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.
23.4.1 Rivets shall be heated uniformly throughout their length, without burning or excessive scaling, and shall be of sufficient length to provide a head of standard dimensions. These shall, when driven, completely fill the holes and, if countersunk, the countersinking shall be fully filled by the rivet. If required, any protrusion of the countersunk head shall be dressed off flush.
23.4.2 Riveted member shall have all parts firmly drawn and held together before and during riveting, and special care shall be taken in this respect for all single-riveted connections. For multiple riveted connections, a service bolt shall be provided in every third or fourth hole.
23.4.3 Wherever practicable, machine riveting shall be carried out by using machines of the steady pressure type.
23.4.4 All loose, burned or otherwise defective rivets shall be cut out and replaced before the structure is loaded, and special care shall be taken to inspect all single riveted connections.
23.4.5 Special care shall be taken in heating and driving long rivets.
23.5.1 In all cases where the full bearing area of the bolt is to be developed, the bolt shall be provided with a washer of sufficient thickness under the nut to avoid any threaded portion of the bolt being within the thickness or the parts bolted together, unless accounted for in design.
23.5.2 Pre-tensioned bolts shall be subjected to initial tension (the proof stress) by an appropriate pre-calibrated method.
23.6.1 Welding shall be in accordance with good practices [6-6(45)], as appropriate.
23.6.2 For welding of any particular type of joint, welders shall give evidence acceptable to the purchaser of having satisfactorily completed appropriate tests as prescribed in good practices [6-6(46)], as relevant.
23.6.3 Assembly and welding shall be carried out in such a way to minimise distortion and residual stress and that the final dimensions are within appropriate tolerances.
23.7.1 Column splices and butt joints of struts and compression members, depending on contact for stress transmission, shall be accurately machined and close-butted over the whole section with a clearance not exceeding 0.2 mm locally, at any place. Sum of all such clearance shall not be more than 30 percent of the contact area for stress transmission. In column caps and bases, the ends of shafts together with the attached gussets, angles, channels, etc; after connecting together should be accurately machined so that clearance between the contact surfaces shall not exceed 2 mm locally, subject further to the condition that sum total of all such clearance shall not exceed 30 percent of the total contact area for stress transmission. Care should be taken that these gussets, connecting angles or channels are fixed with such accuracy that they are not reduced in thickness by machining by more than 2.0 mm.
23.7.2 Where sufficient gussets and rivets or welds are provided to transmit the entire loading [see Section 6(d)], the column ends need not be machined.
Slab bases and slab caps, except when cut from material with true surfaces, shall be accurately machined over the bearing surfaces and shall be in effective contact with the end of the stanchion, the bearing face which is to be grouted to fit tightly at both top and bottom, unless welds are provided to transmit the entire column face.
23.7.4 To facilitate grouting, sufficient gap shall be left between the base plates and top of pedestal and holes shall be provided where necessary in stanchion bases for the escape of air.
23.8.1 Painting shall be done in accordance with the good practices [6-6(47)].
23.8.2 All surfaces, which are to be painted, oiled or otherwise treated, shall be dry and thoroughly cleaned to remove all loose scale and loose rust.
23.8.3 Shop contact surfaces need not be painted unless specified. If so specified, they shall be brought together while the paint is still wet.
23.8.4 Surfaces not in contact, but inaccessible after shop assembly, shall receive the full specified protective treatment before assembly. This does not apply to the interior of sealed hollow sections.
23.8.5 Chequered plates shall be painted but the details of painting shall be specified by the purchaser.
23.8.6 In case of surfaces to be welded, the steel shall not be painted or metal coated within a suitable distance
of any edge to be welded, if the paint specified or the metal coating is likely to be harmful to welders or impair the quality of the welds.
23.8.7 Welds and adjacent parent metal shall not be painted prior to de-slagging, inspection and approval.
23.8.8 Parts to be encased in concrete shall not be painted or oiled.
23.8.9 Contact surface in friction type connection shall not be painted in advance.
Each piece of steel work shall be distinctly marked before dispatch, in accordance with a marking diagram and shall bear such other marks as will facilitate erection.
23.10.1 The steel work shall be temporarily shop erected complete or as arranged with the inspection agency so that accuracy of fit may be checked before dispatch. The parts shall be shop assembled with sufficient numbers of parallel drifts to bring and keep the parts in place.
23.10.2 In the case of parts drilled or punched, through steel jigs with bushes resulting in all similar parts being interchangeable, the steelwork may be shop erected in such position as arranged with the inspection agency.
23.10.3 In case of shop fabrication using numerically controlled machine data generated by computer software (like CAD), the shop erection may be dispensed with at the discretion of the inspector.
All projecting plates or bars and all ends of members at joints shall be stiffened, all straight bars and plates shall be bundled, all screwed ends and machined surfaces shall be suitably packed and all rivets, bolts, nuts, washers and small and loose parts shall be packed separately in cases, so as to prevent damage or distortion during transit.
23.12.1 The inspecting authority shall have free access at all reasonable times to those parts of the manufacturer's works which are concerned with the fabrication of the steelwork and shall be afforded all reasonable facilities for satisfying himself that the fabrication is being undertaken in accordance with the provisions of this section.
23.12.2 Unless specified otherwise, inspection shall be made at the place of manufacture prior to dispatch and shall be conducted so as not to interfere
unnecessarily with the operation of the work.
23.12.3 The manufacturer shall guarantee compliance with the provisions of this standard, if required to do so by the purchaser.
23.12.4 Should any structure or part of a structure be found not to comply with any of the provisions of this section, it shall be liable to rejection. No structure or part of the structure, once rejected shall be resubmitted for test, except in cases where the purchaser or his authorized representative considers the defect as rectifiable.
23.12.5 Defects, which may appear during fabrication, shall be made good with the consent of and according to the procedure laid down by the inspecting authority.
23.12.6 All gauges and templates necessary to satisfy the inspection authority shall be supplied by the manufacturer. The inspecting authority may, at his discretion, check the test results obtained at the manufacturer's works by independent testing at outside laboratory, and should the material so tested be found to be unsatisfactory, the cost of such tests shall be borne by the manufacturer, and if found satisfactory, the cost shall be borne by the purchaser.
The suitability and capacity of all plant and equipment used for erection shall be to the satisfaction of the engineer.
All structural steel should be so stored and handled at the site that the members are not subjected to excessive stresses and damage by corrosion due to exposure to environment.
The positioning and levelling of all steelwork, the plumbing of stanchions and the placing of every part of the structure with accuracy shall be in accordance with the approved drawings and to the satisfaction of the engineer in accordance with the deviation permitted below.
Unloaded steel structure, as erected, shall satisfy the criteria specified in Table 33 within the specified tolerance limits.
Each criterion given in the table shall be considered as a separate requirement, to be satisfied independent of any other tolerance criteria. The erection tolerances specified in Table 33 apply to the following reference points:
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
a) For a column, the actual centre point of the column at each floor level and at the base, excluding any base-plate or cap-plate. The level of the base plate on pedestal shall be so as to avoid contact with soil and corrosive environment; and
b) For a beam, the actual centre point of the top surface at each end of the beam, excluding any end-plate.
| Sl No.(1) | Criterion(2) | Permitted Deviation(3) |
|---|---|---|
| i) | Deviation of distance between adjacent columns | 5 mm |
| ii) | Inclination of a column in a multi-storey building between adjacent floor levels | where is the storey height |
| iii) | Deviation of location of a column in a multi-storey building at any floor level from a vertical line through the intended location of the column base | where is the total height from the base to the floor level concerned and n is the number of storeys from the base to the floor level concerned |
| iv) | Inclination of a column in a single storey building (not supporting a crane gantry) other than a portal frame | where is the height of the column |
| v) | Inclination of the column of a portal frame (not supporting a crane gantry) | Mean: Individual: where is the height of the column |
The straightness tolerances specified in Table 34 have been assumed in the derivation of the design stress for the relevant type of member. Where the curvature exceeds these values, the effect of additional curvature on the design calculations shall be reviewed.
| Sl No. | Criterion | Permitted Deviation |
|---|---|---|
| (1) | (2) | (3) |
| i) | Straightness of a column (or other compression member) between points which will be laterally restrained on completion of erection | 0.001L generally, and 0.002L for members with hollow cross sections; where, L is the length between points which will be laterally restrained |
| ii) | Straightness of a compression flange of a beam, relative to the weak axis, between points, which will be laterally restrained on completion of erection | 0.001L generally, and 0.002L for members with hollow cross sections; where, L is the length between points which will be laterally restrained |
A tension member shall not deviate from its correct position relative to the members to which it is connected by more than 3 mm along any setting axis.
23.13.4.1 All steel materials including fabricated structures, either at fabrication shop or at erection site, shall be handled only by a worker skilled in such jobs; where necessary with load tested lifting devices, having tested wire rope slings of correct size. The devices should be well maintained and operated by experienced operators.
23.13.4.2 Oxygen and acetylene cylinders and their hoses shall have distinctive colours. Cylinders should be stored in upright position in well-ventilated rooms or in open air, not exposed to flames, naked lights or extreme heat and should also be in upright position when they are being used. All gas cutting works shall be done only by experienced skilled gas cutters, equipped with gloves, boots, aprons, goggles and good cutting sets of approved make.
23.13.4.3 While doing any welding work, it should be ensured that the welding machine is earthed and the welding cables are free from damage. The welder and his assistant shall use a face shield or head shield with a welding lens and clear cover glass and their hands, legs and bodies shall be well protected by leather gloves, shoes and aprons. Combustible materials should be kept away from the sparks and globules of molten metals generated in any arc welding. In case of welding in a confined place, it should be provided with an exhaust system to take care of the harmful gases, fumes and dusts generated.
23.13.4.4 In addition to precautions against all the hazards mentioned above, erection workers shall also be protected in the following manner:
a) All workers shall wear helmets and shall also be provided with gloves and shoes. In addition those working at heights shall use safety belts.
b) All structures shall be so braced/guyed during erection that there is no possibility of collapse before erection work is completed.
c) Warning signs such as ‘Danger’, ‘Caution’, ‘440 V’, ‘Do not smoke’, ‘Look ahead’, etc; should be displayed at appropriate places.
23.13.4.5 For detailed safety precautions during erection, reference shall be made to the good practice [6-6(48)].
Rivets driven at the site shall be heated and driven with the same care as those driven in the shop.
Field bolting shall be carried out with the same care as required for shop bolting.
Field assembly and welding shall be executed in accordance with the requirements for shop fabrications excepting such as manifestly apply to shop conditions only. Where the steel has been delivered painted, the paint shall be removed for a distance of at least 50 mm on either side of the joint.
23.14.1 Before painting of such steel which is delivered unpainted is commenced, all surfaces to be painted shall be dry and thoroughly cleaned from all loose scale and rust, as required by the surface protection specification.
23.14.2 The specified protective treatment shall be completed after erection. All rivet and bolt heads and the site welds after de-slagging shall be cleaned. Damaged or deteriorated paint surfaces shall first be made good with the same type of paint as the shop coat. Where specified, surfaces which will be in contact after site assembly, shall receive a coat of paint (in addition to any shop priming) and shall be brought together while the paint is still wet. No painting shall be used on contact surfaces in the friction connection, unless specified otherwise by the design document.
23.14.3 Where the steel has received a metal coating in the shop, this coating shall be completed on site so as to be continuous over any welds and site rivets or bolts, subject to the approval of the engineer. Painting on site may complete protection. Bolts, which have been galvanized or similarly treated, are exempted from this requirement.
23.14.4 Surface, which will be inaccessible after site assembly, shall receive the full-specified protective treatment before assembly.
23.14.5 Site painting should not be done in frosty or foggy weather, or when humidity is such as to cause condensation on the surfaces to be painted.
23.15.1 Bedding shall be carried out with Portland cement grout or mortar, as described under 23.15.4 or fine cement concrete in accordance with good practice [6-6(49)].
23.15.2 For multi-storeyed buildings, this operation shall not be carried out until a sufficient number of bottom lengths of stanchions have been properly lined, levelled and plumbed and sufficient floor beams are in position.
23.15.3 Whatever method is employed, the operation shall not be carried out until the steelwork has been finally levelled and plumbed, stanchion bases being supported meanwhile by steel wedges or nuts; and immediately before grouting, the space under the steel shall be thoroughly cleaned.
23.15.4 Bedding of structure shall be carried out with grout or mortar, which shall be of adequate strength and shall completely fill the space to be grouted and shall either be placed under pressure or by ramming against fixed supports. The grouts or mortar used shall be non-shrinking variety.
A few recommendations are given in Annex F for general information.
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
Analysis and Design Methods
A-1 Advanced Structural Analysis and Design
For a frame, comprising members of compact section with full lateral restraint, an advanced structural analysis may be carried out, provided the analysis can be shown to accurately model the actual behaviour of that class of frames. The analysis shall take into account the relevant material properties, residual stresses, geometrical imperfections, reduction in stiffness due to axial compression, second-order effects, section strength and ductility, erection procedures and interaction with the foundations.
Advanced structural analysis for earthquake loads shall take into account, as appropriate the response history, torsional response, pounding against adjacent structures, and strain rate effects.
For the strength limit state, it shall be sufficient to satisfy the section capacity requirements of Section 6(h) for the members subjected to bending, of Section 6(g) for axial members, of Section 6(j) for combined forces and of Section 6(k) for connections. Effect of moment magnification given in Section 6(j), instability given in Section 6(g) and lateral buckling given in Section 6(h) need not be considered while designing the member, since advanced analysis methods directly consider these.
An advanced structural analysis for earthquake loads shall recognize that the design basis earthquake loads calculated in accordance with Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code is assumed to correspond to the load at which the first significant plastic hinge forms in the structure.
A-2 Second Order Elastic Analysis and Design
In a second-order elastic analysis, the members shall be assumed to remain elastic, and changes in frame geometry under the design load and changes in the effective stiffness of the members due to axial forces shall be accounted for. In a frame where the elastic buckling load factor of the frame as determined in accordance with 10.6 is greater than 5, the changes in the effective stiffness of the members due to axial forces may be neglected.
The design bending moment under factored load shall be taken as the maximum bending moment in the length of the member. It shall be determined either,
a) directly from the second-order analysis; or
b) approximately, if the member is divided into a sufficient number of elements, as the greatest of the element end bending moments, or;
c) by amplifying the calculated design bending moment, taken as the maximum bending moment along the length of a member as obtained by superposition of the simple beam bending moments determined by the analysis.
For a member with zero axial force or a member subject to axial tension, the factored design bending moment shall be calculated as the moment obtained from second-order analysis without any amplification.
For a member with a design axial compressive force as determined from the analysis, the factored design bending moment shall be calculated as follows:
where
A-3 Frame Instability Analysis
Frame instability, as treated here, is related to the design of multi-storey rigid-jointed frames subject to side sway. The elastic critical load factor, may be determined using the deflection method as given in A-3.2 or any other recognized method. This is used to calculate the amplified sway moments for elastic designs and to check frame stability in plastic designs. The elastic critical load factor, of a frame is the ratio by which each of the factored loads would have to be increased to cause elastic instability.
An accurate method of analysis (ordinary linear elastic analysis) should be used to determine the horizontal deflections of the frame due to horizontal forces applied at each floor level, which is equal to the notional horizontal load in 10.3.6. Allowance should be made
for the degree of rigidity of the base as given in A-3.2 in this deflection calculation.
The base stiffness should be determined by reference to 10.3.4.
The elastic critical load factor, is calculated as:
where
largest value of the sway index where, given by:
where
storey height;
In any storey the stiffening effect of infill wall panels
may be allowed for by introducing a diagonal strut in that storey of area , given by:
where
storey height;
width of the braced bay;
sum of the stiffness , of the columns in that storey;
Sum of spring stiffness (horizontal force per unit horizontal deflection of all the panels in that storey determined from:
where
thickness of the wall panel, and
= modulus of elasticity of the panel material.