Determining the Load Capacity of a Structure When As-Built Drawings Are Unavailable (ACI 364.4T-10)

Description

In cases where a structural evaluation is to be performed, knowledge of member sizes, reinforcing details, and material properties is generally required. In cases where this information is not readily available through drawing document records, it should still be obtained. For most buildings, bridges, and structures governed by a building code or similar design standards, the process to establish load capacity typically needs approval of the governing official.

Derivation of load-carrying capacity and other critical behavioral characteristics requires the involvement of a structural engineer with strong experience in this field. Determining the structure’s age may provide some idea of what the building code requirements were at the time of construction. Many historic reinforced concrete struc-tures, however, were designed using systems that are no longer used and are not addressed in modern codes and design practices. Structural configuration and reinforcement may be established using the general techniques described in ACI 364.1(ACI Committee 364 2001) and ANSI/ASCE 11-90 (1994). Visual examination and physical measure-ments are made in conjunction with nondestructive examination and examination after removal of concrete (ACI Committee 364 2001; ANSI/ASCE 11-90 1994).

Material properties (concrete and reinforcement strength) may be determined by physical testing (ACI Committee 228 2003, 2004). Reinforcement size and strength may also be determined through observation of mill imprinted markings on exposed bars (CRSI Manual of Standard Prac-tice). Once the structural system, reinforcement, and material properties are known, the structural capacity may be determined through calculations. Research into the testing and design procedures for historic structural systems may provide valuable clues about reinforcement configuration and structural behavior (CRSI 1980 Manual of Standard Practice).Visual examinations and physical measurements are suggested to define the overall structural system for resisting both gravity and lateral loads. Understanding the lateral load-resisting system is necessary to properly consider the structural effects of lateral loads on elements such as slabs, beams, columns, walls, and foundations.

Once the basic structural configuration is known, the actual layout of reinforcement may be determined. Where the depth of cover is less than 2 in. (50 mm), the size, spacing, and depth of the cover of reinforcement in typical structural members can usually be established using a pachometer. Pachometer surveys may be partially effective where the depth of cover ranges from 2 to 4 in. (50 to 100 mm). Pachometer surveys may be problem-atic in areas of congested reinforcement, such as columns and beams and at areas of metallic surface interference (for example, metallic conduits). Additional exploratory openings may be required, and other methods, such as x-ray or ground-penetrating radar surveys, may be necessary to determine internal reinforcing pattern arrange-ments. The pachometer survey results should be spot-checked by physical measurement of exposed reinforce-ment at exploratory openings