Civil's Guide
Effective Lengths and Preliminary Sizing
Effective Length
Effective length factors are used in the design of steel memebers and factors correspond on the condition of the beam during construction and in its permanent state. For example, a common example in the design of a beam is checking the top flange fully restrained (usually by floor plate/slab) or unrestrained.
The effective lentgh factors are tabulated below:
| Conditions of restraint at supports | Loading conditions | ||
|---|---|---|---|
| Normal loads | Destabalising loads | ||
| Compression flange laterally restrained; beam fully restrained against torsion (rotation about the longitudinal axis) | Both flanges fully restrained against rotation on plan | 0.7L | 0.85L |
| Compression flange laterally restrained; beam fully restrained against torsion (rotation about the longitudinal axis) | Compression flange fully restained against rotation on plan | 0.75L | 0.9L |
| Compression flange laterally restrained; beam fully restrained against torsion (rotation about the longitudinal axis) | Both flanges partially restrained against rotation on plan | 0.8L | 0.95L |
| Compression flange partially restrained against rotation on plan | 0.85L | 1.0L | |
| Both flanges free to rotate on plan | 1.0L | 1.2L | |
| Compression flange laterally unrestrained. Both flanges free to rotate on plan. | Partial torsional restraint against rotation about longitudinal axis provided by connection of bottom flange to supports | 1.0L + 2D | 1.2L + 2D |
| Compression flange laterally unrestrained. Both flanges free to rotate on plan. | Partial torsional restraint against rotation about longitudinal axis provided only by pressure of bottom flange onto supports | 1.2L + 2D | 1.4L + 2D |
Effective Length of cantilevers without intermediate restraint
| Restraint conditions | Effective Length | ||
|---|---|---|---|
| Support | Cantilever Tip | Normal load | Destabalising loads |
| Continuous with lateral restraint to top flange |
|
|
|
| Continuous with partial torsional restraint |
|
|
|
| Continuous with lateral and torsional restraint |
|
|
|
| Restrained laterally, torsionally and against rotation on plan |
|
|
|
Preliminary Sizing of Steel Beams
When starting the design of any stuctural members, there are rules of thumb (span to depth ratio) which are commonly used to determine the size of a members which provide a starting point in a design before refining it through calculations and checks.
When steel beams depths become large/long spans, deflection will govern the design
| Floor Type | Span (m) | |
|---|---|---|
| Composite beam & in-situ composite slab | 6-12 | |
| Steel beam & precast slab | 6-9 | |
| Slimfor beam & precast slab | 6-12 | |
| Composite beams with web openings | 6-12 | |
| Castellated/Cellular beams | 6-12 | |
| Truss | 6-12 | |
| Composite plate girder | 6-12 | |
| Element | Depth | |
|---|---|---|
| Non-composite primary beams | Floor = span/20 | |
| Roof = span/25 | ||
| Non-composite secondary beams | Floor = span/25 | |
| Roof = span/30 | ||
| Composite beams |
| |
Typical Column sizes
The maximum length of a column is dictated by the transportable length on the back of a truck/wagon on the motorway. The maximum lnegth is typically 12m – 15m before a splice connection between 2 columns.
| Number of floors supported by column | Universal column size (UC) |
|---|---|
| 1-2 stories | 152 UC |
| 2-4 stories | 203 UC |
| 3-6 stories | 254 UC |
| 5-12 stories | 305 UC |
| 8-12+ stories | 356 UC |
Section classification of Structural Steel
Structural steel sections are classified as either elastic or plastic which is determined in accordance to Eurocde 3 (BS EN 1993). The thinner the web and plate sections in steel means that the beam will likely buckle locally as the section is slender. This means the section cannot achieve its full plastic capacity. This will impact the capacity of the steel in design scenarios. British standards used to refer the classifications as plastic, compact, semi-compact and slender before changing to class 1,2,3 and 4 in accordance to Eurocodes.
Before beginning the design of any steel member, the section classification must be confirmed, which will allow us to determine it’s capacity when checking against bending, shear, etc.
| Cross-section classification | Method of global analysis | Method of section analysis | Description |
|---|---|---|---|
| Class 1 | Plastic | Plastic | achieves a plastic hinge and has rotatational capacity required for plastic analysis. |
| Class 2 | Elastic | Plastic | achieves a hinge, but limited rotational capacity due to local buckling. |
| Class 3 | Elastic | Elastic | achieves yield strength with elastic distribution, but local buckling prevents development of the plastic moment resistance. |
| Class 4 | Elastic | Elastic plate buckling | local buckling prevents attainment of yield |