If there is deviation of surfaces between each member, it shall be corrected as shown in Table 5.16-1.

Table 5.16-1 Correction of Offset

In principle.'a torque wrench or a bolt tightening tool capable of adjust­ing torques shall be used for tightening bolts.

In tightening bolts, the tightening torques shall be adjusted so as to generate the standard bolt tensions listed in Table 5.16-2. This shall be confirmed by inspection.

Table 5.16-2 Standard Bolt Tensions for Tightening (tf)

Description:

Treatment of friction surfaces for friction joints are of most impor­tance, and this is a necessary measure to secure the required slip fac­tors. The necessary corrections specified in this Article arc as follows:

Black skin on contact faces must be removed to make rough sur­faces. (about 50S)

Primers or anti-corrosive paints should not be applied to the faces of contact in a work shop, and the primers applied in the state of plate before processing should be removed by blast, flame, wire brushes, etc.

. When bolts are tightened at site, rust, dirt and oil on the member­

contact-faces should be removed by a wire brush.

In order to exert a sufficient friction effect between members, it is ab­solutely essential that close contact should be made particularly on fric­tion faces between mutual members to be connected and between a member and a splice plate. Therefore, strain, distortion and bending should by all m^ans be corrected. If member plates are offset due to the unequal thicknesses or some similar reason, some correction should be made by inserting a filler, for example.

Care should be taken in handling and storing the tightening bolts, etc., so that they do not rust or suffer damage to threads. A pair of washers should be used, one for the bolt head side and the other for the nut side. A washer with finished surface to reduce torque should be used only on the rotary side. (Usually on the nut side)

As a rule, tightening should be in accordance with the calibrated wrench method. In case of using a torque wrench, the torque required for giving a standard bolt tension for tightening should be obtained, and whether or not a calibration record in a work shop should be used with no cor­rection depends on the storage conditions at site and the significance of the portions where bolts are used. In case of using a bolt tightening tool with a torque adjustable device, whether or not the torque adjust­ment by a tested calibrator is satisfactory should be checked in advance. In case of using turn-of-nut method, its safety should be checked in advance.

It is necessary to tighten a group of bolts in such an order that all tjie bolts \vork effectively (from the center to the ends). The general prac­tice is to first tentatively tighten all the bolts approximately 60% of the required torques, and then to tighten to bolt tension for tighten­ing. In tightening, the rotation of tapered washers should be avoided, anti it is necessary to take note of the amount of nut rotation, and to prevent negligence or excessiveness of tightening.

As the value of standard bolt tension for tightening generally applied is 10% larger than the value of designed bolt tensions listed in Table 5.16-3, this criterion is also employed in this standard. But specified allowable stresses of bolts are based on designed bolt tensions, and thus this point should be noted in tightening.

Table 5.16-3 Design Bolt Tension (tf)

Measurement of tightening at site is made by tightening torques. The relationship between tightening torque and₄ bolt tension is expressed by the torque coefficient.