominal diameter of bolts,
Bolts in Bearing:Now in case of HSFG bolt, we also consider the bearing failure. So this bearing failure wecan calculate as we have calculated in case of bearing type of bolt.
Where,Vnpb = nominal bearing strength of boltfu = ultimate tensile stressd = nominal diameter of boltt = summation of thickness of connected plates experiencing bearing stress in same directionSo this is what we have discussed earlier also, now also the same formula we are using. Similarly, we can calculate the strength due to bearing using this value of Kb. Where,fub = ultimate tensile stress of boltsd0 = diameter of bolt holep = pitch of fastener along bearing directione = edge distanceSo the design force,
Where, γmb = partial safety factor = 1.25
Bolts in Tension:In case of bolt in exerted in tension we can calculate the tensile strength of the bolt as
Where,Tnf = design tensile strength of boltfub = ultimate tensile stress of boltsAnb = net tensile stress area of boltAsb = shank area of boltfyb = yield stress of boltγm0 = partial safety factor = 1.1γm1 = partial safety factor = 1.25The design tensile force,
So similar way whatever we have done here same process we are following in case of HSFGbolt except the shear strength calculation. Bolts with combined Shear and Tension:If bolts are in combine shear and tension, then we have to check the interaction formula
Where,Vsf = applied shear force at service loadVsdf = design shear capacityTf = externally applied tension at service loadTsdf = design tension capacitySo in this way we can calculate the strength of bolt, bolt means HSFG bolt due to shear dueto bearing due tension and due to combined effect.
Praying forces:Now for high strength friction grip bolt, another force will come which is called prying force.Prying force is the additional force coming into picture in case of tension and if thedeformation is allowed between two plates then prying force develops. Let me show throughsome figure, say one plate is attached with a ‘I’ section, as shown in the figure. So what wecan see here that it has an applied force and in the opposite direction of applied force boltforce is coming. So if the applied force is 2Te then bolt force is Te, this equation is true ifdeformation of the flange is not allowed. If deformation is allowed, then what will happenthat some additional forces at the connection will be developed. This additional force is calledprying force that means due to application of load of 2Te, the bolt is getting force as Te but ifwe allow the deformation of the flange then additional prying force will come into picture. Soif Q is the praying force then bolt force will be (Te + Q). So the prying force will bedeveloped at the connection and to withstand that force bolt will face extra force of amount Qand this Q value has been calculated and reported in clause 10.4.7.
Where,Q = additional force of fastener due to prying actionlv= distance from bolt centre line to toe of fillet weld or to half the root radius of a rolled section(figure. 16 of IS 800)le = distance between prying force and bolt centre line
So in case of HSFG bolt apart from shearing, bearing and tension, prying force may alsocome into picture. So the bolt we supposed to take will have some additional value of Q thatmeans if Te is the external force on bolt then actual force will be Te + Q where Q is a pryingforce. Now we will go through one example of HSFG bolt and we will see how to calculatethe shear strength and other strength means bearing strength etc. for HSFG bolt then it will beclear to us.
Example:An ISA 110 mm ×110 mm ×10 mm carries a factored tensile force of 150 kN. It is to be jointed with a10 mm thick gusset plate. Design the joint using HSFG bolt when (a) no slip is permitted, (b) whenslip is permitted. Assume steel is Fe 410 grade.
Solution:Let us provide HSFG bolts of grade 8.8 and of diameter 20 mm.
So here we have seen that if we consider slip critical and if we do not consider slip, if weallow slip and if we do not allow slip how the values are going to change. So design of highstrength friction grip bolt is exactly similar to bearing type of bolt except the friction. Frictioncomponent if we had that means if we consider the slip resistance then we have to find out,the design shear strength accordingly, other things will be exactly similar to the bearing type.
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