Steel Beams
Steel Beams
Steel beams (Fig 1) are important type of structural elements which play a key role in creating a safe load path to transfer the weight and forces on a structure to the foundations and into the ground. These beams are a member of the structure which is subjected primarily to transverse load and negligible axial load. The cross section of the beam provides superior load bearing support. The superior spanning capability of the steel beam means fewer columns and more usable space.
Steel beam is a type of building material which is used to construct several structures. It can be used in many types of building applications. In some countries, it is the standard for creating columns in home and commercial buildings. These beams can also be placed parallel to the ground to form floors and roofs. They also play a pivotal role in bridge construction, and serve as structural supports for highway ramps and overpasses. The application of steel beams depends on their sectional properties which include mass per unit length, cross sectional area, flange width, depth, web thickness, flange thickness, root radius, section modulus, moment of inertia, radius of gyration, and plastic section modulus.
Steel beam is one of the frequently used structural member whose main function is to transfer load principally by means of flexural or bending action. In a structural framework, it forms the main horizontal member spanning between adjacent columns or as a secondary member transmitting floor loading to the main beams. Normally only bending effects are predominant in a beam except in special cases such as crane girders, where effects of torsion in addition to bending have to be specifically considered.
The cross section of a beam consists of a web and two flanges. The junction between the flange and the web is known as fillet. Beams are produced either by rolling or by fabrication by cutting and welding steel plates. The cross section of the beam has a vertical web in the centre of the beam and horizontal flanges at the top and at the bottom. The flanges resist bending while the web takes on the shear-force. The Euler–Bernoulli beam equation shows that the I-shaped section is a very efficient form of a section for carrying both bending and shears loads in the plane of the web. On the other hand, the cross-section has a reduced capacity in the transverse direction, and is also inefficient in carrying torsion loads, for which hollow structural sections are frequently preferred.
Steel beams are produced as per various national and international standards. These standards have standardized beam sizes which make it easier for engineers, architects, and builders to coordinate during the construction. The national and international standards give the nominal dimensions, mass, sectional properties, and tolerances of the beams. Beams are frequently designated by nominal depth and nominal flange width and mass of the section in kilograms per meter (kgs/m). They are mostly produced from steels of either structural steel grades or micro-alloyed steel grades.
Steel beams are produced in two shapes represented by the alphabets ‘I’ and ‘H’. I-beam, as shown in the name, is a type of beam having a cross section which looks like the alphabet ‘I’. H-beam is named after the alphabet ‘H’ since its cross sectional shape is similar to the letter ‘H’. While the flanges in an I-beam are normally narrower than the height of the web, those in an H-beam are much broader. H-beam is an economical section with more optimized section area distribution, and more strength-to-weight ratio. It is also been called wide flange beam. Beams are produced in two configurations. As per these two configurations, the beams have either tapered flange or parallel flange. Fig 1 shows cross sections of a tapered flange beam, a parallel flange I-beam, and a parallel flange H-beam.
Fig 1 Steel beams
The beams with tapered flange are also called steel joists. These beams normally have narrow flanges and are normally rolled in a rolling mill with only horizontal rolls. The inner surface of the upper and lower flanges of these beams has a slope which makes the flanges thin outside and thick inside. These beams need tapering washers for connection. The use of these beams is restricted since the tapered flanges causes processing difficulties.
The moment of inertia of the section is quite different in case of tapered flange beams since the cross sectional size of these beams is relatively high and narrow. Hence, these beams normally can only be applied directly to the parts with bending in its web plane or to form lattice-type force bearing parts. It is not suitable for the axially compressed structural parts or the bending parts perpendicular to the plane of the web, which makes very limited in application of these beams.
The beams having parallel flanges are called parallel flange beams. They feature a profile which has a central web connecting the two parallel end units (flanges). For each specific beam, the surface of the top (or outside) of each flange is parallel with the surface of the bottom (or inside) of that flange. The flange essentially has a constant thickness. The web is also of a constant thickness. The thickness of the web is not equal to the thickness of the flanges. There are a large numbers sizes and shapes of parallel flange beams which are specified in various national and international standards. Parallel flange beams are rolled in universal rolling mills having two horizontal rolls and two vertical rolls.
Parallel flange beams are also known as universal beams. H-beams are produced only as parallel flange beams and are also known as universal columns. The use of these beams is very common. Parallel flange beams are produced as light, medium, and heavy hot rolled parallel flange beams. Heavy beams are used for heavier loads. Parallel flanges of these beams are easier to connect and do away with the need for tapering washers. Use of parallel flange I-beam sections is advantageous than tapered flange I-beam sections due to the increased lateral stiffness. It is critical that these beams not to be used in any torsion bearing loads, since twisting can warp the beam and rupture the floor.
Increasing the depth of the beam increases the bending strength by the depth cubed, hence universal beams provide a lot of stiffness. However, the web thickness is not to be small to avoid buckling.
The parallel flange beam has the most efficient cross sectional profile since most of its material is located away from the neutral axis providing a high second moment of area, which in turn increases the stiffness, hence resistance to bending and deflection. In a structural steel frame work, it forms the main horizontal member spanning between adjacent columns or as a secondary member transmitting floor loading to the main beams.
The web of a parallel flange beam resists shear forces, while its flanges resist most of the bending moment experienced by the beam. I-shaped beams are designed to carry high loads over long spans. They are a very efficient form for carrying both bending and shear loads in the plane of the web. The thick flanges and thinner web efficiently proportions material to resist the high bending loads from the beam applications. However, I-shaped sections have a reduced capacity in the transverse direction, and are also inefficient in carrying torsional load.
Though I-beams are excellent for unidirectional bending in a plane parallel to the web, they do not perform as well in the bi-directional bending. These beams also show little resistance to twisting and undergo sectional warping under torsional loading. For torsion dominated problems, H-beams or other types of stiff sections are used in preference to the I-beams.
H-beams have equal or near-equal width and depth and are more suited to being oriented vertically to carry axial load such as columns in multi-storey construction, while I beams are significantly deeper than they are wide are more suited to carrying bending load such as beam elements in floors. These beams are cost-effective, flexible and are superior in terms of strength, efficiency, higher axial and bending load-bearing capacities. Parallel flange beams enable complex fabrications in high volumes due to inherent functional advantages of these sections.
Due to the reasonable cross-section shape, H-beams can make the steel function better and bear a higher load. Different from the common I-beam, the flanges of H-beams are wide, and the inner and outer surfaces are normally parallel, which makes them strong in connecting high-strength bolts and other components. With reasonable sizes and complete models, they are convenient for design of several complex steel structures.
H-beam is a high-performance steel section due to its advantage of optimized cross sectional area distribution and reasonable ratio of strength to weight ratio. With the features of wide flange and thin web, H-beam has a large section modulus, high bending resistance. and excellent mechanical properties. These beams are normally heavier than I-beams and are useful as supports for retaining walls and the like. They can also be used as beam sections where head room is of concern. Because of its outstanding properties, H-beam is a popular section for the designers of steel structures.
Bearing pile shapes (H-piles) are similar in shape to the H-beams in that they also have equal or near-equal width and depth and the flange surfaces are parallel. The web surfaces are also parallel. However, they differ from the wide flange in that the bearing pile shapes have equal flange and web thicknesses. The thickness of the web of a particular H-pile shape is equal to the thickness of the flange. H-piles are manufactured and designed to transfer structural loads to the good bearing soils.
When a beam bends, the top of the beam is in compression and the bottom is in tension. These forces are greatest at the very top and at very bottom. Since a parallel flange beam has higher amount of material at the top and bottom sides and smaller material in the web, it provides a structural section which is stiff with use of least material.
I-beams are designed to carry high loads over long spans. The thick flanges and thinner web efficiently proportions material to resist the high bending loads from beam applications. H-beams are stockier, wider, and are designed to carry high axial loads, for applications such as columns and piles etc. These beams are normally heavier than other steel sections and are useful as supports for retaining walls and the like. They can also be used as beam sections where headroom is of concern. H-beams are used for beams, columns, and in bearing pile sections as compression members.
The primary advantage to the H-beam is that it allows the designers of a building structure to distribute a load over a wide area. This means that it can support a larger or wider structure with less risk of a failure. The H-beams also weigh less than a square beam of the same size, but can support a larger load, making them more efficient.
Parallel flange beam sections are most preferred, popular and are widely used in seismic resisting steel buildings worldwide. These sections are used for steel structures due to advantages such as (i) increased lateral stiffness, (ii) 10 % to 15 % cost reduction, (iii) easy to weld and bolt, and (iv) substantial availability of numbers and grades. When used under bending load, steel savings in the range of 10 % to 25 % are achieved, as beams of lower sectional weight can be used.
Tolerances for rolled steel beams
The tolerances for rolled steel beams are shown in Fig 2 and described below.
Fig 2 Tolerances for rolled steel beams
Section height – It is the deviation from nominal on section height (h) measured at the centre line of the web thickness. The deviation is to be within the tolerance limits as specified in the standards.
Flange width – It is the deviation from nominal on flange width (b). The deviation is to be within the tolerance limits as specified in the standards.
Web thickness – It is the deviation from nominal on web thickness (t) measured at the mid-point of the beam height (h). The deviation is to be within the tolerance limits specified as per the standards.
Flange thickness – It is the deviation from nominal on flange thickness (T) measured at the quarter flange width point (b/4). The deviation is to be within the tolerance limits specified as per the standards.
Out of squareness – Fig 2 shows out of squareness. The out of squareness of the beam is not to exceed the limits specified in the standards.
Web off- centre – The mid distance of the web is not to deviate from the mid width position (b1= b2) on the flange by more than specified in the standard.
Straightness – The straightness of the beam is to comply with the standard. The measurement the straightness needs a reference straight edge from which deviations from beam straightness are measured. A rigid string line is an acceptable straight edge provided that deviations in the horizontal plane only are measured. Measurement is carried out as given below.
For Qxx (Fig 2), the beam is laid in the ‘H’ position on a flat surface and the string taken on the outside of the centre of the flange width from the two ends of the unconstrained beam section. For Qyy the beam is laid in the ‘I’ position on a flat surface and the string is taken along the flange tip between the two ends of the unconstrained beam section.
Tolerance on mass – The deviation from the nominal mass of a batch or a piece is not to exceed the limits specified in the standards. The mass deviation is the difference between the actual mass of the batch and the calculated mass. The calculated mass is determined using a density of 7.85 tons per cubic meters.
Tolerance on length – The beam is to be cut to the ordered length with the lenfth tolerances as specified in the standards. It is normally +/- 50 mm or +100 mm where minimum length is specified. The beam length is the longest usable length of the beam assuming that the ends of the beam have been cut square.
Advantages of parallel flange beams
Parallel flange sections are hot rolled steel sections, with parallel or nearly parallel flange with square toes and curves at the root of 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.
Parallel flange beams have several inherent functional advantages which include (i) flexibility, (ii) cost-effectiveness, (iii) excellent durability, and (iv) superior weldability. There are several advantages of using parallel flange beams for various purposes in structural steelwork. 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, with the maximum deflection falling on the centre of the beam. 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. The strength of steel in the beam and its shape both help in reducing the need for several other supporting structures, which can help in saving time during the construction work.
Parallel flange beams help in custom steel fabrication. These beams are very flexible and can be used in various structural steel construction projects. Steel fabrication involves cutting, bending, and shaping of steel, and the fabrication of the steel beam is one of the most efficient processes for meeting deadlines of a project. It is easier to use parallel flange beams during custom structural steel fabrication and welding processes for all types of construction.
Parallel flange beams are recyclable and cost-effective. Since the parallel flange beams are steel products, they 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.
Parallel flange beams are high tensile sections. The high efficiency of the parallel flange beams is primarily due to the better distribution of the materials across the sections. This leads to higher moment of inertia, section modulus, and radius of gyration. Hence parallel flange beams have higher load carrying capacity.
Parallel flange beams provide efficient and economic designs. They have a good utilization area of the section. They are mechanically more efficient because of higher bending strength in case of beams and higher axial load carrying capacity in case of columns. They are structurally more stable since 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.
Parallel flange beams provide easier fabrication because of easier connection of joints by direct bolting on flanges without using tapered washers and easier butt welding of plate at the edge of the flange. These beams are economical and result into substantial saving in material weight when used as compression members (columns) or flexural members (beams).
The advantages of using parallel flange beams are (i) lighter super structure, (ii) upfront saving in costs because of lesser weight of steel, (iii) reduced depth of beams, (iv) higher load carrying capacity for the same depth of columns, and (v) lower transportation, handling, and erection costs because of lower weight of structure.
Different applications of parallel flange beams
Parallel flange beams are good construction materials which are used in a large number of construction projects. Both the I-beams as well as the H-beams have several applications because of their excellent load-bearing capabilities. These beams are used for (i) light weight applications, (ii) flyovers, ramps, and over-passes, (iii) stadiums, (iv) residential and commercial complexes, (v) industrial buildings, (vi) on-shore and off-shore structures, (vii) electrical poles, (viii) ports, (ix) ship building, (x) wagons and truck bed frames, (xi) bearing piles, and (xii) many others. The difference in the cross-section of I-beam and H-beam greatly differentiates their applications. H-piles are used to anchor super-structures to deep foundations, particularly in areas where the soil is not dense enough to support the weight of the building. Examples of some of the applications are described below.
Steel frame structures made from parallel flange beams are frequently used as seismic load resisting systems for building 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 flexural stiffness of the beams and columns.
I-beams are the ideal steel beam for construction projects which need a lightweight load-bearing material. Frequently, these beams are used to construct frames for trolley-ways, elevators and lifts, trailers, and even truck bed frames. These projects do not need the wider range and stronger hold which the H-beams provide. Instead, these projects focus on a lighter, somewhat shorter material which can bring a comparatively similar weight-bearing material onto a smaller, more precise area. H-beams are typically made to be long and wide since majority of its applications need it to cover a larger area. I-beams, on the other hand, can be produced into smaller sections, making them the preferred beam for these smaller applications.
H-beams having thicker walls and flanges are ideal for the construction of platforms of varying sizes. The thicker flanges make it stronger and able to support larger amounts of weight. H-beams are primarily used for platforms which need to be able to support heavier loads.
In case of the floors of a skyscraper, or the upper floor of a warehouse, the floors are required to hold lighter loads. In such a case the weight of the beam can become an issue. For these applications, I-beams are normally preferred.
When it comes to the bridges, or overpasses, it is quite accepted that it is the H-beams which are widely used. This is since the bridges need a certain level of strength and durability which is more evident in H-beams instead of I-beams. The fact, the H-beams are heavier is also disregarded because of how much more support bridges normally have on their own.
For low rising commercial buildings, I-beams are normally used. Their contained strength provides enough support for a few floors and leaves a lot of room for more construction. I-beams are more versatile for these kinds of construction projects. H-beams, however, are more frequently used for large constructions such as skyscrapers, or large industrial complexes. H-beams are normally stronger and have a wider span, bringing more support across a wider area.
Ports, on-shore and off-shore structures need a relatively strong foundation. H-beams are the perfect structural framework for these constructions. Its wider range weight-bearing, makes it ideal for these structures. The same is also applicable for large cargo ships which need to be strong enough to carry an enormous quantity of weight across oceans and seas. Because of the weight that H-beams can carry, they are one of the more popular choices for such things. I-beams can be used for cargo ships as well, however, they serve more of a structural application for the deck instead of the area in which the cargo is placed on.
Leave a Comment