NBC 2016 (SP 7:2016) Volume 1
2016 · PDF page 1060
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Tolerances for fabrication of steel structures shall conform to the accepted standard [6-6(41)]. Tolerances for erection of steel structures shall conform to the accepted standard [6-6(42)]. For general guidance on fabrication by welding, reference may be made to good practice [6-6(43)].
Material shall be straightened or formed to the specified configuration by methods that will not reduce the
properties of the material below the values used in design. Local application of pressure at room or at elevated temperature or other thermal means may be used for straightening, provided the above is satisfied.
The erection clearance for cleated ends of members, connecting steel to steel should preferably be not greater than 2.0 mm at each end. The erection clearance at ends of beams without web cleats should be not more than 3 mm at each end. Where for practical reasons, greater clearance is necessary, suitably designed seating should be provided.
23.2.2.1 The holes for bolts may be made as given in Table 19, unless otherwise specified by the engineer. The hole diameter in base plates shall not exceed the anchor bolt diameter by more than .
23.2.2.2 In friction type of connection clearance may be maintained, unless specified otherwise in the design document.
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
| Sl No. | Nature of Construction and Materials | Minimum Thickness (in mm) of Protection for a Fire Resistance of | |||||
| h(3) | 1h(4) | h(5) | 2h(6) | 3h(7) | 4h(8) | ||
| i) | Hollow protection (without an air cavity beneath the lower flanges):a) Metal lathing with trowelled lightweight aggregate gypsum plaster b) Plasterboard with 1.6 mm wire binding at 100 mm pitch, finished with lightweight aggregate gypsum plaster less than the thickness specified :1) 9.5 mm plaster board2) 19 mm plaster boardc) Asbestos insulating boards, thickness of board:1) Single thickness of board, with 6 mm cover fillets at transverse joints2) Two layers, of total thickness | 13 | 13 | 15 | 20 | 25 | — |
| 10 | 10 | 15 | — | — | — | ||
| 10 | 10 | 13 | 20 | — | — | ||
| — | — | 19 | 25 | — | — | ||
| — | — | — | — | 38 | 50 | ||
| ii) | Hollow protection (with an air cavity below the lower flange):Asbestos insulating board screwed to 25 mm asbestos battens | 9 | 12 | — | — | — | — |
| iii) | Solid protections:a) Concrete, not leaner than 1:2:4 mix (unplastered):1) Concrete not assumed to be load bearing, reinforced 2) Concrete assumed to be load bearingb) Lightweight concrete, not leaner than 1:2:4 mix (unplastered) | 25 | 25 | 25 | 25 | 50 | 75 |
| 50 | 50 | 50 | 50 | 75 | 75 | ||
| 25 | 25 | 25 | 25 | 40 | 60 | ||
Where wire binding cannot be used, expert advice should be sought regarding alternative methods of support to enable the lower edges of the plasterboard to be fixed together and to the lower flange, and for the top edge of the plasterboard to be held in position.
Reinforcement shall consist of steel binding wire not less than 2.3 mm in diameter, or a steel mesh weighing not less than 0.5 kg/m . In concrete protection, the spacing of that reinforcement shall not exceed 200 mm in any direction.
Cutting shall be effected by sawing, shearing, cropping, machining or thermal cutting process. Shearing, cropping and gas cutting shall be clean, reasonably square, and free from any distortion. Should the inspector find it necessary, the edges shall be ground after cutting. Planning or finishing of sheared or gas-cut edges of plates or shapes shall not be required, unless specially noted on drawing or included in stipulated edge preparation for welding or when specifically required in the following section.
Re-entrant corners shall be free from notches and shall have largest practical radii with a minimum radius of 15 mm.
Shearing of items over 16 mm thick to be galvanized and subject to tensile force or bending moment shall not be carried out, unless the item is stress relieved subsequently.
The use of sheared edges in the tension area shall be avoided in location subject to plastic hinge rotation at factored loading.
Gas cutting of high tensile steel by mechanically
controlled torch may be permitted, provided special care is taken to leave sufficient metal to be removed by machining, so that all metal that has been hardened by flame is removed. Hand flame cutting may be permitted only subject to the approval of the inspecting authority.
Except where the material is subsequently joined by welding, no load shall be transmitted through a gas cut surface.
Thermally cut free edges, which shall be subject to calculated static tensile stress shall be free from round bottom gouges greater than 5 mm deep. Gouges greater than 5 mm deep and notches shall be removed by grinding.
23.2.4.1 Holes through more than one thickness of material for members, such as compound stanchion and girder flanges, shall be where possible, drilled after the members are assembled and tightly clamped or bolted together. A round hole for a bolt shall either be machine flame cut, or drilled full size, or sub-punched 3 mm undersize and reamed to size or punched full size.
Hand flame cutting of a bolt hole shall not be permitted except as a site rectification measure for holes in column base plates.
A punched hole shall be permitted only in material whose yield stress ( ) does not exceed 360 MPa and where thickness does not exceed (5 600/ ) mm. In cyclically loaded details, punching shall be avoided in plates with thickness greater than 12 mm.
For greater thickness and cyclically loaded details, holes shall be either drilled from the solid or sub-punched or sub drilled and reamed.
The die for all sub-punched holes or the drill for all sub-drilled holes shall be at least 3 mm smaller than the required diameter of finished hole.
A special plate washer of minimum thickness 4 mm shall be used under the nut, if the hole diameter is larger than the bolt diameter by 3 mm or more.
Oversize hole shall not exceed or in diameter, were is the nominal bolt diameter, in mm.
A short slotted hole shall not exceed the appropriate hole size in width and 1.33d, in length. A long slotted hole shall not exceed the appropriate hole size in width and 2.5d, in length. If the slot length is larger than those specified, shear transfer in the direction of slot is not admissible even in friction type of connection.
Slotted holes shall be punched either in one operation or else formed by punching or drilling two round holes apart and completed by high quality mechanically controlled flame cutting and dressing to ensure that bolt can freely travel the full length of the slot.
Holes for turned and fitted bolts shall be drilled to a diameter equal to the nominal diameter of the shank or barrel subject to tolerance specified in the accepted standards . Preferably, parts to be connected with close tolerance or barrel bolts shall be firmly held together by tacking bolts or clamps and the holes drilled through all the thicknesses at one operation and subsequently reamed to size. All holes not drilled through all thicknesses at one operation shall be drilled to a smaller size and reamed out after assembly. Where this is not practicable, the parts shall be drilled and reamed separately through hard bushed steel jigs.
23.2.4.5 Holes for rivets or bolts shall not be formed generally by gas cutting process. However, advanced gas cutting processes such as plasma cutting may be used to make holes in statically loaded members only. In cyclically loaded members subjected to tensile stresses which are vulnerable under fatigue, gas cutting shall not be used unless subsequent reaming is done to remove the material in the heat affected zone around the hole.
All parts of bolted members shall be pinned or bolted and rigidly held together during assembly.
The component parts shall be assembled and aligned in such a manner that they are neither twisted nor otherwise damaged, and shall be so prepared that the specified camber, if any, is provided.
When holes are drilled in one operation through two or more separable parts, these parts, when so specified by the engineer, shall be separated after drilling and the burrs removed.
Matching holes for rivets and black bolts shall register with each other so that a gauge of 1.5 mm or 2.0 mm (as the case may be, depending on whether the diameter of the rivet or bolt is less than or more than 25 mm) less in diameter than the diameter of the hole will pass freely through the assembled members in the direction at right angle to such members.
Drilling done during assembly to align holes shall not distort the metal or enlarge the holes.
Holes in adjacent part shall match sufficiently well to permit easy entry of bolts. If necessary, holes except oversize or slotted holes may be enlarged to admit bolts, by moderate amount of reaming.
When design is based on bolts with unthreaded shanks in the shear plane, appropriate measures shall be specified to ensure that, after allowing for tolerance, neither the threads nor the thread run-out be in the shear plane.
The length of bolt shall be such that at least one clear thread shows above the nut and at least one thread plus the thread run out is clear beneath the nut after tightening. One washer shall be provided under the rotated part.
When non-preloaded bolts are used in a structure subject to vibration, the nuts shall be secured by locking devices or other mechanical means. The nuts of preloaded bolts may be assumed to be sufficiently secured by the normal tightening procedure.
Washers are not normally required on non-preloaded bolts, unless specified otherwise. Tapered washers shall be used where the surface is inclined at more than to a plane perpendicular to the bolt axis.
Hardened washer shall be used for preloaded bolts or the nut, whichever is to be rotated.
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
All material within the grip of the bolt shall be steel and no compressible material shall be permitted in the grip.