BOLTED CONNECTIONS 1.1 RANGE AND CLASS OF THE SCREW FASTENERS. Steel structures Laboratory

Transcription

1 1. BOLTED CONNECTIONS 1.1 RANGE AND CLASS OF THE SCREW FASTENERS Diameter of bolts for building structures: M10, M12, M16, M20, M24, M27, M30 Class of bolts: 3.6, 4.6, 4.8, 5.6, 5.8, 6.6, 8.8, 10.9, high strength bolts f u f y /f u Materials: - low carbon steel - carbon steel - alloy steel (bolts class 8.8 and 10.9) Methods of production - hot or cold plastic working - mechanical working - mechanical and plastic working - heat treatment (hardening + tempering) high strength bolts -1-

2 Class of nuts: 4, 5, 8, 10, 12. Bolts class high than 4.8 i 5.6 and nuts class 4 should be marked. Tolerances for fasteners. Depending on the accuracy of proper dimensions and geometrical and the surface roughness three grades of making screws are being distinguished: - high grade A - mid - range grade B - coarse grade C In non preloaded connections are used bolts 4.8 i 5.6 grades (B) or (C): d 20 mm 4.8 d > 20 mm 5.6 In preloaded connections are used high strength bolts grade B (mid - range grade). Washers flat washer high strength washer shim washer Drawing symbol of bolts -2-

3 1.2 CATEGORIES OF BOLTED CONNECTIONS CATEGORY A : SHEAR CONNECTIONS BEARING TYPE Shear connection external force direction is perpendicular to bolt axis. External force is transferred by shear of the bolt shank or bearing bolts to connection element. For simplification of the montage, the hole diameter is greater than nominal bolt diameter: for d14 mm d o =d+1 mm for 16d24 mm d o =d+2 mm for 27d44 mm d o =d+3 mm. In elements bolt holes can be punched or drilled. During the punching a steel is deformed and its plasticity is decreased along hole edge. Therefore punching is used for steel plates with thickness less than t25 mm. Nuts should be established so that marking the class is visible. When the surface of element is drawn aside more than 3 from the perpendicular to the bolt axis, the shim washer should be used. Tightening of the bolts: connected elements should be cling together. Tolerance less than 2 mm is acceptable if this is not give in design recommendations. Bolts should be manual tightening till first point of resistance. First point of resistance is defined as a tightening nominal hand wrench with one hand power. -3-

4 EFFECTIVE LENGTH OF THE BOLTS IN SHEAR CONNECTIONS Class of bolts Bolts diameter M12 M16 M20 M24 M30 M20 M24 M27 number of washers Length of the bolts l, Range of the effective length for bolts, mm mm

5 CATEGORY C : SHEAR CONNECTIONS SLIP-RESISTANT AT ULTIMATE LIMIT STATE Shear connection external force direction is perpendicular to bolt axis. External force is transferred by friction between connected elements. The friction force is a result of the bolt preloading. In this connection a high strength grade bolts are used. The friction surfaces should be adequately prepared. Hole diameter and technology, method of washers and nuts tightening is similar like the joint A category. High strength washer should be established phasing side adjacent to head of the bolt and nut. For preloaded bolts the design preload F p, Cd.to be used in design calculations should be taken as: Fp. Cd 0,7 fub As / M 7, where A s is the tensile stress area of the bolt and f ub is the ultimate tensile strength of the bolt. Nominal value of the preload force should be specified in design documentation. At tightening of the nuts, treated part of the bolt and washer under turn off part of connection should be greased by the graphited grease or molybdenum paste. Tightening the bolts in prestressed joints should make connectors successively from the middle of everyone multi-bolts connections, repeating the entire procedure all the way to getting regular stretching the bolts. One of the major problems with the use of bolted joints is the precision, with regard to achieving an accurate preload, of the bolt tightening method selected. Insufficient preload, caused by an inaccurate tightening method, is a frequent cause of bolted joint failure. -5-

6 There are six main methods used to control the preload of a threaded fastener. Specifically: 1. Torque control tightening. Controlling the torque which a fastener is tightened to is the most popular means of controlling preload. The nominal torque necessary to tighten the bolt to a given preload can be determined either from tables, or, by calculation using a relationship between torque and the resulting bolt tension. Bolt diameter Bolt class 10.9 Bolt class 8.8 Preload force Torque moment 1) M o [Nm] F p.cd [kn] Light oiling 2) MoS 2 paste M M M M M M ) at preload force equal 0.5 F p.cd, the torque moment 0.5 M o 2) also by the graphited grease. Preload force Torque moment 1) M o [Nm] F p.cd [kn] Light oiling 2) MoS 2 paste Angle control tightening. The method has been applied for use with power wrenches, the bolt being tightened to a predetermined angle (120 to 240) beyond the elastic range and results in a small variation in the preload due, in part, to the yield stress tolerance. 3. Yield controlled tightening. The electronic wrench can allowed to detect the yield point of the fastener with reasonable precision. 4. Bolt stretch method. The method uses a small hydraulic ram which fits over the nut, the threaded portion of the bolt/stud protrudes well past the nut and a threaded puller is attached. 5. Heat tightening. The bolt is heated and expands; the nut is indexed (using the angle of turn method) and the system allowed to cool. 6. Use of tension indicating methods. This category includes the use of special load indicating bolts, load indicating washers and the use of methods which determine the length change of the fastener. Class of friction surface. Slip factor for pre-loaded bolts: Class of friction surface Slip factor A 0.50 B 0.40 C 0.30 D 0.20 Untreatment Surface preparation A1 Blast with shot of grit without pitting A2 Blast cleaning or shot blasting and spray metallized with Alluminium A3 Blast cleaning or shot blasting, spray metalized with a zin based c A pulverisation, and slippage test Blast cleaning or shot blasting, with alkali-zinc silicate paint with a thickness of 80 microns Wire brushed or flame cleaning and removing all scale or rust -6-

7 TENSION CONNECTIONS PRELOADED OR NON - PRELOADED tension connection external force direction is parallel to bolt axis. In the tension joint, the bolt and clamped components of the joint are pre-load designed to transfer the external tension load through the joint by way of the clamped components through the design of a proper balance of joint and bolt stiffness. The joint should be designed such that the clamp load is never overcome by the external tension forces acting to separate the joint The forces within a connection which result from the deformation of the connected parts are known as prying forces. In bolted tee-connections, these forces cause an increase in the tensile load on the bolts. -7-

8 Steel structures Laboratory 1.3 FASTENERS OF THE THIN SHEET METAL (t< 3mm) sheet metal screw - has sharp threads that cut into a material such as sheet metal, plastic or wood. self-tapping machine screw - is driven into an untapped hole. self-drilling screw - Teks screw - has a drill-shaped point to cut through the substrate to eliminate the need for drilling a pilot hole. plain rivet with two side access. blind rivet - the rivet assembly is inserted into a hole drilled through the parts to be joined and a specially designed tool is used to draw the mandrel into the rivet. sheet metal nail designed to jointing sheet metal with solid steel elements and concrete. -8