Slip and yield resistance of friction type of high strength bolted connections with over-sized holes

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1 IABSE-JSCE Joint Conference on Advances in Bridge Engineering-II, August 8-10, 2010, Dhaka, Bangladesh. ISBN: Amin, Okui, Bhuiyan (eds.) Slip and yield resistance of friction type of high strength bolted connections with over-sized holes Takeshi Mori Hosei University, 2-33 Ichigaya-Tamachi Shinjuku-ku, Tokyo , Japan ABSTRACT: It is sometimes difficult to adjust the hole of the splice plate to the hole of the main plate correctly in a friction type of high strength bolted connection at the construction site. In order to modify this problem and make an erection easy, the over-sized holes is sometimes required and utilized. In this study, in order to examine the influence of over-sized holes on the slip resistance and the yield resistance of the friction type of high strength bolted connection, tensile tests have been performed on the bolted connections in consideration of slip to yield resistance ratio. As a result, following conclusions have been obtained. The slip resistance does not depend on the hole diameter. The yield resistance is decreasing in proportion to the reduction of net sectional area of the main plate due to the over-sized hole. However, the yield resistance can be regarded as about 1.2 times of one calculated from the net sectional area because of stress transmission due to the friction between a main plate and splice plates. 1 INTRODUCTION As for field joints of steel bridges, friction type of high strength bolted connections have been commonly adopted. The friction between main plate and splice plates due to bolt tightening force makes the stress transmission. The limit states of this type of connections are usually regarded as slip between main and splice plates and yielding of main or splice plate. Therefore, these two limit states are checked in the design phase 1)- 3). In a steel bridge erection site, it is sometimes difficult to adjust the hole of main plate to the hole of splice plates because there are several variations of bolt-hole pitch due to the field welding and/or supporting condition. In these cases, the over-sized hole is sometimes required and utilized in order to modify the problem and make an erection easy. Here, the over-sized hole is defined as the hole diameter larger than bolt diameter (M22) by 4.5mm or more. Normal bolt-hole diameter is 24.5mm (bolt diameter + 2.5mm) in Japanese standard 1). In this study, in order to examine the influence of over-sized holes on the slip resistance and the yield resistance of the friction type of high strength bolted connections, tensile tests have been performed on the bolted connections fabricated in consideration of slip to yield resistance ratio. Here, several kinds of specimens are prepared and designed by means of slip to yield resistance ratio (β; ratio of design slip resistance Ps to design yield resistance Py). If β value is less than unity, slip precedes yielding. Yielding precedes slip when the value is larger than unity. 2 METHOD OF TENSILE TESTS FOR CONNECTION WITH OVER-SIZED HOLE 2.1 Specimen The specimens used for tensile tests are butt connections with two friction surfaces as shown Figure 1. The material of plates and bolt is SM490Y and F10T-M22 specified in Japan Industrial Standard, respectively. Bolt hole diameters of main and splice plates are 24.5, 26.5, 28.5mm. Main plate thickness is determined as 16, 19, 22 and 25mm so that the design slip to yield resistance ratio is 0.8, 1.0, 1.2 or 1.4 in case of hole of 24.5mm in diameter. The friction surface of main plate and splice plate is grid-blasted to remove a mill-scale. The arrangements of bolt holes consist of six types as shown Figure 2. From type 1 to 3, hole diameter is varied as 24.5, 26.5 and 28.5mm. Type 4, 5 and 6 have hole diameter of 28.5mm, but the hole of splice plate is 171

2 sifted from the hole of main plate. As for the type 1, 2 and 3 specimens, the plates of all the plate thickness (16, 19, 22, 25mm) have been utilized, but only plate of 25mm thickness has been employed for the type 4 to 6 specimens. 2.2 Method of tensile tests The tensile tests are performed until both slip and yielding are observed using universal tensile test equipment with load capacity of 0kN. The measured items are shown in Figure 3. The gap variation between main plates due to applied load is measured by clip-gauge and the yielding of main plate is observed by strain gauge adhered on the edge side of the plate. The strain gauge is also put on the side of splice plates to confirm the stress transmission from main plate to splice plates. The high strength bolt with strain gauge is adopted to measure the variation of bolt tightening axial force during test. Before the tensile test, the relaxation of bolt axial force is measured on right after fastening and the just before the tensile test (12~24 hours after fastening). The side of main and splice plates are machined by about 5mm in order to remove residual stress due to flame cut. B A A B Figure 1. Shape and Dimensions of Test Specimen Type1 Type2 Type mm Type mm Type5 Type mm mm mm Figure 2. Combination of Bolt Hole 3 RESULTS OF TENSILE TESTS 3.1 Relaxation of bolt axial force The relaxation has been measured 34 days after fastening on the type 1-5 of slip proceeded connection (t=25mm). The axial force reduction of each type was 3.2, 2.2, 2.7, 1.1, and 3.2%, respectively. Therefore, it can be said that the difference of relaxation due to diameter of bolt hole ad shifting bolt has not occurred. In 12 to 24 hours measurement mentioned last chapter, significant variation of relaxation of bolt tightening force due to the difference of type of specimens was not observed. 172

3 3.2 Slip load and yielding load Slip load defined as the load when the slip occurs during the tensile test. The slip can be distinguished by big sound, sudden enlargement displacement of the gap and decreasing load. Yielding load is obtained from the load-strain curves at gauge No.3 shown in Fig.3 and defined as 0.2% offset load. From the load-strain curve, specimens can be classified three types (slip preceded, yield preceded and slip-yield compound) whether slip occurs before or after the yielding. The connection with 16mm thickness is classified as yield preceded regardless of specimen type. The connection with 25mm thickness is classified as slip preceded. The connection with 19mm or 22mm thickness of which slip to yield resistance ratio is almost equal in 1.0 are classified two types; one is slip preceded and another is slip-yield compound in which slip and yield arise almost simultaneously. 3.3 Relationship between bolt-axial force and applied load In a yield preceded connection, the bolt-axial force of the outside bolt decreased suddenly due to the reduction of main plate thickness caused by yielding. In a slip preceded connection, the bolt-axial force decreased as load increased (reduction of axial force just before slip is remarkable) and still decreased after slip and yield of main plate on the outside bolt position. 3.4 Load-strain relationship The load-strain relationship of main plate outer to splices (gauge No.1 in Fig.3) is liner regardless of the type of specimens. In the range of the load of the experiments, the main plate outer to splice plates is not yielded. An example of the load-strain relationship of the yield preceded connections is shown in Figure 4. The load-strain relationship is almost liner up to the load of 460kN, after that main plate yields and strain still increase according to load. Slip is observed when load is around 510kN, and load decrease to kn. This is the typical load-strain relationship of the yield preceded connection. An example of the load-strain relationship of the slip preceded connections is shown in Figure 5. Slip arises at the load equal to about 580kN and both load and strain decrease. Up to this stage, the load-strain relationship is liner and behaves elastic action. Up to the load equal to around kn it is still liner, after that plastic strain arises and main plate yields. This is the typical load-strain relatonship of the slip preceded connection. 4 DISCUSSIONS 4.1 Effect of over-sized hole on slip resistance The relationship between slip resistance and bolt hole diameter is shown in Fig.6. Variation of slip resistance is a little in all thickness. The relationship between slip coefficient factor and bolt hole diameter is shown in Fig.7. The slip coefficient factor is calculated as the load at slip divided by number of friction surface and total design axial forces of three bolts. The slip coefficient factor of the yield preceded connection is slightly low due to the much reduction of bolt-axial force caused by the reduction of main plate thickness after yielding. Meantime, the slip coefficient factor of the slip preceded connection is among 0.45 to 0.48, so no remarkable difference is observed. Therefore, the slip resistance does not depend on the bolt hole diameter as for the slip preceded connection in which the slip is the limit state. The remarkable difference of bolt position to the hole was not recognized in the slip preceded specimens (t=25mm) as well. 4.2 Effect of over-sized hole on yield resistance Relationship between the yield resistance and bolt hole diameter is shown in Figure.8. The yield resistance lowers as the bolt-hole diameter is larger. Relationship between the yield resistance ratio and bolt hole diameter is shown in Fig.9. The yield resistance ratio is calculated as the yield load obtained from tensile tests divided by the yield load calculated by yield stress of the material and net sectional area. The yield resistance ratio is around 1.18 to 1.20 in the yield preceded connection (t=16mm) because stress is transmitted to splice plates by friction between main plate and splice plates. Meantime, the yield resistance ratio in the slip preceded connection is about 1.02 to

4 In both of the yield preceded connection and the slip preceded connection, the yield resistance ratio does not change by bolt-hole diameter so much. Therefore, the yield resistance is decreasing in proportion to the reduction of net sectional area of main plate due to over-sized hole. However, the yield resistance can be regarded as about 1.2 times of one calculated from net sectional area as for the yield preceded connection. Load- 0.2% Offset Stress Gross Sectional Area Net Sectional Area Figure 4. Load Curve(t=16 Load- 0.2 % Offset Gross Sectional Area Net Sectional Area Figure 5. Load- Curve(t= Slip Resistance (kn) t=16 Yield Preceded t=25 Slip Preceded Slip Factor t=16 Yield t=25 Slip preceded Figure 6. Slip Resistance Figure 7. Slip Factor Yield Resistance (kn) t=19 Compound Connection t=16 Yield Preceded Connection t=25 Slip Preceded Connection Figure 8. Yield Resistance 30.5 Yield Resistance Ratio t=16 Yield Preceded Connection t=25 Slip Preceded Connection Figure 9. Yield Resistance Ratio 4.3 Mechanism of stress transmission An example of typical load-strain relationships (gauge No.2) of the yield preceded connection is shown in Figure 10. In this figure, the load-strain relationship calculated from gross and net sectional area is also indicated. The gradient of load-strain curves is steeper than those calculated from gross and net sectional area. 174

5 This fact means that apparent sectional area increment is achieved larger than gross and net sectional area. That is because stress is transmitted to splice plates. An example of load-strain relationships measured on the locations of gauge No.2, 3 and 4 is shown in Figure 11. Each load-strain relationship is liner up to yield load of about 460kN, and strain of main plate (gage No.2) is increasing after yielding. On the other hand, the strains of splice plates (gage No.3 and 4) are decreasing after yielding. These behaviors on main plate and splice plates show that stress transmission to splice plates is decreasing after main plate is yielding and slip occurs due to reduction of bolt axial force caused by reduction of main plate thickness. Main Plate Gross Sectional Net Sectional Gauge 3,4 Splice Plate Yield Load Splice Plate Shared Main Plate t=16 Bolt Hole 28.5 Load Gauge 2 main Plate Main Plate t=16 Bolt Hole Figure 10. Load and Curve Figure 11. Load and Curve (gauge2, 3 4) 5 CONCLUSIONS (1) The slip resistance does not depend on the hole diameter. (2) The yield resistance is decreasing in proportion to the reduction of net sectional area of the main plate due to the oversize hole. However, the yield resistance can be regarded as about 1.2 times of one calculated from the net sectional area because of stress transmission due to the friction between a main plate and splice plates. REFFERENCES 1) Japan Road Associations (2), Specifications of Highway Design (Part 3) 2) American Association of State Highway and Transportation Officials (1998), LRFD Bridge Specifications, Second Edition 3) British Standard (1982), Steel, Concrete and Composite Bridges, Part 3 Code of practice for design of steel bridges 175