Skid beams supply structural integrity for buildings, skid construction, and many other steel structures. The proper design, specification, and assembly of steel beams provide the foundation for a successful steel assembly. In this article, you will learn about the types of steel beams used in skid construction, the beam selection process, associated steel components, and the construction process.
Types of Beams
Three primary types of beams may be used to frame and support a structural skid: i-beams, c-channel, and structural tubing.
I-Beams are also known as W, H, rolled joist, and universal beams. They withstand heavy loads, provide unidirectional bending behavior, and are economical and versatile. As a result, I-Beams see frequent use in skid construction.
The most commonly used I-Section in the United States is the wide flange (W-Shape). Inside surfaces of their flanges are parallel over most of the area. Wide-flange shapes are available in ASTM A992, having a yield strength range from 340-450 MPa.
Also known as the Parallel flange channel, C-channel may serve as a beam and column. C-Channel material derives from aluminum, steel, and stainless steel. They provide a flat web back and lesser radius of gyration than an I-section of the same height.
Unlike I-beams, the C-Channel bending axis does not center on the width of the flanges. Size varies according to the nature of skid construction. Grades of C-channel are designated as per ASTM A276. It frames supports for equipment used in skid construction and ceiling channel system. C-channels are preferred on I-Beams due to their ability to maintain strength tolerances at lower gauge than I-Beam and usefulness over low spans.
Structural tubing possesses a hollow structural steel cross-section and may subsitute for I-beam and C-channel. It is available in different shapes like rectangular, square, and round. Shape selection depends on material requirements, specifications, and application.
Structural tubing provides a high strength to weight ratio. It has a high radius of gyration about both axes. Aesthetically, structural tubing provides a good aesthetic appearance due to its smooth sides, closed sections, and rounded corners.
Structural tubing’s closed sections provide effective resistance against torsional loads. Hollow structural tubing provides uniformity of size, shape, and strength and easy ability to form, drill, bent, and punched.
Strucdtural tubing provides strong fire resistance and resists warping, twisting, shrinking, swelling, and splitting.
Design of Steel Beams
Steel beam design follows these steps:
- Choose the grade of the steel beam:
There are many different grades of steel, but A992 is the most commonly used grade for I Beams.
- Select the shape of the steel beam:
A wide variety of the shapes are available like W-shape, C-shape, L-shape and HSS. It depends on the designer to choose the beam’s shape. The most commonly used is I-Beams.
- Calculate the types of the loads acting on the beam:
The following type of loads provides the basis for beam section.
- Dead loads
- Imposed loads
- Wind loads.
- Vibration loads
- Select the type of design method (ASD or LRFD):
Typically there are two design methods of steel beam selection.
- Allowable stress design method (ASD).
- Load Resistance Factor Design method (LRFD).
It depends on the designer to choose the type of design method. The fundamental difference between LRFD and the allowable stress design method is that the latter employs one factor (i.e., the Factor of Safety), while the former uses one factor with the resistance and one factor each for the different load effect types. LRFD provides more consistent outputs simply because calculations employ more than one factor. LRFD produces more economical results as compared to ASD. It is always preferable to use LRFD method.
I-Beam Selection Process
The most commonly used beam section is I-beam and the factors affecting the selection of I-beam section have been discussed here:
The flange and web compose two essential components of an I-beam design. Sizing these components depends upon the client’s specifications and forces encountered by the beam/skid.
Parameters to Consider
- I-Beams should be selected with accurate web thickness to avoid rippling. Rippling is like a buckling phenomenon produced as a result of “tension field action.” When the thickness of the web is less than permissible limits, the section starts to ripple, causing wrinkles on the surface of the section. Rippling control occurs by keeping the thickness of the web as per the designer’s recommendations.
- Vibration: I-Beam design must minimize vibration. The stiffness and mass of the beam provide the primary vibration resistance.
- Bending: I-Beam must be able to withstand yield stresses upon application of loads.
- Buckling: I-Beam design must meet minimum torsional stresses to avoid any accident.
- Deflection: Deflection should be minimum in I-Beam. Deflection can be reduced by the following measures.
- Decreasing the length of the beam.
- Using moment joints at the end of the beam.
- Increasing beam modulus of elasticity.
- Sharing loads on other beams.
The web component of I-Beam should withstand shear stresses, and the flange should have enough resistance against bending.
After the selection of the grade and shape of the beam, the following steps determine the design of a I-beam:
- Account for all loads that impose upon the I-Beam.
- Draw a bending moment diagram considering all imposed loads on I-beam. Take the value of the maximum bending moment.
- Select the most suitable size of steel I-beam from the standard I-beam table.
- Calculate the area moment of inertia declared as I of that section.
- Find the depth of the selected I-beam.
- Find Stress developed in the selected section of I-beam by the following formula:
f/L = M/I
f is the bending stress
M is the moment at the neutral axis
L is the perpendicular distance to the neutral axis
I is the moment of inertia about the neutral axis x.
Associated Steel Components
A skid, structural frame, or other beam-centric construction project usually requires additional steel components to provide the required structure. These components include cross members, angle iron, grating, stairs, platforms, lifting lugs, and anything else require per client or project specfications.
Cross members add rigidity and strength to a beam structure and connect to perimeter beams as per structural calculations. Angle iron runs perpendicular to cross members and helps reduce floor plate deflection.
Floor plate, also known as skid plate, provides the capability for foot traffic and equipment to transverse the surface. Floor plate material is the same as those of skid beams. Client requirements dictate the required dimensions and style.
Grating provides a collection of identical elements consisting of two sets in which the second is set perpendicular to the first. Grating allows the movement of small particles like air and water to traverse. The size and material selection of the grating varies per required specifications.
Beam Construction Process
Generally, the first step in beam fabrication is cutting the beam to size. Most foundries and distributors provide factories with beams in preset lengths and personnel must cut the beam to the required size. As, part of the beam cutting process, beams frequently require coping. Coping allows beams to fit perfectly by removing part of the web or flange. Coping may be provided via a manual cutting process or a CNC machine.
After cutting and coping the beams, fitting and welding the beams occurs. Generally speaking, beams do not require the degree of precision that pipe or vessel welding requires. Steel beams joined to each other via a circumferential butt weld where two members butt together end-to-end.
Circumferential butt welds provide two options. The stronger of the two is the full-penetration weld, in which welding wire fills up to the wall thickness of the two adjacent members. The second method is the partial penetration weld, in which melted wire simply caps the circumference of the adjoined members like a strong piece of tape. Load-carrying structures typically utilize the full penetration method.
After type of weld used in steel construction is a fillet weld. This type of weld joins two pieces of metal together when they are perpendicular or at an angle to each other. Fillet welds are commonly applied when floor plate, angle iron, or other steel material must join to beams.