On Some Common Errors in Iron Bridge Design

4 It appeared, therefore, that a criticism of existing bridges would be useful not only to the designer of new structures anxious to avoid the defects of the older ones, but also, and perhaps in an even greater degree, to the man who has received a legacy of imperfect structures from his predecessors, which he is clesiious of utilizing as far as possible by judicious repairing and local strengthening, for it is to be noted as a good point of many of our defective bridges that they are like chains, most of the links of which are abundantly strong while occasionally a very weak one is found, which governs the strength of the whole and that thus a comparatively inexpensive local reinforcement may improve the whole structure to a very large and valuable extent. I shall now proceed as briefly as is consistent with clearness to point out what I consider to be the principal errors in structures that have come under my notice, and indicate how their defects may be remedied, if remediable, in existing and avoided in future structures. 1. Disproportion of foundation area to load carried.—If a foundation is too small it gives way partially or wholly, injuring or destroying the structure; if too large it stands but represents waste of money. In every instance however some slight yielding when the load is applied takes place, and it is desirable, espe- cially if continuous girders are employed, that all the supports should yield equally. Hence all foundations should be propor- tioned to the load carried—that is to say, under full load the pressure per unit area on the supporting material should be throughout equal. In calculating this pressure, it is to be remembered that it is not the total load on the foundation surface that is to be considered, but the excess over the load that existed previously. ]<or example, at the great Hawkesbury Bridge, N.S.W., it has been stated, that the pressure on the foundation is 10 tons per square foot, and this is obtained by dividing the total weight of the structure by the area of foundation. But in order to reach the depth required a very large quantity of earth had to be removed, and the foundation was relieved to that extent. The true or effective pressure on. the foundation is therefore the difference between these two amounts, and actually is only 5 tons per square foot. This I submit is the correct way of stating foundation pressure.