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In recent times, steel structures have gained popularity over RCC structures due to their advantages such as higher strength and durability, lightweight, speed of construction, easy fabrication and fire, moisture, and pest resistance.
Accuracy in the erection of steel structure plays a vital role in deciding durability of the structure. Therefore proper erection, welding, bolting, and riveting must be ensured during the erection of steel structures.
In this article, we'll discuss the erection procedure of steel members, joint members, and the tolerance allowed in the steel structure.
Erection of Steel Structure
- A working scheme shall be prepared before the commencement of the erection. It shall also contain rules for safety precautions. The detailed safety guidelines for erection of steelwork is decribed in the article- How to Safely Erect Structural Steelwork? [PDF]
- The anchor bolts used for the fastening of steel structures shall be set in a designed position and grouted along with foundations.
- Alternatively, the anchor bolts should be provided in the concrete foundations with anchor channels and bolt boxes for the purpose of flexibility and grouted after final alignment and leveling of the column.
- The gaps between the bearing surface of the foundation and bottom of the structures must be filled with cement grout.
- The grouting shall be done after the verification and proper positioning of the structures but before encasing the structures with concrete if specified.
- The damaged structural members shall be examined and rectified or replaced as directed.
- The erection of high-tier structures shall be executed only after fastening the relevant lower-tier by the permanent or temporary fastening devices as per the schedule of execution of work and certified for safety.
- The joint and mating surface, including the strips, mating planes, and filler or spacers shall be cleaned of dust, rust, and water.
- The erected structural members must be firmly fastened by drifts and bolts, crossing bars, permanent or provisional tacking, and so on before the erection crane book is removed.
- Trusses with span over 12m shall not be slung at the point of the apex, as they can develop compression stresses in the bottom tie member.
- These types of members shall be lifted by slinging at two midpoints of rafters, braced by a wooden member at a suitable location.
- Once the trusses are placed in position, wind bracings and purlins shall be fixed as early as possible.
- The ends of the trusses, which overtakes the prevailing winds shall have one free end and the other fixed with holding down bolts.
- For steel members of large spans, the free end of the truss shall be provided with suitable rocker and roller bearing whereever indicated.
Erection of Joints in Steel Structure
- The holes to be riveted shall be fitted with temporary bolts and drifts of diameters equal to those of the holes while erecting joint steel members.
- It is necessary to draft the location of bolts on both members for accurate matching of holes.
- The number of bolts and drifts shall not be less than 40 % of the total number of holes.
- The forces applied to drifts shall be the same as approved for rivets.
- The number of drifts shall be 10 % of the number of holes.
- The number, size, and length of tack welds in erection joints bearing erection forces shall be as indicated.
- For the erection joints which do not bear the erection forces the length of tack welds shall be a minimum of 10 % of the designed weld length of the joints.
- The welding, riveting, and final fastening of permanent bolts shall be done only after the inspection of the structural elements for their positions.
- The head bolts and nuts shall perfectly be in touch with the surfaces of structures and washers.
Tolerance Allowed in Erection of Steel Structure
For erecting steel members without a crane, the maximum tolerance for line and level of steel structure shall be +/-3.00mm on any part of the structure.
The steel structure shall not be out of plumb more than 5.00 mm for the 10-meter section in height and not more than 7.00 mm per 30-meter section. These tolerances shall apply to all parts of the structure unless otherwise specified.
The tolerance allowed in the erection of steel structure containing cranes shall be as per Table 1.
Table 1: Tolerance allowed in erection of steel structure using cranes.
|Main columns and Roof posts||A. Shifting of columns axis at foundation level with respect to building line:|
|1. In longitudinal direction||± 5.00 mm|
|2. In lateral direction||± 5.00 mm|
|B. Deviation of both major column axis from vertical between foundation and other member connection levels:|
|1. For a column up to and including 10 m height||± 5.00 mm from true vertical|
|2. For a column greater than 10 m but less than 40 m height||± 5.00 mm from true vertical for any m length measured between connection levels but not more than ± 8.00 mm for 30m length|
|C. For adjacent pairs of columns across the width of the building prior to placing of truss.||± 5.00 mm on true span|
|D. For any individual column deviation of any bearing or resting level from levels shown on drawings.||± 5.00 mm|
|E. For adjacent pairs of columns either across the width of buildings or longitudinally level difference allowed between bearing or seating level supposed to be at the same level.||5.00 mm|
|Trusses||A. Deviation at center of span or upper chord member from vertical plane running through center of bottom chord.||1/500 of the span or 10 mm whichever is less.|
|B. Lateral displacement of top chord at center of span from vertical plane running through center of supports.||1/250 of depth of truss or 20 mm whichever is less.|
The maximum tolerance for line and level of steel structure shall be +/-3.00mm on any part of the structure.
The steel structure shall not be out of plumb more than 5.00 mm for the 10-meter section in height and not more than 7.00 mm per 30-meter section.