Rules-of-Thumb are often dismissed as the lazy man's route to design, but it's worth remembering that in ancient times, structures were built according to form and proportion, the builders knew and understood the relationships between span and depth, together with height and cross-sectional area, and this understanding enabled them to build structures, which were not only breathtaking in scale and beauty, but were functional and resilient. The fact that many of them are still standing today is testament to their skills.

The experienced draftsman or engineer will still apply these ‘rules’ which will enable him or her to be able to set-out an entire structure and assign member sizes and connection details simply ‘By-Eye’ and final calculations will probably confirm, or be fairly close to, his or her intuition.

But with the advent of proper structural engineering theory, these old rules have largely been disregarded and forgotten, but the one that has survived, which is probably the foundation of all engineering theory is:

If it looks wrong, it probably is!

The ‘Select-and-Check’ principle is still applied widely today, and in order to reap the benefits of that experienced ‘eye’ we reproduce here some of the more common and useful ‘Rules-of-Thumb’ applying to:

  • Beams
  • Columns
  • Portal Frame Buildings
  • Flexible Connections
  • Moment Connections
  • Bracing
  • Bolts and Bolting

Beams

Span to Depth Ratio (L/h)

Member L/h
Floor Beam - Primary 15 / 20
Floor Beam - Secondary 20 / 25
Composite Beam - Primary 15 / 20
Composite Beam - Secondary 20 / 25
Castellated of Cellular Beam - Primary 15 / 20
Castellated of Cellular Beam - Secondary 20 / 28
Truss or Lattice Girder - Primary 10 / 15
Truss or Lattice Girder - Secondary 15 / 20
Plate Girder – Heavy Construction 10 / 15
Plate Girder – Light Construction 15 / 20
Crane Girder – Up to 10 T. Crane 12 / 15
Crane Girder – 25 T. + Crane 10 / 12

Beam Splices

  1. The beam splice should not be located at the point of maximum deflection.
  2. The splice flange-plate thickness should not be less than the beam flange thickness.
  3. The splice web-plate thickness should not be less than the beam web thickness

Columns

... General

  1. Columns in industrial structures should be orientated so that the web is in the direction of the shortest span.
  2. Columns in commercial or residential structures should be orientated so that the flanges face outward to facilitate the installation of Cladding or Curtain-Walls.

... Base Plates

  1. The Base-Plate thickness should not be less than the column flange thickness for axially loaded columns. Base-Plates that must resist significant bending-moments will be thicker. Assume bolt-diameter x 1.75
  2. The Base-Plate size should be the column size + 200 mm – rounded up to the nearest standard size
  3. The Base-Plate holes should be the bolt-diameter + 5 mm.
  4. The minimum Edge-Distance for bolts should not be less than 2 x the bolt-diameter. For Base-Plates that must resist significant bending-moments, assume bolt-diameter x 2.5

... Column Splice

  1. The column splice should not be located at the point of maximum deflection but should be located as close as possible to a point of lateral restraint.

... Cap and Base Splices

  1. The cap and base plate thicknesses should not be less than 1.5 x the column flange thickness and the bolts should be located as close to the flanges as practical.
  2. The column ends should be faced for bearing to provide full contact with the plates

... Flange and Web-Plate Splices

  1. The splice flange-plate thickness should be at least as thick as the column flange
  2. The splice web-plate thickness should be at least as thick as the column web
  3. Ideally, the column ends should be faced for bearing, ensuring full contact between the abutting column ends.
  4. If the column ends are not faced, then the splice-plates and bolts must be designed to transmit the full column load. If the column is likely to go into tension – slip-resistant bolts would need to be used.

Crane-Columns

  1. The spacing of Crane-Columns should not be greater than the height to the underside of the Crane-Girder above the ground
  2. The distance between the leg-centers of latticed-columns should be between H/7 – H/10, where H = the height to the top of the Crane-Girder
  3. The Crane-Columns should be orientated so that the webs are in the direction of the crane travel. The upper roof portal-columns should be orientated so that the flanges face inward (opposite to the crane travel)

Portal Frame Buildings

  1. For pitched Portals, the eaves haunch length should be between 7.5% to 10% of the span
  2. The haunch angle to the horizontal should be between 30° and 45°
  3. The haunch should be cut from a section which is equal to, or larger than the rafter section
  4. The column section should be at least one or two serial sizes greater than the rafter section, with a section depth (h) greater that the eaves height / 25

Longitudinal Spacing of Frames in Industrial Buildings (Portal-Frames or Truss-Girders)

Frame-Span Frame-Spacing
0 – 15m 6m
15 – 25m 7.5m
25m + 9m

Flexible Connections

... End-Plates

  1. The End-Plate thickness should be equal to bolt-diameter / 2 for beams up to 500 mm deep. For beams greater than 500 - increase the plate thickness to the next standard size.
  2. Horizontal Bolt-Hole edge-distances should not be less than 1.5 x the bolt-diameter – Vertical edge-distances should be increased to 2 x the Bolt Diameter.
  3. Recommended Bolt-Diameters should be M20.
  4. Hole-Diameters should be bolt-diameter + 2 mm.
  5. Welds should be 6 mm. Fillet for End-Plates up to 12 mm thick

... End-Cleats

  1. The Cleat leg-thickness should be equal to bolt-diameter / 2 for beams up to 500 mm deep. For deeper beams increase the leg thickness to the next serial size.
  2. Horizontal Bolt-Hole edge-distances should not be less than 1.5 x the bolt-diameter – Vertical edge-distances should be increased to 2 x the Bolt Diameter
  3. Recommended Bolt-Diameters should be M20.
  4. Hole-Diameters should be bolt-diameter + 2 mm.

... Fin Plates

  1. The Fin-Plate thickness should not be less than the bolt-diameter / 2
  2. Bolt-Hole edge-distances should not be less than 2 x the bolt-diameter
  3. Recommended Bolt-Diameters should be M20
  4. Hole-Diameters should be bolt-diameter + 2 mm.
  5. Welds should be 8 mm Fillet.

Moment-Resting Connections

... End-Plates

  1. End-Plate thickness should be equal to the bolt-diameter
  2. For beams less than 400 mm deep: use M20 bolts. For larger beams, use M24
  3. Welding should be fillet-welds (see Welding)

... Flange-Plates

  1. The Flange-Plate thickness should be at least as thick as the incoming beam flange-thickness, rising to, flange thickness + 10 mm for large beams or girders.
  2. For beams less than 400 mm deep: use M20 bolts. For larger beams, use M24
  3. Welding should be fillet-welds, provided adequate penetration can be achieved (see Welding)
  4. The shear-plate should be at least the same thickness of the incoming beam web – rounded-up to the nearest standard size.

... Moment-Trees

  1. The Flange Splice-Plate thickness should be at least as thick as the incoming beam flange-thickness, rising to, flange thickness + 10 mm for large beams of girders.
  2. For beams less than 400 mm deep: use M20 bolts. For larger beams, use M24
  3. Welding should be fillet-welds, provided adequate penetration can be achieved (see Welding)
  4. The stub-length will usually fall between the range or 600 – 1000 mm. when measured from the column flange or web.
  5. The shear-plate should be at least the same thickness of the incoming beam web – rounded-up to the nearest standard size.

Bracing

  1. Bracing should be set-out so that the centers-of-gravity of the component-members intersect at a common node point. (see item 3)
  2. Vertical-bracing connecting to the column flange may be re-aligned so that the node point is formed by the intersection of the bracing member to the upper or lower flanges of the beam. (see Chapter 9 - Vertical-Bracing)
  3. When using bolted angle-bracing, the setting out-point should be located at the center-line of the bracing leg.
  4. When using welded angle bracing, the setting-out-point should be located at the centroid (center-of-gravity) of the angle leg.
  5. Tension-struts may require the use of slip-resistant bolts

Bolts and Bolting

  1. For all ‘flexible’ or ‘Shear’ connections, use M20 Bolts
  2. For moment-resisting connections, use M20 bolts for beams less than 400 mm deep, and M24 bolts for beams greater than 400 mm.
  3. Bolt-hole diameters should be equal to the bolt-diameter + 2 mm.
  4. For foundation base-plates the HD bolt-hole should be equal to the bolt-diameter + 5 mm.
  5. For slotted-holes, thru’-hardened washers should be used under both the nut and bolt.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top