This Section refers to a series of Load Capacity Tables (see pink and green pages) giving load carrying capacities of generic cold formed steel sections. Values are given for the based on limitations of both bending resistance and deflection.
6.1 Generic sections
The generic sections are symmetric C and Z sections, with flange widths as given in Table 6.1. The sizes have been selected to be typical of those marketed. Back to back C sections are used as "double" sections. Properties for most of the sections are given for steel grade S280 (pink pages); some sections are also given for steel grade S350 ( green pages). These generic sections are representative of sections of similar size and thickness produced by a number of manufacturers, and can be used for scheme design in a particular application. Manufacturer's data should be used for final design.
Table 6.1 List of generic lipped C sections of grade S280 and S350 steel included in Load Capacity Tables
Depth
Load Capacity Tables (Green pages)
Beam Column Beam Column
100
Table 6.2 List of generic lipped Z sections of grade S250 and S350 steel included in Load Capacity Tables
Depth
Load Capacity Tables (Green pages)
Btop Bbot Beam Column Beam Column
100
-6.2 Load capacity tables for beams
For each section size and steel grade, a single load-span table is given. The table gives values of maximum working load per span for a range of spans and restraint conditions, at the limits of bending resistance, of total deflection and of web crushing. For any particular situation, the minimum of the three values is to be taken as the maximum working load that may be supported by each span of the beam. Deflections tend to govern the choice of section as the span/depth ratio of the member increases.
6.2.1 Bending resistance limits
The bending resistance of a beam depends on the distance between points of lateral restraint, as explained in Section 3.3.4. The bending resistance in derived in the tables is that given by BS 5950-5 for the particular section and restraint conditions. Most beams used in floors are continuously restrained, if fixed regularly to the flooring.
The load values in the table are the values of total working load per span which, when multiplied by a factor of 1.6 (see Section 1.2), cause a bending moment at midspan that is equal to the bending resistance. (That is WL/8 for distributed loads and WL/4 for point loads.) The factor of 1.6 is conservative in all cases, and becomes increasingly so as the ratio of dead to imposed load increases. No distinction is made between single and multiple spans. This is a ‘safe’
assumption, based conservatively on the same moment at failure.
For C sections, values are given for both single and double sections. For Z sections, values are only given for single sections.
6.2.2 Deflection limits
Beam deflections depend on the section stiffness, the beam span, and whether it is simply supported or continuous. Load values in the tables correspond to the total working load per span at which the total deflection is equal to the limits of either span/250 or span/350 (see comment in Section 3.5.2).
Deflections are calculated using a section stiffness based on the average of the gross and the effective second moments of area of the section. The working load is unfactored when calculating these deflections. Deflections for the double span case are assumed to be 60% of the value for the single span case.
The limit of span/250 corresponds to the 'normal' limitation for beams in roofs.
The limit of span/350 should be used for lightweight floors. If it is assumed that the self weight component for floors is about 30% of the total load, this limit correspond to approximately L/450 under imposed load. Furthermore, the deflection under total load should not exceed 15 mm in order to satisfy the natural frequency limit of 8 Hz (see Section 3.5.1).
6.2.3 Web crushing limits
Additionally, values are given for each section for the load at which web crushing will occur at supports or at a point load when no web stiffener or cleat is present. The crushing resistance depends on the support width and the lateral restraint to the top flange. Web crushing resistance is in accordance with BS 5950-5, depending on the location of the load and the restraint to the flanges.
As for bending resistance, the load values are working loads which, when multiplied by a factor of 1.6, will equal the crushing resistance of the web. For single spans, the total load W is twice the crushing resistance. Continuous or multiple spans result in increased reactions at the internal supports. Thus, an additional stiffening cleat is required for all continuous beam applications.
6.3 Load capacity tables for columns
For each section size and steel grade, a single load-height table is given. The table gives values of the maximum axial working load for a range of effective lengths for both axes of the section.
Values are given for single and double C sections. Double sections are assumed to be attached together at regular intervals to form an equivalent I section. No tables are given for Z sections, since they are not normally used to support axial load.
The load values in the table are the values of working load which, when multiplied by a factor of 1.6, are equal to the compressive resistance of the column. The column resistance is derived in accordance with BS 5950-5, as discussed in Section 3.4. The factor of 1.6 is conservative in all cases, and becomes increasingly so as the ratio of dead to live load increases.
In deriving the axial resistances, torsional restraints are assumed at the positions of the major axis restraints and at the column ends. This affects the torsional-flexural buckling mode of failure.
For the concentrically loaded case, the load eccentricity is assumed to be zero.
For the eccentrically loaded cases, the load is assumed to be applied at the face of the section. For members in walls, the eccentric load is typically taken to be applied at the wall face, but concentrically on the other axis. For isolated columns, load is typically taken to be applied eccentrically to both axes, i.e. at a corner of the section. Values are given for combinations of four different effective lengths about the y-y or minor axis and two different effective lengths about the x-x or major axis. Values are also given for the cases where continuous restraint is provided about each axis, in combination with the range of effective lengths about the other axis. (In the plane of walls, minor axis restraint can be provided by straps.)
6.4 Guidance on selection of cold formed steel sections
For efficient structural performance, the following general guidelines on member selection are given:
(1) For compliance with normal deflection limits, the ratios of member span, L to depth, D should be:
L/D # 20 simply supported floor beams L/D # 24 continuous floor beams L/D # 30 simply supported roof beams L/D # 35 continuous roof beams.
(2) Restrain the compression (upper) flange of beams by attachment of flooring or roof sheeting or other members.
(3) Limit the effects of local buckling by keeping the flange widths to steel thickness ratio (b/t) to within the following limits:
b/t # 40 lipped sections
b/t # 12 plain sections (no lips)
b/t # 60 lipped sections with intermediate stiffener
(4) Limit the effects of web buckling or web crushing by keeping the web depth to thickness ratio, (D/t), within the following limits:
Single span:
D/t # 120 for single sections under uniformly distributed loads D/t # 80 for single sections under point loads
D/t # 150 for double sections back-to-back Double span:
D/t # 80 for single sections
D/t # 120 for double sections back-to-back
(5) Avoid heavy point loads applied directly to the top flange of the member.
Allow at least 50 mm end bearing for heavily loaded beams, and 100 mm bearing for internal supports on double span beams (or, preferably, use a stiffening cleat).
(6) Restrain studs in their y-y (minor axis) direction at one or two points along their length, or use double C sections). The unsupported height H divided by the member depth should be less than 35.
Designs outside the above suggested limits are permitted, but this can result in relatively inefficient structural performance. Designing within these limits does not imply that structural calculations are unnecessary, but rather that the selected section will be structurally efficient for the loads and span conditions under consideration.
6.5 Example of use of load-span tables for beams
Consider a floor spanning 4.5 m, with beams at 600 mm spacing:
Imposed load = 2.5 kN/m2
Self weight of floor = 0.5 kN/m2 (assumed) Total working load = 3.0 kN/m2
Load on beam = 3 × 0.6 × 4.5 = 8.1 kN
Choose deep lipped C section with L/D = 20 (approx) - say 200 mm deep For D/t = 120, t = 1.67 ; say 1.8 mm thick:
From Table 12:
Bending resistance of section:
Max. load for full restraint = 10.2 kN ~ OK Deflection of single span beam.
For * # L/350: max. load = 7.3 kN, which is not adequate.
Deflection of double span beam.
For * # L/350: max. load = 12.2 kN, which is adequate.
Web crushing: Support width = 50 mm
For unrestrained section, max. load = 8.2 kN ~ OK
Therefore, 200 mm × 1.8 mm thick C section is adequate, provided that the beams are double span.
For single span beams, try 2.0 mm thick section.
From Table 13:
Bending resistance of beam:
Max. load for full restraint = 11.6 kN ~ OK Deflection of single span beam.
For * # L/350: max. load = 8.3 kN, which is adequate Web crushing: Support width = 50 mm
For unrestrained section, max. load = 10.1 kN ~ OK
Therefore, 200 mm × 2.0 mm thick C section is adequate as a single span beam
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