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Rules of thumb  


Martin Pratchett
New Member
Joined: 3 years ago
Posts: 2
19/02/2021 3:22 pm  

Hi all,

I’m putting together some rules of thumb that can be published and downloaded for anyone to use. The ones I’ve accumulated so far are below, please add in any that you can think of, or let me know if you think any of mine are incorrect.

concrete + reinforced masonry

Column sizing 

  • Check beam steel, if the beam bars are the same size or larger than the column, it’s probably too strong
  • Exterior columns should not be smaller than 520mm, if they are then there is normally insufficient cover

Beam sizing

  • Depth = span/20 for standard tributary areas

Slabs and foundations

  • Saw-cut centres for concrete ground slabs should not exceed 30 times slab depth when unreinforced or 45 times slab depth when reinforced with steel mesh
  • When designing slabs or foundations over 1m thick, heat buildup can become problematic and should be addressed.

Concrete panels

  • Singly reinforced panels typically required HD12@300crs centrally located. Check for ductile failure.


  • Try to use a single grade of the same size bar. For example, avoid using D12 and HD12 on the same site as it’s easy to confuse the two.

Retaining walls

  • HD16 at 400crs is the most steel that can fit into 20series walls. Cover and brittle failure govern with higher ratios.
  • 20 series block will typically go to 2m high with no backslope or surcharge, 25 series to about 2.8m.
  • Width of the footing is typically 60-80% of the wall’s height if there is no heel. Width may reduce to 40-60% in low seismicity regions. Check soil crushing at the toe.
  • The depth of the footing should be deeper than that of the wall.

Placing concrete

  • Concrete is a brittle material such that tensile strength is only 10% of its compressive strength, hence why we use reinforcement in structures
  • Extra water added to fresh concrete for workability will reduce strength and increase shrinkage and cracking potential
  • Ensure the design enables the contractors to vibrate the concrete properly. Excess air in concrete due to poor compaction will reduce strength by 5-7% for every extra percentage of entrapped air
  • Compressive strength development of concrete increases with maturity, and is the product of temperature and time
  • Ponding is critical to prevent cracking. Concrete is most vulnerable to cracking at early ages, particularly when evaporation exceeds 0.5L/m3 or thermal swings exceed 15 0C
  • Timing of cracking of concrete is often diagnostic when assessing the likely cause (e.g. plastic, thermal, heat of hydration and shrinkage)



  • Span / 25 for standard tributary areas
  • Strength of a beam goes up to the depth’s square, and deflection works to the cube of the depth.



  • Span/10 for SG8
  • Span/12-15 for HySpan
  • Joist spacing generally requires a maximum of 450mm for domestic situations due to timber flooring’s capacity.


  • If a triple stud is required to support the load, consider a steel post. It indicates that it is critical to the structure.

Retaining Walls

  • Typically use round poles for walls over 1m high
  • The 1, 1, 1, 100 rule nearly always applies. For every metre up go 1m down, space the poles 1m apart. For every meter up, the short end diameter of the pole = 100mm. So a 1.5m wall = 150mm pole.
  • Check the strength of the waling planks. Double planking generally required for walls over 2m.
  • Check strength reduction due to soil interaction for hole diameter over 350mm
This topic was modified 3 months ago by Martin Pratchett

Jamie Nguyen
Active Member
Joined: 3 years ago
Posts: 6
24/02/2021 4:05 pm  

1,1,1, 100 rule I disagree with (maybe different soil conditions or loads you’re referring to)

Generally in auckland in average ground conditions, embedment depth is 1.5x our wall height as rule of thumb. Also a 150mm pole in my opinion is too small for a 1.5m wall. Generally looking at least 175 or usually 200mm depending on pole spacing. 

Loads increase exponentially with height.
I use Height x sqrt(1.5) x 100 for quick approximate pole diameter…