Re: N Subramanian Design Of Steel Structures.pdf
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ISMB sections are the only I-sections that are normally produced in India on account of caliber rolling method. These sections are used for beams as well as columns. ISMB sections have relatively narrow and sloping flanges and a thick web compared to wide flange sections (see Figure). The ISMB beams are not economical especially for compression members, because of excessive material in the web and the lack of lateral stiffness due to narrow flanges. Also since the available sections are limited, when a section is slightly inadequate, the choice is limited to either the next available section (which may be 25-45% heavier in weight) or built-up sections through welding, which is time-consuming and expensive.
Parallel flange sections are hot-rolled steel sections, with parallel or nearly parallel flanges with square toes and curves at the root of the flange and web. Structurally these beams are more efficient than the conventional I-beams with taper flanges. The load-carrying capacity of parallel flange I-beams under direct compression is much higher than that of tapered flange beams. Also, connections to the flanges are simpler since no tapered washers, etc. are needed. Further, these beams proved to be very popular with the construction industry for reasons of considerably reducing the cost of fabrication and erection.
As per IS 12778:2004, hot rolled parallel flange sections are classified (Fig. 1b) as narrow parallel flange beams (NPB), wide parallel flange beams (WPB), and parallel flange bearing pile sections (PBP). All above sections having yield stress 250MPa [12] are most commonly produced and used for steel structures in India These wide parallel flange beams and columns are manufactured in India M/s Jindal Steel and Power Limited (JSPL), at Raigarh, Chhattisgarh. The sectional properties of some of these beams are given in Appendix A of Subramanian,2016. (Other sectional properties may be obtained from the manufacturers).
Parallel flange beams have several inherent functional advantages which include (1) flexibility, (2) cost-effectiveness, (3) excellent durability, and (4) superior weldability. There are several advantages of using parallel flange beams for various purposes in structural steelwork.
- Flexibility: These beams are available in an extensive range of weights, dimensions, and sizes, as well as in different section depths, flange widths, and web thicknesses. These beams are designed to handle uniform loads across the beam length. This increases the tension on the sides of the beam. With the weight applied on the flange, the entire mass is distributed evenly, causing less stress to pass through the web.
- Parallel flange beams bear high loads: The design of these beams makes them capable of bending, rather than buckling, under high stress. These beams can withstand massive loads of structures. Since H beams have higher section modules for the same weight, they result in the economy. (Savings of the order of 10-24% can be achieved).
- Ease of Fabrication: Parallel flange beams provide easier fabrication because of the easier connection of joints by direct bolting on flanges without using tapered washers. In addition, gussets can be easily welded to the inner surface of the beam flange. Unlike tapered flange beams, H beams can be readily butt welded and sound welding is assured.
- Parallel flange beams have higher load-carrying capacity: The high efficiency of the parallel flange beams is primarily due to the better distribution of the materials across the section. This leads to a higher moment of inertia, section modulus, and radius of gyration. Hence parallel flange beams have higher load-carrying capacity.
- Parallel flange beams are recyclable and cost-effective: Like any other steel product, Parallel flange beams can be recycled several times. The special feature associated with their use is that their strength is never compromised the longer they are used. Recycling structural steel can also help to reduce costs, saving on production expenses, materials, time, and energy.
- Efficient and economic designs: They have excellent sectional performance, with high bending and buckling resistance due to the H shape arrangement of flanges and the web. They are more efficient because of higher bending strength in the case of beams and higher axial load-carrying capacity in the case of columns. They are structurally more stable since a higher radius of gyration lowers the slenderness ratio and allows withstanding of buckling to a greater extent. Further higher strength-to-weight ratio leads to lighter structures and foundations.
- Better Seismic Resistance: Steel frame structures made from parallel flange beams are frequently used as seismic load-resisting systems for buildings in seismic regions. These structures are rectilinear assemblies of columns and beams which are typically joined by welding or high-strength bolting or both. Resistance to lateral load is provided by flexural and shearing actions in the beams and columns. Lateral stiffness is provided by the flexural stiffness of the beams and columns.
These NITTETSUHYPER BEAMs were meant to replace welded H-shapes; the web height and the flange width of a size series are constant regardless of the flange and web thickness. As seen in Fig. 4 of Ogawa et al.(2012), the method consists of expanding the distance between the flange inner faces using a rolling mill with skewed rolls and finishing the rolling through a universal mill with variable-barrel width horizontal rolls. An additional method was developed whereby uniform flange width is obtained through edger rolling using variable-caliber depth edger rolls. Yet another method was developed for controlling residual stress by water cooling the flanges during and after rolling. As a result, NITTETSUHYPER BEAMs of 177 different sizes in 26 size series up to 900300 mm with high dimensional accuracy were made available.
Thereafter, using FEM analysis based on a software called NSCARM, an efficient roughing rolling method for large sectional areas was developed, and thanks to this method, the sizevariety of the NITTETSUHYPER BEAM was further expanded to 592 sizes in 47 size series up to 1,000 400 mm Ogawa et al.(2012). This method also incorporates advanced temperature control technology in the rolling process to produce webs thinner than those of conventional I-beams. These thinner webs open the way to more economical design since more material is needed only in the flanges (Subramanian, 1982).
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