In this article we will discuss about:- 1. Bolt Installation – Snug Tight Versus Fully Tensioned 2. Threads Included and Threads Excluded Conditions of Bolt 3. Bolt Assembly 4. Common Types 5. Grade Classification 6. Clearances for Holes 7. Tacking Fasteners 8. Load Transmission 9. Failure.
Bolt Installation – Snug Tight Versus Fully Tensioned:
Common bolts can be installed only in snug tight condition. High strength bolts can be installed either in snug tight condition or fully tensioned condition. For a snug tight installation all the plies should be in contact.
This state can be attained by a few strokes with an impact wrench or by personal effort on an ordinary wrench. For a fully tensioned installation the bolt is subjected to tension to nearly 70 per cent of its tensile strength.
General Specifications for Fully Tensioned Installation:
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Fully tensioned installations are justified in the following conditions:
(i) Joints subjected to stress reversal,
(ii) Joints subjected to tension or combined shear and tension,
(iii) Column splices in tier structures over 40 m in height,
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(iv) In structures of carrying capacity 50 kN, in roof truss splices connections of truss to columns, column splices, column bracings, knee braces,
(v) Connection of beams and girders to columns, and
(vi) Connection for supports of running machinery or live loads producing impact or stress reversal.
Threads Included and Threads Excluded Conditions of Bolt:
Bolts in bearing are designed with shear plane passing through the threads or with shear plane not passing through the threads. Very often a threads excluded condition is assumed in designing the connection. This may be justified while connecting a thin plate, but may not be justified while connecting thick plates. Bolts are generally made with the same thread length for all lengths of bolts.
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The minimum thickness required to achieve threads excluded condition may be taken from the table given below:
Bolt Assembly:
A bolt is a metal rod or pin of mild steel. It has a standard head at one end. The shank of the bolt is provided with threads cut at the other end in order to take a nut. A bolt assembly consists of the bolt, washer and a nut as shown in Fig. 4.9.
For a satisfactory bolted connection, it is necessary that the parts connected are clamped tightly between the bolt and the nut. Nuts should be locked in position after fitting them using a locking device if available, so that they are not liable to become loose due to vibrations, shocks or impact.
ADVERTISEMENTS:
It is worthwhile to slightly blur the top of the thread after it is fitted, in order to lock the fitted nut. Steel washers should be provided under the bolt head and under the nut to prevent direct bearing of the bolt head on the connected member. The bolt shank should project beyond the nut by one or two threads at least.
Common Types of Bolts:
Bolts meant for connection of steel components are of the following types:
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1. Black Bolts (Ordinary Unfinished Bolts):
This is the common unfinished rough bolt with hexagonal head. These bolts are made from black round bars. The bolt head is formed by forging. The diameter at the root of the threads is 1.5 mm to 3 mm less than the diameter of the shank. These bolts have lower strength. They have limited applications and are used in works of minor importance. These bolts are not suitable where members are subjected to slips, shocks and vibrations.
2. Turned Bolts (Precision Bolts):
These are also made from black round bars. These are turned down to the exact diameter with precision. The bolt head on the inner side and also the flat face of the nut are well machined. Washers used with these bolts are also machined on both faces.
In situations where slips, shocks and vibrations are undesirable these bolts are fitted into precisely drilled holes. These bolts are also called close tolerance bolts. These bolts are of two types, viz., Precision, A Grade bolts and semi precision B Grade bolts.
3. Ribbed Bolts:
These are bolts with round head and they have projecting longitudinal ribs over their shanks. As the bolt is driven, the projecting ribs cut the edges of the whole resulting in a tight and firm fit. Such bolts are preferred for connecting members subjected to stress reversal.
4. High Strength Friction Grip Bolts (HSFG Bolts):
These bolts are made of high tensile steel. While fitting, these bolts are adequately tightened using special torque wrenches so as to introduce a predetermined tension in the shank. This enables additional shear resistance developed between the contacting connected plates as a result of friction. These bolts have a larger diameter head and are provided with additional marks of identification. These bolts are used where a genuine need exists.
Advantages of HSFG bolts are given below:
(i) The joint is absolutely rigid since the plates do not slip.
(ii) The bolts which are in high tension provide considerable clamping force on the connected plates due to which large frictional resistance against slipping of the plates is available. The strength of the joint becomes very high.
(iii) Due to high frictional resistance available due to high clamping force shearing and bearing stresses do not occur in the bolts.
(iv) Even outside the bolt hole frictional resistance is available. Hence the load transmitted through the net section is reduced.
(v) Stress concentration in the holes and close to them is eliminated. The fatigue strength is increased. The joint can withstand pulsating or alternating loads.
(vi) The bolts are in high tension close to proof load and hence the nuts are unlikely to get loosened.
Tightening of HSFG bolts to produce a given pre load is generally controlled by the following methods:
(i) Torque Method:
A calibrated manual wrench or a power wrench is used to produce a given torque. The torque produced must be related to the required pre load.
(ii) Turn of Nut:
First a preliminary tightening is done using an ordinary podger spanner, sufficient to bring the surfaces into contact. The nut and the bolt shank end are marked. The nut is then turned further with respect to the shank, one half to three quarters of a turn so as to produce a tension which generally exceeds the minimum proof load of the bolt.
(iii) Direct Tension Indication:
This is a special method using a load indicating washer (coronet) or a load indicating bolt (Lib) to provide a direct indication of the tension in the bolt.
The HSFG bolts are meant to be used with hardened steel washers to protect the connected parts. The contacting surfaces must be free of rust, grease, mill scale etc., which are likely to prevent solid contact between the surfaces and bring down the friction coefficient. It is important to ensure that the bolts are tightened to the required tension; otherwise slip is liable rendering the joint to the state of an ordinary non-preloaded bolt joint.
Bolts are made in a wide range of diameters and lengths. But certain sizes are preferred and are easily available. The sizes of bolts generally used are. 10, 12, 16, 20, (22), 24, (27), and 30. The sizes shown in brackets are not generally preferred.
Grade Classification of Bolts:
The grade classification of a bolt is indicative of the strength of the material of the bolt. The two grades of bolts commonly used are grades 4.6 and 8.8.
For a 4.6 grade 4 indicates that the ultimate tensile strength of the bolt = 4 × 100 = 400 N/mm2 and 0.6 indicates that the yield strength of the bolt is 0.6 × ultimate strength = 0.6 × 400 = 240 N/mm2.
Similarly, for a 8.8 grade bolt, ultimate tensile strength = 8 × 100 = 800 N/mm2 and the yield strength of the bolt = 0.8 × 800 = 640 N/mm2.
The yield stress and ultimate stress for the standard grades of bolts are given in the table below:
The diameter of a bolt hole is about 1.5 mm to 2 mm more than the diameter of the bolt. The IS 800 code has given the following specification regarding the size of bolt holes and spacing of bolts:
Clearances for Holes for Fasteners:
The standard clearances for holes are the normally provided clearances.
The I.S. 800-2007 code has given the following specifications regarding the clearances to be provided for standard size, over size and short and long slotted holes:
(a) Standard Clearance Hole:
Except where fitted bolts, bolts in low-clearance or oversize holes are specified, the diameter of the standard clearance holes for fasteners shall be as given in the table below.
(b) Over Size Hole:
Holes of size larger than the standard clearance holes, as given in the table below may be used in slip resistant connections and hold down bolted connections, only where specified, provided the oversize holes in the outer ply is covered by a cover plate of sufficiently large size and thickness and having a hole not larger than the standard clearance hole (and hardened washer in slip resistant connections).
(c) Short and Long Slots:
Slotted holes of size larger than the standard clearance hole, as given in the table below may be used in slip resistant connections and hold down bolted connections, only where specified, provided the oversize holes in the outer ply is covered by a cover plate of sufficiently large size and thickness and having a hole of size not larger than the standard clearance hole (and hardened washer in slip resistant connection).
Minimum Spacing of Bolts:
The distance between centres of bolts shall not be less than 2.5 times the nominal diameter of the bolt.
Maximum Spacing of Bolts:
The distance between the centres of two adjacent bolts shall not exceed 32 t or 300 mm, whichever is less, where t is the thickness of the thinner plate.
The distance between the centres of two adjacent bolts (pitch) in a line lying in the direction of stress, shall not exceed 16 t or 200 nun whichever is less, in tension members and 12 t or 200 mm whichever is less in compression members, where t is the thickness of the thinner plate.
In the case of compression members where forces are transferred, through butting faces, the distance shall not exceed 4.5 times the diameter of the bolt for a distance equal to 1.5 times the width of the member from the butting faces.
The distance between the centres of any two consecutive bolts in a line adjacent and parallel to an edge of an outside plate shall not exceed 100 mm + 4 t or 200 mm whichever is less, in compression and tension members, where t is the thickness of the thinner outside plate.
Edge and End Distances:
Edge distance is the distance at right angles to the direction of stress from the centre of a hole to the adjacent edge. The end distance is the distance in the direction of the stress from the centre of a hole to the end of the element.
The minimum edge and end distances from the centre of any hole to the nearest edge of a plate shall not be less than 1.7 times the hole diameter in the case of sheared or hand flame cut edges and 1.5 times the hole diameter in case of rolled, machine-flame cut, sawn and planed edges.
Bolts may be arranged to act in single or double shear (Fig. 4.11).
The maximum edge distance to the nearest line of bolts from an edge of any unstiffened part should not exceed 12 tɛ, where ε = (250/fy)1/2 and t is the thickness of the thinner outer plate.
This would not apply to bolts interconnecting the components of back to back tension members. Where the members are exposed to corrosive influences, the maximum edge distance shall not exceed 40 mm plus 4 t where t is the thickness of the thinner connected plate.
Load Transfer to a Bolt:
In most of the connections, the direction of the load is at right angles to the axis of the bolt as shown in Fig. 4.12. The bolt in such a case is subjected to shear load.
In some cases the bolts may be so located that the direction of the load is parallel to the axis of the bolts as shown in Fig. 4.13.
In some cases of load transfer a bolt may be subjected to shear as well tension.
Bolt hole diameters and the minimum edge distances corresponding to bolts of various sizes are given in the table below:
Tacking Fasteners:
These are additional or intermediate fasteners provided when the distance between the centres of end fasteners is large. Tacking fasteners shall have spacing in a line not exceeding 32 times the thickness of the thinner outside plate or 300 mm, whichever is less.
Where the plates are exposed to the weather, the spacing in line shall not exceed 16 times the thickness of the thinner outside plate or 200 mm, whichever is less. In both cases, the distance between the lines of fasteners shall not be greater than the respective pitches.
The above specifications shall also apply to compression members. In tension members composed of two flats, angles, channels or tees in contact back to back or separated back to back by a distance not exceeding the aggregate thickness of the connected parts, tacking fasteners with solid distance pieces shall be provided at a spacing in line not exceeding 1000 mm.
For compression members , tacking fasteners in a line shall be spaced at a distance not exceeding 600mm.
Load Transmission by Different Types of Bolts:
Load Transmission by Black Bolt:
Consider the black bolt connecting two plates as shown in Fig. 4.14. In this connection clamping action on the plates is negligible. Due to load the plates slip so as to come into contact with the edges of the hole [Fig. 4.14 (b)]. The load transmission is by bearing on the bolt and shear on the shank.
Load Transmission by Turned Bolt:
In this connection since the bolt fits in the reamed hole the plates cannot slip. The load is directly transmitted by bearing and shear on the bolt. Due to the elimination of slip only little local bearing stresses are induced. This joint has a greater strength than the joint with black bolt.
Load Transmission by High Strength Friction Grip Bolts:
Consider the connection of the plates by the HSFG bolt shown in Fig. 4.16. In this case by tightening the nut a tension T is transmitted to the bolt to reach a tensile stress equal to 0.8 to 0.9 times the yield stress. As a consequence of this initial tension in the bolt, the two plates get tightly clamped together. If the tensile force in the bolt is T, a compressive force T becomes the clamping force.
The plates are prevented from slipping due to frictional resistance F whose limiting value is µT. where µ is the coefficient of friction between the plate surfaces. If the pull P applied to the plate is less than µT, the plates do not slip at all and the load transmission from one plate to the other takes place entirely by frictional resistance only. If the applied force P exceeds µT, slip between the plates occurs resulting in shearing and bearing stress in the bolt.
Failure of a Shear Joint:
There are four ways in which a shear joint can fail.
The possible modes of failure are the following:
1. Shear failure of the bolt shank.
2. Bearing failure of the plate or the bolt.
3. Shear failure at the end of the plate.
4. Tension failure of the plate.
To prevent the failure modes 1 and 2, sufficient bolts of suitable number must be provided.
To prevent the failure mode 3 sufficient end distances must be provided.
To prevent the failure mode 4, the plate must be designed to provide sufficient effective sectional area.