Handbook-Testing & Inspecting

To protect one’s self against all eventualities, however, is usually not economical. In an area in which earthquakes

are frequent, considerable expense in building ”earthquake-proof” buildings is justified, whereas such expense would be entirely unjustified in an area in which earthquakes are not known. In designing or planning the construction of a part, therefore, all of the service conditions for which it is economically justifiable to provide are taken into consideration. It is not quite enough, however, for the part to be built just strong enough to meet those requirements. The rain may come down just a little bit faster or the wind may blow just a few miles an hour harder than had been experienced before and all of the work would have been lost just as if these factors had never been considered. After the requirements of the part are determined, therefore, they are multiplied by what is known as a safety factor. Depending upon the circumstances, this factor may be anywhere from 1-1/2 to 12. The designer of a bridge, for instance, calculates the strength required to accommodate all of the people or automobiles that might be on it at one time, even in winter when the bridge might be loaded down with ice and snow and a strong wind blowing. Because some part might not be quite up to the strength expected or because just as the bridge was completely loaded under the worst conditions some accident like a tugboat bumping one of the pipes might occur, the strength required is multiplied by, say, 2. The bridge therefore is built twice as strong as it is expected will be required under the worst possible conditions. Whether the factor of safety is 2, 3, or even 6, is a matter that is determined by experience. An understanding of the factor of safety is important to the welding operator because if the weld that he makes in a structure has defects in it or otherwise does not match up to the strength that the designer expected him to obtain, he is using up at least part of that factor of safety. Of course, one of the reasons for having a factor of safety is to allow for such circumstances, but it is easy to see how several poor welds in a structure could quickly use up the entire allowance for safety, even though the structure did not break down immediately. To make sure that materials and workmanship come up to requirements, inspectors are kept on the job and inspection and testing procedures are established and carried out.