In every case the use to which a building is to be put will in a great measure determine the character of its general structural features. A building may or may not require to be heated in winter; it may or may not need to be well lighted, and it may require a heavy or a light construction. With these general facts in mind we are in position to consider the questions of general design.
In determining the size of an iron building, in case the ground room is unlimited, it is well to locate first the machinery to the best advantage to turn out products at the minimum cost, and afterwards to decide on the size and shape of the building to suit the machinery. If, however, the amount of ground is limited, this cannot be done, as the building will cover only the limits of the lot. It is well also to consider whether or not the proposed building is to be a permanent one. If for temporary purpose, unit strains may be taken high and the first cost of construction cut down to the very lowest limit. If, however, the building is to be permanent, this is not desirable, as it is frequently found that heavier loads and greater strength is required than at first anticipated. It is the practice of the writer, after making his original design, to go over the plans a second time and rearrange and omit all unnecessary pieces, at the same time adding bracing where it may be found necessary. Stiffness in the whole construction should be one of the principal ends. There are more steel buildings throughout the country going to ruin on ac count of insufficient bracing than perhaps from any other cause. Steel will easily stand being strained nearly up to the elastic limit without serious injury, but lack of stiffness is liable, not only to destroy the frame itself, but the covering and glass, at the same time cause shafting to get out of line, and traveling cranes to bind and run untrue. It is frequently the intention in constructing a building to plan for future extension at. one or both ends. Provision can well be made for this by placing regular trusses at the ends and putting in temporary posts up to the height of the bottom chord to support the end purlins. Then when extension is desired the end covering can be removed and these posts taken out to be used again in the new end.
The wall construction most commonly employed in mill buildings is: Solid brick walls; iron columns with brick curtain walls, or iron columns and purlins covered with corrugated iron. Concrete filling between steel columns has occasionally been employed for side walls, but it is somewhat more expensive than brick filling on account of the temporary timber-work required to keep it in place while hardening and also because a light permanent iron frame is necessary to hold the windows in position. Of the forms of construction named, corrugated iron is the cheapest and most easily renewed, but it cannot be used for buildings which are to be heated. Machine shops, electric light stations, and similar buildings must have solid walls, and if the height is not great or if the loads are not excessive, brick walls will be the cheapest construction. Brick walls make a rigid construction suited to withstand the action of cranes and heavy machinery. In case the walls are required to be very thick under trusses, the most economical construction will be iron columns with curtain walk. This construction is usually less rigid than solid brick walls, and it is consequently not so serviceable for buildings having heavy traveling cranes. In special cases, where sand and gravel are plentiful, and where bricks are expensive, a concrete filling between the wall columns is less expensive than brick, and is in every way just as serviceable. Where columns with curtain walls are employed, the tops of the columns should be connected by steel struts to keep them in position. Columns with bracing between them in a vertical plane make as stiff a construction as can be secured, and consequently are well suited for heavy cranes and machinery.
The form and section of column employed varies greatly. For light loads four angles latticed together, as shown by Fig. 2, is a construction frequently used, and the column must be given sufficient width to take the bending strains from the knee braces. If brick-work is to be built into the columns, their width must be made to suit the size of the brick. It may sometimes be desirable to have one or more large bays or wide panels in the walls, in which case the ends of the roof trusses coming over these bays must be supported on side or wall girders attached to the columns on each side of the bay.