ARTICLE#002 COLD-FORMED STEEL (CFS) BOXED LINTEL DESIGN
are commonly used above the load-bearing wall window and door openings to resist floor gravity and live loads. This article will go through the provisions North American specification for the design of cold-formed steel structural members, CSA S136 for boxed lintel design, and is limited to boxed lintels joined at discrete points along the axis of the member. Thus, built-up beams are not covered in this article.
Boxed lintels are either detailed supporting the underside of the floor slab or directly above the window. The first of the two options is better, mainly to avoid localized web-crippling issues due to the load path, which is the transfer of gravity loads from the floor slabs, through to the cripple studs, and finally bearing on the boxed lintel. In turn, causing an issue when dealing with significant gravity loads. Web stiffeners will have to introduce into the boxed lintel at crippling stud locations, resulting in a significant labour cost increase during fabrication.
Another benefit is that the load path is simplified for wind resistance, it becomes clear that the track header will be the wind resisting element. Whereas, when the box lintel is located above the opening, the wind resistance can be argued to be the weak axis strength of the boxed lintels, in addition to the track header strength. I guess it depends on the mood of the designer that day?
In the image above, the end bearings are shear connectors to prevent the ends from failure pre-maturely.
Load combinations from NBCC 2010 will be used for this example. The boxed lintel's analysis model is a simply supported beam, braced at its ends.
Span length = 2,300 mm
Total dead load = 4.50 kN/m
Total live load = 5.76 kN/m
Total factored load = 1.25(4.50 kN/m) + 1.50(5.76 kN/m) = 14.27 kN/m
Deflection limit for live load = L/360
We will try 2 - 800S200-68.
Factored shear resistance, Vr = 2 x 19.10 kN = 38.20 kN
Factored moment resistance, Mr = 2 x 10.10 kN-m = 20.20 kN-m
Moment of inertia, Ix = 2 x 3.390 x 10⁶ mm⁴ = 6.78 x 10⁶ mm⁴
Web crippling, Pwcr = 2 x 7.69 kN = 15.38 kN
Factored shear force, Vf = wL/2 = 14.27 kN/m x 2.3 m / 2 = 4.91 kN
Factored moment force, Mf = wL²/8 = 14.27 kN/m x (2.3)² m / 8 = 9.44 kN-m
Required moment of inertia, Ix = 5wL⁴/384E(L/360) = [5 x 5.76 kN/m x (2.3)⁴ m] / [ 384 x 203 mPa x (2.3 m/360) ] = 1.62 x 10⁶ mm⁴
Shear, Vr ≥ Vf = 4.91 kN ≥ 38.20 kN
Flexure, Mr ≥ Mf = 20.20 kN-m ≥ 9.44 kN-m
Web-crippling, Pwcr ≥ Pwcf = 15.38 kN ≥ 4.91 kN. The web crippling force is equal to the shear force in this example.
Deflection, Ix ≥ Ix,req = 6.78 x 10⁶ mm⁴ ≥ 1.62 x 10⁶ mm⁴
Combined bending and shear
Combined bending and web-crippling will not be checked as there is no bending moment at the ends of the boxed lintel.
Combined bending and torsional load will not be checked in this example as the top and bottom tracks act as restraints and are typically adequate to resist torsional loads. However, when there is significant torsional load present (i.e. boxed lintel supporting masonry veneer), combined bending and torsional load is usually the failure mode, therefore additional care must be taken by the designer to achieve a clear load path to resist the torsional force.
Our selection was a little bit overdesigned but for this example, it will suffice. In practice, you should probably resize this boxed lintel before your senior strolls on over to your desk to lecture you.
WHAT TO DO WHEN CFS LINTELS AREN'T STRONG ENOUGH?
When CFS lintels are not adequate to resist the required loads the designer has the option to introduce structural steel W or HSS beams in lieu of the CFS boxed lintel. It is important that the fabricator is aware of this decision as the structural steel material will become part of the CFS wall fabrication process. The design of the structural steel lintel in lieu of the CFS boxed lintel is not covered in this article and is similar to structural steel design methodology for a simply supported beam bearing on posts found in CISC S16.
1. Batt insulation is typically required in all closed members exposed to the exterior, please take note of the wall assembly. This is quite a pain to fix later on-site.
2. An important note, often forgotten by designers is to check if a boxed lintel is required for non-load bearing walls due to the header material self-weight (CFS + gypsum board + insulation + finishes).
3. When specifying stiffeners, it is important that the flange size is at least one size greater to ensure adequate fabrication room for welds and connections.
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