What is the allowable settlement of foundation?

The settlement of a foundation that is acceptable both structural and esthetic point of view is called allowable settlement which generally includes a rational factor of safety. As esthetic issue is considered to set acceptable settlement, architect often take part in determining allowable settlement. The structural engineer and architect should consult with foundation engineer about expected

• Loading conditions
• Allowable settlement of building
• Primary selection of foundation typeIf above information is not available, geotechnical engineer has to calculate allowable settlement of building to determine proper type of foundation. Many studies has been conducted to determine settlement of buildings and it is ascertained that allowable total settlement and differential settlement depends on• Complexity and flexibility of buildings

• Construction materials

• Construction type.

According to Coduto (1994) the factor affecting allowable settlement are follows:

Construction type:

Different types of construction have different level of tolerance against settlement. For example, buildings having wood frame including wood siding will have more tolerance than unreinforced masonry buildings.

 

Occupancy:

In dwelling house, even a tiny crack is considered unacceptable; on the other hand, in a structure used as warehouse, storage or industrial purposes, a large crack may be even not noticed.

Type of finishing:

When sensitive finishing is provided into a building, like tile, a small cracks or movement will provided visual disturbance.

Structural integrity:

When a structure is designed as rigid body, a settlement of a particular footing under any portion of it relative to other footings, will not result significant affect as some load will be transferred to nearby footing. In case of flexible structure, settlement of any footing is significant as movement occur before any considerable transfer of load to nearby footings, which indicates that a rigid structure will not subjected to significant differential settlement relative to flexible one.

Coduto also interestingly concluded settlement that in most case structures, particularly buildings, settlement is governed by serviceability and aesthetic requirements not by structural. Visual hazard like jamming doors-windows, cracks and other identical problems appears first long before structural integrity is hampered.

Determination of allowable settlement:

Evaluation of allowable settlement of a building is very complex. Engineers often try to correlate settlement with observed behavior of buildings. The relation between performance of structures and settlement of foundation that may cause damage has been present by Skempton & Macdonald in 1956 based on study on 98 buildings. In this study, it has been found that 40 buildings had been subjected to different degree of damage due to varying range of settlement and 58 buildings were unaffected. This study is still a major reference for determining settlement which is concluded below: 

 

• • Cracking in load bearing walls of bricks or brick partition walls in buildings frame is generally occur when foundation is subjected to angular distortion more than 1/300. Structural damage in beams and columns is generally occur when foundation is suffered angular distortion more than 1/150.•They have provided a correlation, Δ = 350δ/L [where Δ = maximum differential settlement measured in inches and δ/L = maximum angular distortion]. This correlation has derived by establishing a plot for maximum angular distortion versus maximum differential settlement as shown in above figure.For maximum angular distortion of 1/300 (δ/L), in a frame building cracking in brick panels or in brick wall of load bearing building will occur at maximum differential settlement (Δ) of 32 mm (1.25 inches).

 

• The limiting values of angular distortion to open cracks (1/150 and 1/300) mentioned above were provided based on observation of load bearing structure and reinforced concrete and steel frame buildings have traditional brick panels as partition or peripheral walls, but not having diagonal bracing. These criteria can be taken as mere guide for regular construction work of typical foundation of such buildings; but in some cases suppressed by aesthetic or for other considerations.

 

Behavior of slab-on-grade foundation:

Slab-on-grade foundations with light reinforcement have been included in above figure with other types of frames. The study shows that wall panels of gypsum board may be cracked at an angular distortion more than 1/300 (Day in 1994). Surprisingly the value of angular distortion 1/300 seems to be applicable both brick panels and gypsum wall panels supported by wood frame.

The relationship, Δ = 350δ/L can be used in buildings supported on slab-on-grade foundation reinforced lightly as shown in above figure. Using limiting value δ/L as 1/300 when cracks opens in panels of residential building having wood-frame supported on concrete slab-on-grade, the estimated differentiation displacement of slab is found 32 mm (1.25 inches); that means when maximum differential settlement exceeds 32 mm, such type of wall panels supported on described foundation become cracked.

Same correlation has also been established by grant et al.; but cracks are expressed as damage. He used a term rate of settlement which defines very rapid or very slow settlement. The available data studied in this research, showed that maximum angular distortion (δ/L) corresponding to damage of buildings found to be same for that cases of fast or slow settlements.

Following figure shows values of angular distortion and respective type of damage (damage in wall panels and structure) provided by Bjerrum in 1963. This provides identical values of δ/L values as specified by Skempton and Macdonald and later Grant et al. the addition information offer by this figure is about threshold value of maximum angular distortion for buildings that have overhead cranes and sensitive machinery.

 

Limiting differential settlement:

Differential settlement of foundation must be checked unless building founded on solid rock. Differential settlement of 20 mm (3/4 inch) or less has been used as general guide to design foundation. Probably this limiting value is based on Terzaghi’s observation. Terzaghi has studied several European buildings in 1938 from which he summarized settlement more than 25 mm (1 inch) were resulted all building to be cracked. But buildings having wall length varying for 12 m (40 feet) to 30 m (100 ft) were not cracked. when differential settlement were equal or below 20 cm (3/4 inch).

Sowers provided allowable displacement foundation dividing them in three categories namely

• Total settlement
• Tilting
• Differential movement

This allowable displacement values indicates that a more flexible structure like general steel framed buildings or building having significantly rigid foundations like raft foundations are capable to sustain larger differential movement and total settlement as well.

 

Total settlement:

For structures supported on load bearing wall, the maximum settlement values are (2.5-5) cm. for chimneys, silos, mats this value lies in between (8-30) cm, for framed structure (5-10) cm. All type of structures discussed above may be subjected to non-uniform settlement.

Other limiting factor like considering drainage, the maximum settlement should be 15-30 cm and considering access (30-60) cm.

Tilting:

• Tilting of towers, chimneys- maximum settlement 0.004L
• Machine operated within the structure:

• Drainage on floors- (0.01 – 0.02) L
• Crane rails: 0.003L
• Goods Stacking- 0.01L

 

Differential movement:

 

• Continuous brick wall of significant height- 0.0005-0.001L
• Wall cracking in one-storied brick mill-0.001-0.002L
• Gypsum plaster cracking -0.001L
• Framed building of reinforced concrete-0.0025-0.004L
• Reinforced concrete structure curtain walls-0.003L
• Continuous steel framed structure-0.002L
• Simple steel framed structure-0.005L

 

 

Where L= difference between two adjacent columns that have settlement differently or any points that have subjected to differential settlement. Here Sowers provided higher values for regular settlement or more tolerant buildings and lower values stated for critical structure and for nonuniform settlement.

 

Foundation damage based on crack width:

 

Patch repair of moderately  damaged foundation (5~15 mm)

A summary of foundation damage category based on crack width, maximum differential settlement and maximum angular distortion were presenter by Burland et al. in 1977 which was later supported and refined by Boone, 1996 and Day, 1998. In the previous section we have provided limiting values of different types of settlement, exceeding which crack opens in different component of buildings of different types. Here we are providing information about cracking width and resulting foundation damage.The relation between differential settlement and angular distortion was also based on the expression, Δ = 350δ/L.

Very Severe damage of foundation [crack width >25mm]

In determining severity of foundation damage of an existing building, the category may be influenced by many factors which include crack width, maximum distortion and differential settlement. When basis of damage category is only single parameter like width of cracks, it may be erroneous as there may have patch or hidden cracks or when secondary or non-relevant factors like shrinkage cracking results opening of cracks. As an example, cracks in walls may be even not noticed under wall paper, which is only visible when crack reappear after additional foundation settlement.