Section Design Rules of Thumb

Sat 20th Aug 2022 by ilyas

A few common rules of thumb for initial sizing of elements are provided for reference below:

Steel

Beams

Type	Typical span-depth ratio	Typical spans
Primary Beams / Castellated Beams	L/15 ~ L/20	4 ~ 12 m
Secondary Beams	L/20 ~ L/25	4 ~ 20 m
Long-span Parallel Roof Truss	L/12 ~ L/20	10 ~ 100 m

Columns

There's no simple rule of thumb for columns, but for braced frames where the Columns are only part of the gravity system, some typical maximum sections are specified below:

203 UC	2 to 3 storeys
254 UC	Up to 5 storeys
305 UC	Up to 8 storeys
354 UC	From 8 to 12 storeys high

※ How to Design Steel Columns?

Concrete

Beams

Type	Simply supported	Continuous	Cantilever
Rectangular Beam	L/12	L/15	L/6

※ Design and sizing of RC Beams. Explained.

Slabs

Type	Simply supported	Continuous	Cantilever
Flat Slab (no drop panels)	L/28	L/36	L/10
Flat Slab (drop panels)	L/33	L/38	L/10
Two-Way Slab	L/25 ~ L/35	L/35 ~ L/40	-
One-Way Slab	L/22 ~ L/30	L/28 ~ L/36	L/10
PT Slab (Office)	L/36	L/40	L/10
PT Slab (Residential)	L/40	L/45	L/12

Columns

A useful approximation for the size of a concrete Column is as follows:

Equivalent concrete stress,

f subscript c equals 0.44 f subscript c k end subscript plus rho over 100 open parentheses 0.67 f subscript y k end subscript minus 0.44 f subscript c k end subscript close parentheses

Area of concrete required,

A subscript r e q end subscript equals fraction numerator N subscript E d end subscript over denominator f subscript c end fraction

Where fck is the concrete characteristic compressive strength, ρ is the reinforcement ratio (typically taken as 0.02 (i.e. 2%) for initial design), and fyk is the steel rebar characteristic yield strength.

This applies where columns are not the primary means of resisting lateral loads.

It is very important at concept stage to account for fixed end moments that could occur due to unbalanced loads in the columns. Approximate factors applied to the axial load NEd can account for these end moments, as follows:

Inner Columns......................1.25
Edge Columns.......................1.50
Corner Columns...................2.00

Note (1): In Eurocode, columns are considered to be stocky (not suspectible to slenderness effects) as long as their aspect ratio, H/B or H/D (clear height divided by the least horizontal dimension) is less than 15.

Note (2): Under the Eurocode provisions, when the column plan aspect ratio (B/D or D/B) exceeds 4, it is considered to be a wall.

※ When do slenderness effects become significant for concrete columns? Explained in 1-minute.

Walls

Guidance coming soon.

Timber

Beams

Guidance coming soon.

Columns

Small timber posts must have a minimum 100 mm depth.

Walls

For CLT walls, refer to TRADA pre-scheme design guidance.

Masonry

Guidance coming soon.

Last Update 3/01/25 07:23 JST

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Section Design Rules of Thumb

Steel

Concrete

Timber

Masonry

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