Course No. 724 Senior Professional Course ( Bridge & General )
Project Report On High Strength Friction Grip Bolts For Railway Bridges
By Vinod Bampal DyCE/D/CCG, Western Railway
IRICEN, PUNE Indian Railway
Index
Contents
Page No.
1.
Introduction
1
2.
Types of High Strength Friction Grip Bolts
2
3.
Load transfer mechanism for Friction Grip Bolts
4
4.
Clamping Forces induced by “HSFG Bolts”
5
5.
Advantage of “HSFG Bolts” over “Riveting”
5
6.
Conclusion
7
7.
Recommendations
7
8.
References
8
High Strength Friction Grip Bolts for Railway Bridges
1.
Introduction :-
The “Connections” are critical components of steel structure as they have the potential for greater variability in behaviour and strength. They are more complex to design than the members and are usually the most vulnerable components in the structural system, consequent of geometric imperfection, complexity of connection geometry and residual stress and strains. The “connection” are normally made either by riveting / bolting or by welding. Normally the “Riveted Connections” are of bearing type and the “Bolted connections” are of both bearing type as well as friction type. In our Railway bridges, “Riveted Connections” are being used in all type of situation. But it is seen that the “Riveted Connections” fail where repetitive dynamic loads are experienced. In the “repetitive dynamic loads” situation, the “HSFG Bolts” are better substitute of Rivets.
2.
Types of High Strength Friction Grip Bolts :-
AS per IS-1367-1979, the “HSFG Bolts” shall be of two property class 8.8 or 10.9, which have different strength and other characteristics. The dimensional details of these “HSFG Bolts” (M16 to M36 dia bolts) are furnished in the IS-3757-1985. Similarly the dimensional details of high strength nuts and washers are also furnished in the IS-6623-1985 and IS-6649-1985 respectively.
Dimensions for High Strength Structural bolts
As per the IS-4000-1992, the design forces of these “HSFG Bolts” are furnished as under. Maximum Permissible Applied forces in bolts in kN. Size of Bolt
Stress Area of Bolt, in mm2
Tension in Bolts of Property class Class 8.8
Shear in bearing type joints for bolts of Property class Class 8.8
Class 10.9
Class 8.8
Class 10.9
Shank
Thread
Shank
Thread
Class 10.9
Shank
Thread
M16
201
157
56.7
78
40.2
31.4
52.3
40.8
M20
314
245
88.2
122
65.2
50.8
81.6
63.7
M24
452
353
127
176
93.8
73.2
117
91.8
M30
706
561
202
280
146
116
148
146
M36
1017
817
294
407
211
169
264
212
3.
Load transfer mechanism for Friction Grip Bolts :-
When a “HSFG Bolts” is tightened against the two plates, it is basically clamping two plates together and this clamping pressure, induced by “HSFG Bolts”, induces a frictional resistance between the plates. When a tensile force is applied to these two plates, than the frictional resistance is induced between these plates due to clamping pressure, which resist the relative movement/slip between the plates and the tensile force is transferred from one plate to another plate. The “HSFG Bolts connections” are designed such that under the service load the force on plates does not exceed the frictional resistance so that the relative slip is prevented between these two plates during it’s service life. When the applied tensile force on these two plates exceeds the frictional resistance between the plates, only then the plates slip and the “HSFG Bolts” comes in the contact with the plates and the further excessive forces are transferred through “HSFG Bolts”.
The tensile force which a joint can safely resist, T = Sf P / nf Where P = clamping force on “HSFG Bolts” Sf = slip factor, 0.45 for steel plates nf = safety factor, 1.4 for normal load condition and 1.2 for wind / EQ load condition. For two interface in one joint, the tensile force doubles T = 2 x Sf P / nf
4.
Clamping Forces induced by “HSFG Bolts” :-
The “HSFG Bolts” are designed for the tensile forces and the total clamping force, induced by the “HSFG Bolts” is given as under. Clamping force induced by “HSFG bolts” P = 0.9 x Ar f y Where Ar = Area at the root of the thread of bolt f y = yield stress of “HSFG bolts” In case “HSFG Bolts” are subjected to combined action of shear and tension, the following relation has to be satisfied. Fs Ps
+
0.8
Ft Pt
1.0
Where Fs = Applied shear Ft = Applied tension Ps = Shear capacity of HSFG bolt Pt = Tension capacity of HSFG bolt 5.
Advantage of “HSFG Bolts” over “Riveting” :-
To under stand the advantage of “HSFG Bolts” over the “Rivets”, first we must understand the load transfer mechanism of the “Rivets” and the “HSFG Bolts”. In case of the Rivets, the entire tensile force on one plate is transferred to another plate only through Rivets and for this we take advantage of the shear / bearing strength of the Rivets. During transfer of the force, the entire force on one plate will converge on the Rivet, thus increasing the stresses many fold at Rivet hole and than the entire force is transferred to another plate through the Rivet, by taking advantage of the shear strength of the Rivet. Although the increased stress on the plate is within the permissible limits but in the repetitive dynamic loading condition, stress range are increased considerably near the Rivet hole area and thus this “Rivet connection” will fail much earlier than the member/plate due to fatigue. A figure showing the stresses induced in the “Rivet connection” is shown as under.
In the case of “HSFG bolts”, the entire tensile force on one plate is transferred to another plate through frictional forces between these two plates, which is induced due to the clamping force of the “HSFG bolts”, not by the HSFG bolt’s shearing strength (Rivet connection case) since there is no relative slip between the two plates. The tensile force/ stress are transferred smoothly only through surface of the plates and in fact during transfer process, the stress in the connection are reduced, since we have two plates together to take care of the applied tensile force/stress. This way the “HSFG bolt connection” is much stronger then the original member/plate, since the stress in the “HSFG bolt connection” is lower then the member/plate. During repetitive dynamic loading, due to less stresses in the “HSFG bolt connection”, the member/plate will fail earlier than the “HSFG bolt connection” due to fatigue. A figure showing the stresses induced in the “HSFG Bolts connection” is shown as under.
In the repetitive dynamic loading condition, the stresses/ “stress ranges” induced in the “Rivet connection” are much larger that the “HSFG bolt connection”.
In fact the stresses in the “HSFG bolt connection” is lesser than the stresses in the member/plates . As per the S-N curve in the AREMA code, the plain member/plate will come under ‘A’ category and “HSFG bolt connection” & “Rivet connection” will come under ‘B’ and ‘D’ category respectively. For the same tensile forces on the plates, due to decrease in the fatigue stresses ranges, the fatigue life of “HSFG bolt connection” (80 million cycles) will increased by 258 times over the “Rivet connection” (0.31 million cycles). In fact the fatigue life of “HSFG bolt connection” will increased by 1.14 times when it is compared with the plain member/plates (70 million cycles). It means that the “Rivet connection” will fail in fatigue, 258 times earlier then the “HSFG bolt connection” and in fact the member/plate will also fail earlier then the “HSFG bolt connection”.
6.
Conclusion :-
The “HSFG bolt connection” can take more load, so the number of the required “HSFG Bolts” are reduced compare to the “Rivet connection”. In the repetitive dynamic loading condition, the stresses/ “stress ranges” in the “HSFG bolt connection” are much lower then “Rivet connection”, the fatigue life of the “HSFG bolt connection” will be much more than the “Rivet connection”. In fact the member/plate will fail earlier then the “HSFG bolt connection” in fatigue condition. So the “HSFG bolt connection” is much suitable at the location where dynamic forces are excessive and there are frequent failure of “Rivet connection”.
7.
Recommendations :-
1. In Railway Bridges, on the stringers and its connection with cross girders, there are excessive dynamic forces due to high axle loading. So it is strongly recommended to fabricate the stringers and its connection with cross girders
with “HSFG Bolts” for the safety as well as economic consideration. Similarly other connections in the Open Web Girder, can be also be made of “HSFG Bolts” where more dynamic forces are experienced or at crack prone areas like verticals etc. 2. All the repairing of the fatigue prone members of Railway bridges shall be done with “HSFG Bolts”. 3. All the retrofitting of Railway bridges for higher axle loads, shall be done with the “HSFG Bolts”, since this will increase the fatigue stresses limits, thus making Railway bridges fit for higher axle l oading. 4. In the welded plate girders, the splice j oints shall be of “HSFG Bolts”, in stead of “Rivets”.
References :-
8.
1. 2. 3. 4. 5. 6. 7.
IS-3757-1985 for the details of High Strength steel. IS-1367-1979 for “HSFG Bolts” (M16 to M36 dia bolts) IS-6623-1985 for High strength nuts IS-6649-1985 for High strength washers IS-4000-1992, for the design load/stresses for “HSFG Bolts” A technical note on “HSFG bolts” by Dr. Saha Chaudhari. “Design of Steel Structures” by Sri A.S. Arya
