The behaviour of most modern structures is very complex and not always adequately definable for analysis even when new.
When the structure to be analysed is several years old it is not uncommon to find that the original drawings have been lost. Even when drawings and calculations are available the understanding of structural behaviour may not be adequate for assessment of changes in use or loading.
At various stages throughout the life of a structure, problems of movement. vibration, stability or deterioration may occur. Again the complexity of many structures makes an understanding of their true behaviour difficult to reach. In many of these cases careful use of monitoring may be of assistance.
The simplest method of movement monitoring, with which most building professionals are familiar is the glass telltale. This has the advantage of low cost and simplicity of installation but the disadvantage of low resolution. When sufficient movement occurs the glass breaks and assessment of movement must be made between the broken faces. For most purposes this information is inadequate.
Questions left unanswered may include:
When did any movement occur?
To a greater accuracy, how much movement has occurred?
What was the pattern of movements and are movements still occurring?
Does the movement occur in a reversible, elastic manner over a short time?
Greater accuracy can be achieved by using more elaborate tell-tales or demountable gauges. The costs of staff and, possibly, access generally prevent sufficient readings being taken manually to obtain much in the way of a movement history.
With the advent of reasonably priced computers and electronic gauges the problems of accuracy, access and frequency of reading can be overcome. Very short movement durations present few problems.
Transducers are readily available for measuring:
Stresses and deflections
Rotations (both accelerations and velocities)
Loads (static and dynamic)
Temperature and humidity
The use of 'intelligent' data logging can eliminate the gathering of data between discrete events while retaining data from the period immediately before the event. Sensitivity levels can be set to ignore minor signals. Alternatively all data can be gathered over a continuous period and any selection or sorting carried out after the data collection is complete.
Presentation of final data for reporting is aided by computer data capture. It is a simple matter to provide the Clients Engineer with a full set of data on diskettes or CDs for further analysis or input to other programs.
Instrumentation of a structure or component is a highly specialised task and each project will have its own solution. In addition to the basic considerations of appropriate gauging, the effects of environmental influences must be included in the submission. This can include aggressive environments, such as in a chemical plant, exposed to severe weather environments and/or large scale wetting by seawater, or the more determined application of a destructive human mind.
A full discussion of the aims of each project and a site visit ensure the satisfactory implementation of the test program and completion of the required data collection.
Projects in which GBG Structural Services have been involved vary from long term monitoring of building movement by manually read gauges to monitoring of trunk road bridges with remote electronic reading.