Bolted connections in steel structures Design| Failure of bolt connection

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What is bolted connection?

A bolt is a small piece of metal equipment made up of high-strength steel. It consists of a head and a shank. The size of the bolt is designed according to the diameter of the shank. The shank is threaded at the end to receive the nut.

Commonly available nominal diameters of High strength friction grip bolt (HSFG) is 16mm, 20mm, 24mm, 30mm, and 36 mm.

Design of bolted connection as per is 800

Advantages of bolted connection

  • It is a faster way of installing any members on site.
  • The connection of the bolt does not require electricity which makes it a noiseless connection.
  • Bolted connections in steel structures is simple to apply on the members ie. no skilled human power is required.
  • It is not necessary to carry extra equipment to install connect bolt.
  • Bolt connection can be easily replaced, retighten, and removed if there is any damage in any parts of the structure.
  • The members connected with bolts can carry the load immediately after installation.
  • There is no chance of fire in the structure since bolt connection is the cold process of fabrication.

Disdvantages of bolted connection

  • Bolted connections in steel structures spoils the appearance of the structure.
  • There may be a slip of bolt after repeatedly applying of load for a long period of time.
  • Bolt required a hole in the plate for connection, so it reduces the area of the plate where the bolt has to be connected. It makes the plate weaker than the previous one.
  • The bolt should be protected with pains or chemicals to prevent it from corrosion.
  • Nuts and bolt connections may loosen if there is too much vibration due to load or hammering.

Difference between bolted and welded connection

S.NBolted ConnectionWelded connection
1.No skilled human resource is requiredIt requires skilled human resources
2.It is a cold process. i.e fire does not involve in itIt is a hot process. Fire involves in it
3.It can be replaced and removed if requiredIt can not be removed and replaced but can be repeated on the welded portion
4.It is noiseless while working which does not require special equipment for fabricationIt creat noise while welding and requires special equipment for fabrication
5.The connection may loose due to the continuous vibration of the structureThe welded connection remains stable for a long time even after vibration
6.It is cheaper to for structure since bolt and nuts can be replaced and reuse.It is expensive for structure since welded materials can not be replaced and reused properly

Failure of bolted connection

  1. Bearing failure
  2. Net-Tension Failure
  3. Shear out failure
  4. Cleavage failure
  5. Cleavage tension failure
  6. Bolt failure
Design of bolted connections in steel structures | Failure of bolt connection

How to prevention bolted connection from failure?

The failure of bolted connection can be prevented by applying high strength plate and bolt. It should be tightened with the help of special equipment. All of the bolts should be tightened equally otherwise connections may be loose due to those loosened bolts.

Design of bolted connections in steel structures as per IS 800

Some terms used to design bolted connection are given below,

Design of bolted connections in steel structures | Failure of bolt connection
Bolted connections in steel structures

Pitch of the bolts:- The center to center distance between the bolts in the direction of applied load is known as the pitch of the bolts.

Gauge distance:- The distance between two adjacent bolts perpendicular to the applied load is known as gauge distance.

Edge distance:- The distance between the end of the plate and the center of the hole of the bolts is known as edge distance.

End Distance:- The distance between the end of the plate and the nearest bolts hole is known as edge distance.

Staggered Distance:- The center to center distance between the staggered bolts measured obliquely on the plate is known as staggering distance.

For Minimum pitch

The minimum pitch should be 2.5 times the normal diameter of the bolt to avoid tension failure.

For Maximum pitch

  1. For compression members, the maximum pitch should be 12 tm or 200 mm whichever is lesser (In the direction of stress) 
  2. For tension members, the maximum pitch should be 16 tmin or 200 mm whichever is lesser (In the direction of stress) 
  3. For both, 32 tmin or 300 mm whichever is lesser (perpendicular to the direction of stress) 
  4. In the case of compressing in which forces are transformed through butting faces, the distance should not exceed 4.5 times the diameter of the bolts for a distance from the butting faces equal to 1.5 times the width of the member. 
  5. The distance between centers of any two consecutive bolts in a line adjacent and parallel to an edge of an outside plate not greater than (100mm + 4t) or 200 mm whichever is less in compression or tension member. 
  6. When bolts are staggered at equal intervals and the gauge does not exceed 75 mm, the distance specified in (i), (ii), (iii), and (iv) between the center of bolts may be increased by 50%. 

Note:-

  • The diameter of a bolt hole should be taken as the nominal diameter of the bolt plus 1.5 mm unless specified. 
  • In calculating shear & bearing stress the effective diameter of a bolt shall be taken as its nominal diameter.

Permissible stress in bolts

S.NDescriptionAxial Tension (N/mm2 )Shear(N/mm2)Bearing(N/mm2)
1Close tolerance and turned bolt120100300
2Bolts in clearance holes8080250

Bolted connection calculation example

Q1. A boiler shell is made up of a 14mm thick Fe-415 plate. If the joint is a double-jointed lap joint with M16 bolts of grade 4.6 at a distance of 50 mm, determine the designed strength of the joint per pitch width. Is it a safe design if the internal diameter of the bolts is 1 meter and steam pressure is 1.2 Mpa.

Solution,

Since the boiler is subjected to hoop tension, the pitch of the joint is as shown in the figure below. In such a case the strength is worked out per gauge width of the joint and checked for safety and efficiency.

Design of bolted connections in steel structures | Failure of bolt connection

Given, Strength of plate per 50 mm width,

Diameter of bolt (d) = 16 mm

Diameter of bolt hole (do) = 18 mm

Strength of plate per 50 mm width,

t = 14 mm, b=p=50 mm, fu = 410 Mpa

No. of bolts in double bolted joint per 50 mm width n=1

Therefore, An = (50-1*18) *14 = 448 mm2

or, Design strength of plate per 50 mm width,

T = (0.9 * 410*448)/1.25 = 132250 N = 132.250 kN

Strength of bolts per 50 mm width,

Since it is lap joint, shear planes at shanks= 0

As there are two bolts per pitch width considered,

nn= 2

Where, Pr is applied pressure and D is the diameter of the boiler.

= (1.2*1000)/(2*14) = 42.857 N/mm2

Therefore, force per 50 mm length

= 42.857 * 14 * 50

= 30,000 N

= 30 KN

Factored design action = 1.5 * 30 = 45 KN

Design strength = 57.494 KN > Design action

Hence the design is safe. Answer

Q.2. Two cover plates, 10 mm and 18 mm thick are connected by a double cover butt joint using 5 mm cover plate as shown in the figure below. Find the strength of the joint. Given M20 bolts of grade 4.6 and 415 plates are used.

Design of bolted connections in steel structures | Failure of bolt connection

Note:- Thickness of thinner plate (t) = 10 mm

Therefore, the Design strength of a bolt,

= (124250.5/1.25) = 99400 N

So, Design Strength of 6 bolts in bearing

= 6 *99400

= 595400 N

= 596.4 KN>557.896 KN

Therefore, Strength of bolts in connection = 557.896 kN

Strength of plates in the joint = Strength of thinner plate at weakest section

The design strength of plate,

= 395568 N

= 395.568 kN < 597.897.895 kN

The design strength of the joint = 395.568 kN Answer.

I hope this article on “Bolted connections in steel structures” remains helpful for you.

Happy Learning – Civil Concept

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