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Designing With Your Thumb

Designing With Your Thumb

Not so long ago the aim of structural analysis was to develop simple, elegant, and reliable methods of design. The idea of splitting a member into thousands of tiny elements and calculating their individual behaviours would have been laughed at. Of course now finite element analysis is common place, and the thought of designing a complete project without using a computer is unheard of.

Of course all of these engineering rules of thumb are just taken from experience and observation, and every element should be properly designed before construction.

The design tables and empirical rules devised before, however, are not without merit. Although no longer accepted design practice, they provide a valuable tool for getting the ‘feel’ of a design way before the detailed calculations have been finished

So, here are five of the many odd ‘engineering rules of thumb’ I’ve picked up and adopted over time.

Instant Bridge Design

Integral reinforced concrete bridges are becoming more and more prolific. Given their potential for low maintenance (no joints) they are now preferred both by the Highways Agency and Network Rail.

In 2006 Braun researched the typical span to depth ratios for integral reinforced concrete bridge designs. Using these findings its possible to sketch a bridge from the span alone.

Typical span/depth ratios for highway bridges:

Construction Abutment Deck (With Haunch) Deck (Without Haunch)
Reinforced Concrete 12-18 20-25 18-21
Pre-Stressed Concrete 15-19 24-30 20-25
Steel Composite 15-19 25-35 21-25

Typical span/depth ratios for rail bridges:

Construction Abutment Deck (With Haunch) Deck (Without Haunch)
Reinforced Concrete 10-15 20-25 16-18
Pre-Stressed Concrete 15-20 20-25
Steel Composite 15-18 25-30 18-21

Statics at a Glance

Here are two short-cuts for static analysis:

  • The end rotation of a well designed, simply supported beam is 0.2c.
  • Splices in continuous beams should be at the quarter points.

Bricking It

The following height to thickness ratios can be used to find a starting size for masonry walls:

  • Solid Bearing Walls: 1:20
  • Bearing Walls with gaps: 1:18
  • Non-bearing exterior walls: 1:18
  • Non-bearing interior walls:1: 36

One Line Floor Slab

A good estimate for the depth of a concrete floor slab is the same as a 1/180 of its unsupported perimeter.

Revealing Diameters

The flow through a pipe is roughly 40% of its diameter, and the minimum fall should be 1: the pipe diameter to ensure self-cleaning velocities.

Of course all of these engineering rules of thumb are just taken from experience and observation, and every element should be properly designed before construction. But when it comes picking a starting point, a few golden rules go a long way.

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