The paper deals with allowable differential settlement of structures and the prediction of such settlements. Since the terminology used to described foundation movements is often confused and even misleading, a consistent set of definitions is given in the firest part of the paper. An approach (apparently first used by Polshin and Tokar, 1957) to assess allowable differential settlements is outlined which is based on the assumption that the onset of visible cracking is associated with a critical value of tensile strain. The critical value of tensile strain is not a constant since it depends on a number of factors such as the function of the building and the texture of the finishes. However, values which appear to be acceptable for many purposes lie in the range of 0.05 to 0.1 percent for brickwork and blockwork set in cement mortar and 0.03 to 0.05 percent for reinforced concrete. The approach is used to study the damage criteria for simple beams and indicates that the allowable deflection ratio depends on the length to height ratio, relative stiffness in shear and bending, the degree of tensile restraint built into the structure and the mode of deformation of the structure. Using a critical value of tensile strain equal to 0.075 percent, good agreement is obtained between the theoretical damage criteria for simple beams and a large number of observations on a wide range of types of building. It is concluded that while the allowable limit of angular distortion of 1/500 proposed by Skimpton and MacDonald (1956) is satisfactory for frame buildings, it is unsafe for load-bearing walls undergoing sagging, and the criterion proposed by Polshin and Tokar (1957) is more appropriate. The study demonstrates that unreinforced load-bearing walls are particularly vulnerable to damage when subjected to hogging when even the criterion of Polshin and Tokar is unsafe. For most buildings it is the relative deflections that occur subsequent to the application of the finishes that cause damage. The ratio between the immediate and total settlement is therefore important. For overconsolidated clays the ratio is observed to be about 0.6 while for normally consolidated clays it is generally less than 0.2. It is concluded that the approach to the problem of allowable settlements should be based on assessing the global stiffness of a structure (including its finishes) and then estimating the relative deflections that will result from its interaction with the ground. The relative deflections can then be compared with the damage criteria given in the paper. /Author/

  • Corporate Authors:

    Building Research Establishment

    Building Research Station, Garston
    Watford WD2 7JR, Hertsford,   England 
  • Authors:
    • Burland, J B
    • Wroth, C P
  • Publication Date: 1975-4

Media Info

  • Features: Figures; References; Tables;
  • Pagination: 44 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00127405
  • Record Type: Publication
  • Report/Paper Numbers: CP 33/75
  • Files: TRIS
  • Created Date: Dec 16 1975 12:00AM