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Hello I am going to discuss about Lug Angle. Lug Angle is an angle with short lengthwhich is required to share the load of the main angle. Sometimes the main angle carries ahuge amount of load and to make connections of this type of angle section with the mainmember, we need a large number of large number of bolts. So if we use a large number ofbolt or large length of weld then the size of the gusset plate become very high and if size ofthe gusset plate become high material required for gusset plate will be large and it will beuneconomic.So to reduce the length of the joint or the size of the joint to a certain amount sometimes weprovide lug angles at the beginning of the joint to share the load from main angle to thegusset plate. So basically lug angle is connected with the outstanding leg of the main angleand some percentage of load main angle is transferred to the lug angle and then that loadagain is transferred to the gusset plate through connected leg of the lug angle.
I can show a diagram, say suppose an angle sections are connected with a gusset plate, rightsuppose this is an angle section. Now if this angle section carries a huge load then I need lots of bolts and thus gusset plate size is increased. Now to save the material sometimes we maybe able to reduce this length by putting an additional angle connected with the main angle inthis directions.
Therefore, in place of sharing whole forces as it is sharing a percentage of load, so number ofbolts required for this section will be less and therefore we can reduce the length. Howeversometimes it may not be possible because as we are providing extra angle, so some extramaterials we are going to use here and also eccentricity may generate because of theconnections. Therefore, it has certain disadvantages but sometimes we use.
So before going to design, I am going to provide the codal provisions. In clause 10.12, IS800:2007, general procedure of design of lug angles are provided.. So when we will be goingfor designing we have to keep in mind these codal provisions. It is two type one we canprovide angle section, another is we can provide channel section. The main member may beangle or main member may be channel sections.So if main member is angle section then it is told that lug angle is connected to theoutstanding leg of the main angle. Then when unequal angle is used the load gets distributedin the ratio of gross areas of connected leg to the outstanding leg.
That means if an unequal sections are used say suppose this is 75 and this is 50, then load tothis leg will be in the ratio of 50 : 75. So less load will be shared by this angle and more loadwill be shared by this angle. So with this ratio it will be shared, this is what codal provisionssay.Then it says that lug angle and their connections to gusset or other supporting member shallbe capable of developing strength not less than 20 percent in excess of force in outstandingleg of main members. So, whatever load is coming here excess to that 20 percent more loadhas to be carried by this.So when we are going to design a lug angle we have to take the load coming according to theproportion plus 20 percent of that. The attachment of lug angle to the main angle shall becapable of developing strength not less than 40 percent in excess of the force in outstandingleg angle, this also we have to keep in mind.
If the main member is channel section, then we should remember that the lug angle, as far aspossible should be disposed symmetrically with respect to the section of the member. So lugangle should be provided at the top and bottom in a symmetric way. Then lug angle and theirconnection to gusset or other supporting member shall be capable of developing strength ofnot less than 10 percent excess of the force in flange of the channel. And similarly attachmentof the lug angle to the members shall be capable of developing strength of not less than 20percent in excess of that force. So this is what we have to keep in mind while designing thelug angle.
And then other few criteria we have to remember that is in no case fewer than two bolts,rivets or equivalent weld length be used for attaching lug angles to gusset plates or othersupporting member. That means when we are attaching the lug angle with the gusset plate orany other supporting member at least two number of bolt or rivet has to be provided, right.And the effective connections of the lug angles should as far as possible be terminated at theend of the member connected. The fastening of lug angle to the member shall preferably startin advance of direct connection of member to gusset or other supporting members, so thesethings we have to keep in mind when we are going to design this lug angle.
ExampleA tension member carrying a factored tensile load of 180 kN has to convert through a gussetplate of 10 mm thick using 16 mm diameter of ordinary bolt of grade 4.6. The availablelength of the gusset plate for making connection is 250 mm. Design the member & itsconnection. Also design the lug angle if required.
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