Diagnostics of Structural Condition

Why the technical condition is assessed

POLYGONAL offers technical inspection of buildings and structures. The reliability of a new structure is built in through calculation and quality control. An existing one is judged differently: from data on how it has performed in service, check calculations and tests in its working position. With time the question comes up whether the structure can keep being used for its purpose in specific conditions, and for what forecast period, set by operation or reconstruction. A technical inspection answers it by checking the reliability and durability of the structures. Today the procedure is called “assessment of the technical condition of structures”.

Inspection is needed because structures wear out physically and become obsolete, operating conditions change, and building codes do not account for time explicitly.

After an inspection, the condition is classed by whether the structures can perform the service functions set by regulatory and design documentation:

• sound: the norms and standards are met in full;
• serviceable: the design requirements and norms are partly not met, but the first group of limit states is not breached, and breaches of the second group do not restrict normal operation of the production;
• limited serviceability: the process can run only if the structures are monitored and both the duration of use and the process parameters are limited;
• unsafe: the first group of limit states is breached.

Methods of assessing technical condition

The main question is how to interpret the results of the work that establishes the condition of structures in service. The technical condition of a structure is assessed by two main methods:

• comparing the defects and damage with the permitted values set in norms or designs;
• calculating what the defects and damage do to the load capacity of the structures.

The first method belongs to technical diagnostics, which means 100% inspection of the object. In construction that is practically impossible: there are too many structures, the material is not continuous (unlike in mechanical engineering), and there are no rigorous methods for checking serviceability. So the object is divided into the material of the structures, the joints between elements, and the structure with its elements. For the material and joints, diagnostics has the means to carry it out; for structures, only partly. The first method is best seen as a development of the second: the types of deviation are known in advance, their effect has already been calculated, and the limiting permitted values are given in tables. This is how, for example, structures are accepted at manufacture.

Where the second method is the main one, the assessment procedure has these parts:

• a field survey of the elements, to find their true condition and operating conditions;
• check calculations based on the survey results;
• assessment of technical condition: whether the structures can be used and under what conditions;
• repair and strengthening that rely on the residual load capacity.

Field studies are about collecting data on technical condition. The properties of materials are refined, because design properties are probabilistic, while real ones may deviate and change over time through fatigue and ageing. Loads and actions are refined too, since real ones may differ from those assumed in the design. Technical diagnostics or field surveys find defects and damage and give quantitative characteristics of the structures.

Visual survey: where damage most often occurs

Diagnostics combines research, calculation and analysis that complement one another. A specialist organisation decides the list and completeness of the procedures case by case. The programme of visual and instrumental surveys is drawn up to get the fullest information on faults, defects and damage from the smallest scope of work, instrumental and laboratory work especially.

In industrial buildings and structures, damage is most likely in these places:

• in general: areas where heavy loads are stored, the most heavily loaded columns, walls, foundations and supports, places with wet soil, places exposed to vibration or impact loads;
• foundations: zones of wet soil (especially soil wetted by aggressive liquids), vibration, impact loads and surcharges; the area next to heavy extensions and close-lying pits; places with uncontrolled dewatering or groundwater lowering;
• columns: the junction with the foundation (the most stressed zone), brackets, joints of precast columns along the height, areas near the floor (aggressive liquid, impacts from vehicles and handling equipment), connections to floor and roof beams;
• beams and floor slabs: zones of maximum bending moment and shear force, points where concentrated forces are transferred, areas exposed to vibration, impact, aggressive liquids, gases and dust, and joints;
• roofs: places of increased wetting and damage from the room side, where process dust builds up, and areas with dense or wet insulation;
• walls: places of increased wetting with freeze-thaw cycles, joints of panel walls, junctions with the floor and slabs.

A visual survey looks for these typical defects and damage:

• design shortcomings: the design scheme does not match real conditions, or there are deviations from design norms;
• manufacturing defects from the plants that made the structures;
• defects of structure erection and building construction;
• mechanical damage from breaches of operating conditions;
• damage from static, dynamic and thermal actions the design did not foresee;
• damage from aggressive external effects of the industrial and surrounding environment.

Diagnostic procedures and check calculations

For full diagnostics, it makes sense to plan the following alongside the field surveys and laboratory tests:

• analysis of changes in the main design and calculation assumptions that have arisen during service;
• analysis of defects and damage, and of how the properties of materials, soils and foundations have changed;
• adjusting the calculation models of elements, structures and foundations to the defects and changes found;
• check calculations using the adjusted models;
• assessment of the technical condition of elements, structures and foundations against the criteria developed;
• assessment of the building’s technical condition as a whole, based on the condition of its elements, structures and foundations.

To find what has changed, compare the design (standard) figures with the actual ones for: functional purpose; level of responsibility according to the consequences of failure; standard and design values of loads and actions (self-weight, atmospheric, process, seismic); features of the design situations; degree of aggressiveness of the natural and industrial environment; and engineering-geological and hydrogeological conditions. Standard self-weight loads are determined from measurements and a check determination of material density (error no more than ±5%). Process loads come from equipment data sheets, atmospheric loads from the nearest weather stations.

Check calculations cover the design situations that can realistically arise in the remaining service life. For each one the design schemes, types of load, coefficients of working conditions and reliability, and the list of limit states are refined. Calculations follow the design norms in force. Norms to which the building was designed but which have since been cancelled may be used only with the written consent of the organisation that developed the new norms.

The physical and mechanical properties of load-bearing and enclosing structures are determined by standard non-destructive methods (ultrasonic, plastic deformation and others) or from samples taken for laboratory tests. Samples come only from secondary, unstressed parts of elements, and the places they were taken from are properly repaired or strengthened. The number of determinations depends on the condition of the structures, and the reliability of the standard strength values must be at least 0.95. The degree of wear of sections is found by measuring the cross-section area directly where the element is most defective or damaged and where it is most stressed; layers of material attacked by corrosion are not counted.

Categories of technical condition

Depending on load capacity and service properties, a structure falls into one of four conditions:

• I (normal): actual forces in elements and sections do not exceed the permitted values by calculation, and there are no defects or damage that impede operation or reduce load capacity or durability;
• II (satisfactory): in load capacity and operating conditions it matches condition I, but defects and damage may reduce durability, so protective measures for the structure are needed;
• III (unfit for normal use): the structure is overloaded or has defects and damage showing reduced load capacity; check calculations and analysis of the damage show that its integrity can be kept during strengthening;
• IV (unsafe): as for condition III, but the integrity of the structure cannot be kept during strengthening, especially if brittle failure is possible. People must be moved out of the zone of possible collapse, the structure unloaded at once and other safety measures taken.

The condition of the building as a whole follows from the condition of its load-bearing and enclosing structures. I: no structures in conditions II, III or IV. II: no structures in conditions III or IV. III: no structures in condition IV. IV: at least one structure in the unsafe condition. With proper justification, separate parts of a building, singled out by function or structure, can also be assessed on their own.