should be allowed, as a great many of the difficulties in erection are due to lack of clearance; besides, giving reasonable clearance will permit of more rapid shopwork and the avoidance of many errors in the field due to inaccuracies thereof.

It is often advisable to provide, in addition to the usual allowance for expansion, a small amount for erecting the metal, due to what is called the "growth of steel." For viaducts such adjustment should be provided about every 400 feet; and similarly for mill-building work it is necessary to consider effects of adjustment, even if it is decided not to take care of expansion, while in other structures the joints for expansion will also serve the purpose of adjustment.

The expansion points for stringers or elevated railroad-girders, where pockets are used, sometimes have not enough space behind the end stiffeners of the expansion girders to allow for the insertion of rivets through the end connections of the fixed girders. It should be remembered that the expansion stringers and the fixed-end stringers are erected before the rivets in the connection of the fixed stringers are driven.

When columns are set to stone bolts, which have been imbedded in masonry, the holes should be 3/4" or 1" larger than the diameter of the bolt, so as to provide adjustment to take care of the inaccuracies in setting the bolts in the concrete.

A common mistake in design is to proportion the members with too small a width, causing considerable trouble in packing the pins and in making room for the verticals, pin plates, etc. Another bad feature of narrow chords is that practically all rivets around the connections must be countersunk because of close space, and the ends of the posts must be cut away for clearance, thereby weakening the said ends. By adopting chords of larger widths much better details can be used around the pins at panel-points.

When two or more truss spans are identical, or when they are similar and have the same field connections, the field holes should be reamed to an iron templet, in place of reaming them while the members are assembled. This will facilitate the delivery of the work, and will make identical members throughout the structure interchangeable. The advantages in the field are evident, less time being spent in sorting and finding material.

In the designing of details extreme care should be exercised in arranging all joints and connections, so that the work cannot only be built at the shop for the least cost in labor and material, but also that it may be erected most economically and with a minimum of risk. In the case of bridgework, all connections should be so detailed that spans can be connected and made self-sustaining and safe in the shortest possible time.

Unless for special reasons, it is usually customary to begin the erection of pin- connected spans at the center panel, as this panel has adjustable members and the trusses can be squared up there before proceeding. The details should, therefore, be so arranged that the center panel can be completed and made self-sustaining before the traveler is moved to the next panel. It is the usual custom for the erection to proceed from the center panel toward the fixed end, and after this half of the span is erected, to proceed toward the roller end. Top chord sections in any particular panel are put in place after the posts and bars are erected; and it is especially desirable in heavy work that the details be so arranged that these chord sections can be lifted above the posts and set directly in place without being moved on end or sideways. Therefore, plates connecting two adjoining chord sections in heavy work should always be shipped loose.

Wherever possible, in all truss spans the floor connections should be so arranged that the floor system can be put in place either before or after the trusses have been erected in their final position. It is usually customary, where local conditions will permit, to put the floor system in place first and erect the trusses afterward. This method of procedure has a great many advantages over that of raising the trusses first, viz.: there is a  large  saving  in  falsework,  as  longer  panels  can  be  used,  putting  bents