Course on Design of Steel StructuresProfessor Damodar Maity

Department of Civil EngineeringIndian Institute of Technology Kharagpur

Lecture 05Module 1

Introduction to Bolt Connection

In the previous lecture various aspects of different connections which includes rivet

connections, bolt connections and welded connections are discussed. As rivet connection is

becoming absolute nowadays, we have not concentrated on rivet that much. We concentrated

only on the bolt connections in details. In case of bolt connections when we are going to

design we need to know certain terminology that we have already discussed. This lecture

focused on the bolt connections in particular where the different pattern of bolt connections is

used. For different pattern of bolt connections, how the strength is going to vary, that will be

discussed.

(Refer Slide Time: 1:20)

In case of bolt connections or in case of weld connections also we have different type of

arrangements and depending upon the arrangement of bolts and plates, we designed the bolt.

Again depending upon the mode of load transmission, nature and location of load, we

designed the bolt connection.

(Refer Slide Time: 1:58)

Now coming to depending upon arrangement of bolts we have two type of joint we can make,

one is lap joint another is butt joint. So in case of lap joint we have seen earlier that we can

make a single bolting, chain bolting and staggered bolting. And in case of butt joint similarly

we can make single bolting, chain bolting and staggered bolting.

(Refer Slide Time: 2:40)

Fig. 1 Chain bolting

So coming to the different bolting pattern first let us come to the chain bolting. In case of lap

joint the two plates are overlapped together and are joined by different ways of connection

like bolt, rivet or welding. Now in case of chain bolting the bolt will be in same line and also

in same row and column as shown in fig. 1. The failure pattern for this type of bolting will be

discussed later.

Fig. 2 Zig-zag bolting

Now in case of zig-zag bolting, the position bolts are in a zig-zag way as shown in fig. 2. For

this type bolting the failure may occur in various direction or path. So we have to calculate

which will be the weakest section and according to that we have to calculate the lowest

strength of the connection and accordingly we will design the connection.

Fig. 3 Diamond bolting

In case of Diamond bolting, the number of bolts are increased towards the center as shown in

fig. 3. Similar type of failure as zig-zag bolting may occur in this case also. We generally

prefer diamond bolting because it is much more efficient.

(Refer Slide Time: 5:52) 7:44

Fig. 4 Different types of bolting depending upon arrangement of bolts and plates

Depending upon arrangement of bolts and plates there are various type of bolt connections as

shown in fig. 4. In single bolted lap joint and triple bolted lap joint two plates are jointed

together by one and three bolts respectively without any cover plate. But in case of single

bolted single cover butt joint single bolt per member and one cover plate is used, where in

Single bolted lap joint Triple bolted lap joint

Single bolted single cover butt joint Single bolted double cover butt joint

Double bolted double cover butt joint

single bolted double cover butt joint double cover is used. Again in double bolted double

cover joint 2 bolt per member and double cover is used.

(Refer Slide Time: 7:59)

Fig. 5 Bolts with different shear

Now depending upon the mode of load transmission we can define a bolt by single shear,

double shear, or multiple shear as shown in fig. 5. When the two plates are joint together

through bolt then only one shear plane is there that is why this is called single shear and if

three plates are connected through bolt, there will be 2 shear plane, hence it is called double

shear. When the number of shear planes is more than two then it is called multiple shear.

(Refer Slide Time: 8:14) 9:19

Now depending upon nature and location of load we also can find out different type of

connections like direct shear connection, eccentric connection, pure moment connection and

moment shear connection.

(Refer Slide Time: 9:56)

Fig. 6 Direct shear connection

Single shear Double shear Multiple shear

In case of direct shear connection, we can see that two plates are overlapped together and

connected through bolt as shown in fig. 6, where bolts may fail due to shear so we have

design accordingly.

Fig. 7 Eccentric connection

In fig. 7 a typical eccentric connection is shown where, beam and column are connected

through gusset plate. Now if we have a load P and eccentricity e as shown in the fig. 7 then

such type of connection exerts moment because this is an eccentric connection. So for

eccentric connection here we can see that the bolt will experience stresses due to the direct

load P the moment P×e.

Fig. 8 Pure moment connection

In pure moment connection the loading is symmetric as shown in fig. 8. So here only moment

will be developed in the connection, so this is called pure moment connection.

Pe

P

P

(Refer Slide Time: 14:05)

Fig. 9 Moment shear connection

In case of moment shear connection, both moment and shear are developed due to the

arrangement of the connection as shown in fig. 9.

(Refer Slide Time: 15:03)

Now we will discuss about the different type of failures because when we are going to design

a bolt, when we are going to find out the strength of a bolt, or strength of the connection as a

whole we have to find out what are the type of failure may occur and to arrange those failures

what are the mathematical equations are developed and how we will calculate the strength.

The failure of bolt connection may occur due to the failure of bolt or the failure of the plate.

The bolted joint may fail in any of the following seven ways, out of which some

failure can be checked by adherence to the specifications of edge distance. Therefore,

they are not of much importance, whereas the others require due consideration.

Fig. 10 Possible modes of failure of bolted connections

(Refer Slide Time: 16:16)

As discussed earlier, there are different types of shearing failure which may occur in bolted

joint. In single shear only one shear plane is developed in the shank portion of the bolt

wherein double shear two shear plane are developed in the bolt.

(Refer Slide Time: 18:20)

Now we will see bearing on plate and bolt and how bearing failure happens. So if two plates

are connected with bolt then due to bearing on plate it may fail that means it may fail by

crushing as shown in the above figure. So this is one type of failure which is called bearing

failure.

Another type of failure may come in to picture also which is called shear tear-out of plane

that means this is somewhat it will look like as shown in the above figure which may fail by

crushing due to shear tear-out of plate. So such type of failure may occur when force P is in

the direction as shown in the figure.

Another failure may come which is called tension failure of plate because when two plates

are under tension and we are going to join through bolt by insertion of hole then along that

hole line tension failure of plate may happen. Suppose we have a bolt as shown in the figure,

where the two plates may tear out as shown in the figure due to failure of plate. So this is

another sort of failure, so different modes of failure of bolt joints we are observing.

(Refer Slide Time: 21:43)

And another failure is called block shear failure. When a plate is connected with another plate

say for example a gusset plate and a force P is acting and if bolts are inserted as shown in the

figure then the block shear failure may occur as a hole.

(Refer Slide Time: 23:31)

Now we have to remember certain things for an efficient connection using bolt that is one is

the stress concentration results in a considerable decrement in the tensile strength so we have

to try to avoid the stress concentration. Then loose fit of the joint can reduce the stiffness

which may result in excessive deflections. So that has to be taken care that means we have to

tighten the bolts properly so that the loose fit does not occur and joint does not get reduced

instantly.

Then vibration cause loosening of nuts which can jeopardize the safety of the structure. So

this has to be taken care while designing a joint using bolt.

(Refer Slide Time: 24:29)

And certain criteria we have to remember for designing the joint under axially loaded

members. First of all, the length of joint should be as small as possible to save material on

cover plates and gusset plates.

(Refer Slide Time: 24:51)

Say for example we have a gusset plate connected with truss member as shown in the above

figure. So how do we connect all the members at a particular point? This type of joining is

possible; we have to join in such way that cg of each members pass through a particular point

otherwise eccentricity may develop then the moment due to eccentricity may come into

picture. The length of joint should be as less as possible to reduce the material amount of

material that means we have to join through a gusset plate. Now if we provide more number

of bolts then we will not be able to reduce the length of joint that means we cannot reduce the

amount of material required for gusset plate, that is why we have to keep in mind that as less

as possible the length of joint should be. Another thing is that the center line of all members

meeting at a joint should coincide at one point only otherwise the joint will twist out of

position. The number of bolts should be increased gradually towards the joint for uniform

stress distribution in bolt. For this type of connection, we prefer diamond bolting where the

number of bolts increased towards the center.

(Refer Slide Time: 28:05)

Then the arrangement should satisfy the pitch, gauge and edge distance requirement, that

means what will be the pitch distance, what will be the gauge distance, what is the edge

distance all the things has to be satisfied. Next is the strength of joint reduces due to the bolt

holes. So the reduction in area due to bolt holes can be minimized by arranging a zig-zag

form or diamond form and the arrangement can be made in such a way that the strength can

be increased.

So in short while designing a connection using bolt we have to remember certain criteria one

is the length of the joint should be as less as possible. Now if we increase the diameter of

bolt, we should not require large number of bolts. We have to try to reduce the number of

bolts so that the number of holes become less so the tensile strength of the member becomes

high as well as because of less number of bolt the length of the joint will be less and thereby

the gusset plate require for joining plates will be less. So the amount of gusset plates can be

minimized through this.

Another thing is the members meeting at a point should be designed in such a way that cg of

all the members are coinciding at a particular point, otherwise due to eccentricity the

additional moment may generate that also has to be keep in mind. Another important thing

which have to keep in mind that whether we are satisfying the pitch and edge distance

properly because as per codal provisions we have to consider the minimum pitch and

maximum pitch. So the pitch distance whatever we will provide we have to be in between

maximum and minimum as prescribed in IS 800: 2007.

So with this I would like to conclude todays lecture saying once again that when we are going

to design a bolt connection we have to keep in mind what type of connections we are going to

handle and accordingly what type of load may come and whether we are going for lap joint or

butt joint, whether we are going for eccentric connections or concentric connection we have

to see and accordingly we have to calculate the external forces and then we have to find out

the bolt strength once we get bolt strength we can find out the number of bolt and we can

design accordingly with the use of codal provisions for pitch distance, edge distance and

gauge distance.

In next class we will discuss the bolt strength due to different failure means different aspects

like bearing failure, shearing failure, tearing failure what will be the bolt strength that we will

calculate as per the codal provisions, thank you very much.