#043 Design Errors in Construction and factors for it

Common design and detailing errors in construction arises due to either inadequate structural design or due to lack of attention to relatively minor design details. These types of design errors are discussed below:

(1) Inadequate structural design

Due to inadequate structural design the concrete is exposed to greater stress than it can handle or strain in concrete increases more than its strain capacity and fails.

The symptoms of such kind of failures due to inadequate structural design shows either spalling of concrete or cracking of concrete. Excessively high compressive stress due to inadequate structural design results in spalling of concrete. Also, high torsion or shear stresses results in spalling or cracking of concrete. High tensile stresses also results in cracking of concrete.

To identify the inadequate design as cause of the structural damage, the structure shall be inspected and locations of the damage should be compared to the types of stresses that should be present in the concrete. For rehabilitation projects, thorough petrographic analysis and strength testing of concrete from elements to be reused will be necessary.

Prevention: Inadequate structural design can be prevented by thorough and careful review of all design calculations. Any rehabilitation method that makes use of existing concrete structural members must be carefully reviewed.

(2) Poor design details:

Poor design details can cause localised concentration of high stresses in structural members even if the design is adequate to meet the requirements. These high stresses may lead to cracking of concrete that allows water or chemicals to pass through the concrete. Thus poor design detail may lead to seepage through the structural members.

Poor design detail may not lead to structural failure, but it can become the cause of deterioration of concrete. These problems can be prevented by a thorough and careful review of plans and specifications for the construction work.

Types of poor design detailing and their possible effects on structures are discussed below:

(a) Abrupt changes in section: Abrupt changes in section may cause stress concentrations that may result in cracking. Typical examples would include the use of relatively thin sections rigidly tied into massive sections or patches and replacement concrete that are not uniform in plan dimensions.

(b) Insufficient reinforcement at corners and openings: Corners and openings also tend to cause stress concentrations that may cause cracking. In this case, the best prevention is to provide additional reinforcement in areas where stress concentrations are expected to occur.

(c) Inadequate provision for deflection: Deflections in excess of those anticipated may result in loading of members or sections beyond the capacities for which they were designed. Typically, these loadings will be induced in walls or partitions, resulting in cracking.

(d) Inadequate provision for drainage: Poor attention to the details of draining a structure may result in the ponding of water. This ponding may result in leakage or saturation of concrete. Leakage may result in damage to the interior of the structure or in staining and encrustations on the structure. Saturation may result in severely damaged concrete if the structure is in an area that is subjected to freezing and thawing.

(e) Insufficient travel in expansion joints: Inadequately designed expansion joints may result in spalling of concrete adjacent to the joints. The full range of possible temperature differentials that a concrete may be expected to experience should be taken into account in the specification for expansion joints. There is no single expansion joint that will work for all cases of temperature differential.

(f) Incompatibility of materials: The use of materials with different properties (modulus of elasticity or coefficient of thermal expansion) adjacent to one another may result in cracking or spalling as the structure is loaded or as it is subjected to daily or annual temperature variations.

g) Neglect of creep effect: Neglect of creep may have similar effects as described for inadequate provision for deflections. Additionally, neglect of creep in prestressed concrete members may lead to excessive prestress loss that in turn results in cracking as loads are applied.

(h) Rigid joints between precast units: Designs utilizing precast elements must provide for movement between adjacent precast elements or between the precast elements and the supporting frame. Failure to provide for this movement can result in cracking or spalling.

(i) Unanticipated shear stresses in piers, columns, or abutments: If, through lack of maintenance, expansion bearing assembles are allowed to become frozen, horizontal loading may be transferred to the concrete elements supporting the bearings. The result will be cracking in the concrete, usually compounded by other problems which will be caused by the entry of water into the concrete.

(j) Inadequate joint spacing in slabs: This is one of the most frequent causes of cracking of slabs-on-grade.