Rules of thumb for steel structures

By Dr Hennie de Clercq,
Executive Director, SAISC

INTRODUCTION
Rules of thumb have a proud history in engineering. In fact, there was a time when they constituted almost the whole body of engineering ‘theory’. The old master craftsmen and those who called themselves architects etc. only had their experience, and those of others, to go on, and such experience got laid down in rules such as that a dome will be unlikely to collapse if it is built to certain proportions. With the advent of proper engineering theory and the power of the computer, the old rules have been largely forgotten, although they will always survive in what is perhaps the most powerful of all the rules of thumb:

If it looks wrong, it might just be wrong.

A corollary to this rule is that a highly experienced engineer or structural steel draughtsman will typically be able to draw a final structure, and assign quite accurate member sizes, without making a single calculation. We are here trying to capture just a little of that internal computer that tells these people what ‘looks right’.

The rules below can be used for any of the following purposes:
 The engineer can get a reasonable estimate of the required size of a member to resist a given force, even without recourse to a computer or handbook.
 The initial size of a member can be guessed for input into calculations or a computer model.
 The initial configuration chosen for structures can be closer to the optimal or to values that will not cause problems.
 They allow a quick check of the order of magnitude of member sizes, quantities, etc.

The approach with the shortcut calculations and the other rules of thumb listed below has been to aim to err on the conservative side. There is no guarantee that they are correct however, and people are encouraged to use and test them and to give feedback to the SAISC at info@saisc.co.za.

SHORTCUT CALCULATIONS

Basic assumptions

The formulae below, are based on the following assumptions:
 Grade S355JR steel is the standard
 Only I-sections are used for beams
 The symbols have the following meaning:
A = cross sectional area, in mm2
b = width of flange or leg length of angle, in mm
Cr = factored compressive resistance, in kN
d = outer diameter of a circular hollow section, in mm
h = depth of section, in mm
I = moment of inertia, in mm4
KL = effective length of column or span of beam, in m
m = mass of steel element, in kg/m                                                                                                                                                      mreq = required mass of section in kg/m
Mr = factored moment resistance, in kNm
Mu = ultimate (factored) bending moment, in kNm
r = radius of gyration, in mm
w = unfactored uniformly distributed load on beam in kN/m

Formulae
See table 1.

GENERAL GUIDELINES
Recommended span over depth (L/h) ratios for beams and trusses:

General proportions of steel structures

 Plate girder web thickness about depth/160.
 A brace should have a capacity in the order of 2% of the force in the main compression member or in the compression flange of a beam or girder.
 Stacks, towers and laced columns (for example transfer towers, furnace pre-heater towers): ratio of height to smallest plant dimension should be smaller than 10.
 Portal frames for single storey industrial steel buildings without cranes:
• For pitched portals, eaves rafter haunch length 7,5% to 10% of span and haunch cut from section equal to or bigger than rafter section.
• For acceptable deflections in pitched portals with haunches, rafter depth to be bigger than span/75.                                              • Stanchion section one or 2 serial sizes bigger than rafter section, with depth bigger than eaves height/25
• For acceptable deflections in propped portals rafter depth to be bigger than span/55.
 Spacing of frames in industrial buildings (portal frames or trusses) without cranes:
 Spacing of columns in industrial buildings with heavy cranes to be approximately equal to the height of the crane girder above the ground.                                                                                                                                                                                             Distance between legs of latticed crane columns: H/7 to H/10 where H = height to top of crane girder.
 Vertical leg depth of angle roof bracing ≥span/70. Span may be taken as distance between points where braces are hung from the purlins. Assorted rules
 The amount of pre-camber in a plate girder should be equal to the deflection under its total permanent loading. As a rule of thumb, let precamber equal span/500.
 Precamber of lattice girders and trusses: span/600.
 Number of purlin sag bars for different spans (L) of purlin and two widths b of top flange of purlin:                                                 Under typical serviceability loads the elastic strain in tension and compression members is about 0.9mm per meter of length.
 To facilitate erection on typical projects, keep the mass of any member to less than 6 tons.                                                           In industrial buildings, expansion joints and full sets of roof and vertical bracing are required for every 60m to 75m of building length.
 Thickness of a base plate for concrete cube strength 25MPa about 30% of distance from face of column to edge of minimum area required to spread load on base.

Estimating

 Weight of steel (kg/m2) in multi-storey building: 35 plus 1,6 times number of storeys.
 Mass of light industrial single-storey buildings without cranes typically less than 25kg/m2
 Number of 20mm shear studs on a composite beam with 100% shear connection equal to 2 times mass of beam in kg/m.
 For estimating purposes, allow 10% of light and medium steelwork (excluding purlins and girts) for connections, splices, column cap and base plates, plus 1% for Grade 8.8 bolts and 0,5% for Grade 4.8 bolts.