Inspection of activities on-site during construction is a high-risk operation that requires expertise and experience. Site inspections are of paramount significance as they aim to minimize the risk of accidents arising within the construction structure – even though they never guarantee non-occurrence of accidents. They can only be carried out by those familiar with the structure of the works and the constraints of the construction methods.
A project’s construction phase is the period where the design is implemented. Unforeseen variables influence the choices taken, so modifications to the original design could occur. Workmanship is also essential as it means that good practice can be implemented on site by the structural model used in design. It is important to ensure that design assumptions have not been violated for materials on which the partial variables used in design differ based on the degree of workmanship/ supervision.
It is usually the responsibility of the main contractor, assisted by the designers, to ensure that the works are done in accordance with the design specifications.
The principal way workmanship is assured is through supervision and oversight by the main contractor. Any inspection carried out by the client’s design team, including by the structural engineer, will be in addition to the contractor’s monitoring of the progress of the works. Nonetheless, it is good practice for such supplemental inspections to occur.
Before attending the site, it is vital that the inspecting engineer knows the nature and intent of the site inspection; the analysis will be focused entirely on a sample of the works available at the time. In order to accomplish the purpose of the inspection, where intrusive work is necessary, the inspecting engineer must have a thorough specification of what to do.
Principles of Site Inspection
Prior to carrying out a site inspection. It is necessary to consider the extent of what is to be inspected and the contractual requirements imposed upon the structural engineer . They are responsible for the formal approval of the site work for and on behalf of the contractor, or they carry out spot checks to ensure that the building complies with the specification for and on behalf of the contractor. Also before carrying out a site inspection, the structural engineer must already have a clear understanding of the authority they have within the construction site.
There are three points to consider for buildings under construction.
The Design of the Project: Calculations, specifications and drawings, and on-site conformity with these in both materials and design.
The Specification of Materials: Structural and ancillary, as defined by the designer.
The workmanship: Performance and quality of on-site jobs and of all prefabricated materials delivered to the site, e.g. prefabricated elements such as steelworks.
These three components must be addressed when examining the component of a construction project during a site inspection. When visiting a site involving a building that is undergoing substantial modification to its construction, two more points need to be considered:
The state of the present structure: determine how robust the structure is and recognize the time during which it has been in place.
Deterioration/lack of maintenance: assess what deficiencies are present and how they would affect new works, e.g. if existing structures have been altered in the past and are still being expanded or remodelled as part of new works.
When focusing on a specific aspect of the work ( e.g. steel frame), inspection becomes even more manageable and practical. This promotes a more detailed and reliable review, which is of higher importance. However, the scope and extent of the inspection must be accepted in advance.
Scope of Site Inspections
The scope of the inspection and the contractual obligations of the inspecting engineer need to be understood if the inspection is to be worthwhile. In the case of an inspecting structural engineer, they will assess the structure against the design. Typically, the design and construction documentation will include drawings , specifications and other auxiliary reports, such as the movement and tolerance criteria and the results of soil surveys.
The quality of the construction of the structural elements is based on the design documentation which sets out the benchmark against which all the elements are assessed. Any deviation from the design objective must be assessed and its impact on the integrity of the structure determined before any action is taken to address the issue, if necessary.
At a minimum, the inspecting structural engineer is required to access the structural aspects under construction against the following criteria:
- compliance with the design intent
- its size and location, level, setting-out and verticality
- condition (e.g. is there any damage?)
- its integrity and the suitability of interfaces with other elements (e.g. joints)
- its performance compared to reasonable expectations and the design intent
There are also other issues that need to be tackled during the inspection, but they normally come under one of the headings listed above. It is common practice to test each component (or a fair sample of components) against these criteria; if any are not met, the problems must be identified as soon as possible. The earlier a problem is established, the quicker it is to fix it. This is particularly true in the case of building projects, as modifications and remedial work are difficult to carry out after the feature has been built in a relatively advanced state.
Typical Components to Inspect
The aspect of the structural element to be inspected depends on the material from which it is made. Each material has distinctive properties that must be taken into account and assessed during the inspection. The accompanying information explains what needs to be checked for the materials usually used to construct the structural elements of a structure. However, the information provided in this post is general and should not be treated as complete.
Different structural forms are subject to different workmanship problems, and differing sensitivities to failure to meet design intent / robustness requirements. This might include, for example, components that are out of plumbing or out of tolerance and joint seating.
Once the site clearing has been carried out, construction on the field continues. It is at this stage that the construction of the substructure is launched. Structural and geotechnical engineers are expected to visit the site to ensure that the design objective is fulfilled or achieved.
Owing to the number of unknowns within the soil strata, it is not rare for the foundations to be substantially changed to suit the on-site conditions uncovered during construction. This is a critical stage of a construction project and needs careful review. Any modifications to the design as a result of experiencing unpredictable features within the soil must be carefully checked – and documented – prior to implementation.
Reinforced Concrete Structures
Reinforced concrete elements are typically inspected at least twice during construction. The first instance is at a point where the reinforcement is assumed to be complete and ready to accept concrete. At this point, the steel reinforcement is revealed and can be easily examined (Figure 2). Reinforcement must be measured against the following criteria:
- concrete cover to the reinforcement
- the integrity of the reinforcement cage
- lap lengths between spliced sections of reinforcement
- the location of laps within the reinforcement
- congestion of the reinforcement
- the orientation of the reinforcement
- installation of cast-in items
- the size and type of reinforcement
- the spacing and location of reinforcement, including shear links
- Positioning of spacers between the formwork and reinforcement
- the quality and arrangement of formwork surfaces
- the quality and arrangement of formwork surfaces.
Of these requirements, the concrete cover to the reinforcement may be the most difficult to accomplish. The installation of reinforced steel cages in the formwork typically results in a drift in its movement as it is being constructed. This is permitted in tolerance limits, but it is necessary not to allow it to slip below tolerances, as the concrete reinforcing cover is the key mechanism by which protection against fire and corrosion is accomplished.
After the concrete has been poured and has been cured for a fair length of time, a second inspection shall be carried out. The objective is to evaluate the finish concrete and to ensure that the concrete mix is carried out as intended. Things to be considered when inspecting reinforced concrete after the concrete has been poured include:
- The presence of cracks
- Plastic concrete settlement.
- Exposed aggregate/lack of fines
- Reinforcement of exposed steel
- Cube strength test certificates
Steelwork frames are largely prefabricated off-site. Most parts, such as trusses, are built ready for assembly and then elevated once they arrive. This prefabrication means that, technically, many of the components have already undergone some sort of examination and evaluation before being delivered. However, they still need to be erected and installed on site. At this point, the inspecting structural engineer must review the steelwork against the design intent. A specific set of checks unique to structural steel needs to be undertaken:
- The proportion of the elements and their location
- Assembly of the steel connection
- The number, degree, size and density of the bolt.
- Applying anti-corrosion paint (if necessary)
- Intumescent paint application (if required)
- Integrity of the components and the connections
- Excluded areas of stain, such as the composite beam top flange,
- Welds’ weight, position and integrity
- Material certificates checking the steel quality used in the creation of steel frame components
- Web openings and the presence of stiffeners
- If these are positioned inside architecturally appropriate areas, the finish for welds and bolt positioning
Additional inspections must be done on the shear studs that are welded to the beams, and on the interfaces between the beams and the floor structure that the beams supports, if composite steel beams are used. The additional measures are:
- Shear studs size and arrangement: are they in pairs, or in a single row?
- Number of shear studs according to design purpose
- The integrity of the weld between the shear stud and the steel beam is tested using the ‘ring and pull’ procedure.
- The bearing of the floor framework on the steel beam; this is extremely important for concrete prefabricated slabs
Three factors are of vital importance when timber components are present inside a structure: the quality of the wood, the connections throughout the frame and the finishes to shield the timber from moisture. The design purpose of a timber structure can be compromised by undermining any of these three variables. Because they are typically the responsibility of specialist contractors, their specification and construction must be reviewed. When inspecting structural timber elements, the structural engineer should assess:
- The location and size of the elements
- Presence of knots and breaks within the timber components
- The moisture content of the timber
- Arrangement of connections
- The status of completion of the connections; i.e. are all the bolts present and tightened in compliance with the standard (not too loose or too tight)?
- State of existing structural timber components, including rot / boring insect damage
Site Inspection Reports
Haven completed a site inspection, it is vital that a report is written and shared as soon as possible to ensure that all corrective action is documented and implemented and that findings are registered. Site records differ in format, but should contain as a minimum:
- the purpose of the inspection
- location of the inspection
- the date and time of the inspection the weather conditions
- A list of attendees at the inspection
- a list of items/areas inspected and observations made
- A list of corrective steps and the form of documentation that has been discussed in the follow-up inspection
- proposed and agreed on amendments
- Photographs/pictures are provided by a delivery list.
Any arrangements entered into with other parties during the inspection must be registered. In situations where costs are concerned, the examining engineer may not operate beyond the remit; if possible, the activities performed that could incur an expense to the contractor or the client must be negotiated with all appropriate parties or their representative before being ordered to do so.
Also see: Site Investigations
Institution of Structural Engineers (2008) Guide to surveys and inspections of buildings and associated structures, London: IStructE Lt
Institute of Clerks of Works & Construction Inspectorate (2018) Clerk of Works and Site Inspector Handbook, London: RIBA Publishing
Institution of Structural Engineers (2010) Appraisal of existing structures (3rd ed.), London: IStructE Lt
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