Allowable Foundation Settlement for Different Structures
The allowable settlement is defined as the acceptable amount of settlement of the structure and it usually includes a factor of safety.
The allowable settlement depends on many factors, including the following:
The Type of Construction
For example, wood-frame buildings with wood siding would be much more tolerant than unreinforced brick buildings.
The Use of the Structure
Even small cracks in a house might be considered unacceptable, whereas much larger cracks in an industrial building might not even be noticed.
The Presence of Sensitive Finishes
Tile or other sensitive finishes are much less tolerant of movements.
The Rigidity of the Structure
If a footing beneath part of a very rigid structure settles more than the others, the structure will transfer some of the load away from the footing.
However, footings beneath flexible structures must settle much more before any significant load transfer occurs. Therefore, a rigid structure will have less differential settlement than a flexible one.
Aesthetic and Serviceability Requirements
The allowable settlement for most structures, especially buildings, will be governed by aesthetic and serviceability requirements, not structural requirements.
Unsightly cracks, jamming doors and windows, and other similar problems will develop long before the integrity of the structure is in danger.
Table below shows the allowable foundation displacement into three categories: total settlement, tilting, and differential settlement.
It indicates that those structures that are more flexible (such as simple steel frame buildings) or have more rigid foundations (such as mat foundations) can sustain larger values of total settlement and differential movement.
|Type of Settlement||Limiting factor||Maximum Settlement|
|Total settlement||Drainage||15 – 30 cm|
|Access||30 – 60 cm|
|Probability of non-uniform settlement:|
|1. Masonry walled structures||2.5 – 5 cm|
|2. Framed structures||5 – 10 cm|
|3. Chimneys, silos, mats||8 – 30 cm|
|Tilting||Stability against overturning||Depends on H and L|
|Tilting of chimneys, towers||0.004L|
|Rolling of trucks etc.||0.01L|
|Stacking of goods||0.01L|
|Drainage of floors||0.01 – 0.02 L|
|Differential settlement||High continuous brick walls||0.0005 – 0.001 L|
|One-storey brick mill building, wall cracking||0.001 – 0.002 L|
|Plaster cracking||0.001 L|
|Reinforced concrete building frame||0.0025 – 0.004 L|
|Reinforced concrete building curtain walls||0.003 L|
|Steel frame, continuous||0.002 L|
|Simple steel frame||0.005 L|
Where, L = distance between adjacent columns that settle to different amounts, or between two points that settle differently. Higher values are for regular settlements and more tolerant structures. Lower values are for irregular settlement and critical structures. H = Height and W = width of structure.