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Module 1: Compression Members

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Video 1
Now I will be discussing about the design compressive strength of a double angles section.Now double angles section is oftenly used because many cases appear when the single angleis not capable of taking that much load and also in case of single angle the radius of gyrationabout the minor axis is very low compared to its X axis, or Y axis, or major axis. Therefore asradius of gyration is quite low about in minor axis, so strength of the angle section is quitelow because it may buckle about its minor axis.And if radius of gyration is low then slenderness ratio will be high and if the slenderness ratiohigh then the design compressive stress will be less and therefore the design strength of themember will be less. Therefore with the given load sometimes we go for double anglesection. Double angle section may be placed by back to back with gusset plate means twosides of gusset plate two angle section can be provided or in the same side of gusset plateangle sections can be provided. So this is how one can place the angle sections to makedouble angle section.
Now to know the design strength of the angle section first we have to know the effectivelength of the double angles which is given in clause 7.5.2. So the codal provision says thatdepending on the degree of restraint provided the effective length will be consider as 0.7 to0.85 times the distance between intersections and the effective length KL, in the planeperpendicular to that of the end gusset shall be taken as equal to the distance between centreof intersections. That means here K value will be 1, so this is what the codal provision says.So when we will be going to calculate the strength of a double angle section we have tofollow this codal provision 7.5.2 to find out the effective length and thereafter the slendernessratio and then compressive stress, the allowable compressive stress. Therefore we have toknow the effective length of the double angle section means whether it is according to thedegree of restraint means whether the length is in the plane perpendicular to that of endgusset or other one
So angle sections may be provided like this, say suppose two angle sections I want toprovide. Now that may be possible if I use two angle section on the opposite side of gussetplate, this is one way we can make, the other way we can provide like two angle section insame side of the gusset plate.Also the two angles can be provided like star formation also however it depends on therequirement means what type of architectural requirement is there depending on that we canprovide such type of double angle section. And also we will see if I provide angle section opposite side to the gusset plate or same side of the gusset plate then at which formation thestrength will be more and that can be check through one workout example.
Video 2
Example: A discontinuous strut of length 4 m consists of two unequal angles ISA 100×75×8and is connected to a 10 mm thick gusset plate by its longer leg. Determine the strength if it isconnected on the:i) Opposite side of the gusset plateii) Same side of the gusset plate
Strength of the member when Angles placed on(i) Opposite sides of the gusset plate: 258.46 k N(ii) Same side of the gusset plate: 148.78 k NSo what we could see for the same angle if it is placed opposite side to the gusset plate itsstrength is quite high compared to this that means we will try to prefer always the angles tobe placed opposite side of the gusset plate. So that the strength can be achieved morecompared to the earlier case means when angles are placed same side of the gusset plate.