In buildings where the walls are of solid masonry construc tion and of moderate height, it is not necessary to consider the effect of wind pressure, as the dead weight of the masonry and the stiffness afforded by cross walls and partitions are sufficient to resist the effect of the wind, under ordinary conditions. With the light steel skeleton buildings carried to the height of the modern buildings, the elasticity of the steel frame makes it necessary, under certain conditions, to consider wind pressure. The walls being merely thin coverings, and the partitions also thin and not bonded to the walls, it is apparent that the frame itself must pro vide all the resistance.
The effect of wind blowing against the exposed surface of a building is (1) To produce an overturning moment tending to tip the whole building over, (2) To shear off the connections of the columns to each other, and to cause the floors to slide horizontally, (3) To slide the whole building horizontally on its founda tion, (4) To twist or distort the frame.
In buildings of usual proportions of height to base, the dead weight, even in the skeleton type, is sufficient to resist a bodily overturning. Some buildings have been built, however, that are almost of the character of towers or monuments, where this effect must be considered, and provision made for it, by anchoring to the foundations. The action under such conditions will be under stood by referring to Fig. 160 which shows the outline of a nar row building, having columns only in the walls. The building would tend to tip about the side opposite to that upon which the wind is blowing, and the columns on the wind side would be in tension, due to the action of the wind. If the load on these columns due to the weight of construction and a small percentage of the live load, to cover weight of fixtures in the old buildings, were less than this tension, the difference would constitute the strain on the anchorage. If the building were safe against over turning, it would ordinarily be safe against sliding bodily, as will be seen from the following consideration : Suppose a = the width of base le = the height above ground p = the wind pressure per square foot w = the dead weight necessary to resist overturning f = the allowable coefficient of friction on the foundations L = length of building Then assuming the whole surface acted upon by the wind, and the weight of the building acting through its center of gravity As the allowable coefficient can safely be taken at .40 this means that for the sliding tendency to be considered the width of base must be .40 or more of the height.
Buildings in which the overturning effect would need to be considered would have a base much narrower than .40 the height
so that it is safe to say a narrow building, if safe against overturn ing, would be safe against sliding.
A further point in this connection is, that ordinarily, the columns do not stop at the ground level, but extend below and therefore have the resistance of the adjacent ground against sliding.
The tendency to shear the connections, and to twist and dis tort the frame, are ordinarily the most important features of wind pressure and these effects are always present in a high building exposed to wind. The connections necessary for framing the floors and columns may sometimes of themselves be sufficient to provide for these strains ; in other cases special provision must be made.
Wind Bracing. Where special provision has been made it has generally been by vertical bracing between columns, either in the form of diagonal members, similar to the web members of a truss, or by portal bracing in the form of a stiffened plate arched between columns, or by knee braces between the columns and the horizontal members. A modification of the two latter forms has of late years resulted in using a deep girder at the floor levels, in the walls between columns. These different types of bracing are illustrated by Figs. 160 to 163. Their calculation will be con sidered later. There is always some vibration in high buildings exposed to a severe wind, as has been shown by plumb lines hung in shafts from the top of the building.
The wall covering being carried by the steel frame has greatly changed the methods of erecting a building. Now, the frame is Carried up a number of stories, perhaps to its full height, before any work on the walls is commenced. It may then be started at the sixth floor just as well as at the first. The frame is also used as anchorage for the derricks used in erection. The designer or draftsman has, perhaps, little to do with the methods used in erection, but a thorough knowledge of the conditions and general practice which prevails should enable him to arrange the framing so as to facilitate and aid in the rapidity of the erection. It is not often that a complete system of diagonal braces can be used in the exterior walls, on account of interfering with the window openings ; they are sometimes introduced in the interior walls or partitions. Portal bracing while formerly used to some extent is but little used now. Knee braces and deep stiff girders or struts at the floor levels, are the more common types of bracing. Portal braces, while forming a rigid frame without interfering with the openings in walls, have the disadvantage of being difficult of erection, expensive, and they induce heavy bending strains in the portal itself and in the columns.