Columbia University Libraries Digital Collections: The Real Estate Record

Real estate record and builders' guide: [v. 99, no. 2551: Articles]: February 3, 1917

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REAL ESTATE AND NEW YORK, FEBRUARY 3, 1917 FEATURES OF SKYSCRAPER CONSTRUCTION UNDER THE ZONE RESOLUTI.ON ■\Y/E are on the verge of a new era in •^ tlie design of tall buildings in New York. The ne.\t few years will be marked in the city's building history as those which saw the establishment of a new skyscraper architecture. The intro¬ duction of structural steel and the caisson foundation has been potent in revolu¬ tionizing the appearance of Manhattan's buildings, but the curb, which in the in¬ terest of better light and ventilation, the Board of Estimate has lately put upon the heights made possible by these sys¬ tems, will effect equally great changes in the appearance of new skyscrapers. The restrictions placed upon the height of buildings and the sizes of yards and courts by the Resolution, commonly known as the Zone Law, adopted by the Board of Estimate on July 25, 1916, vary in different parts of the city, but the only ones we need consider as having an im¬ portant bearing upon future large struc¬ tures are those that prevail in Manhattan south of 59th street. In this section we find the "Two and One-half-times District," the "Two-times District" and the "One and One-half- times District," each of which designates in multiples of the street width the height to which a wall may rise upon the street line. It will be sufficient to con¬ sider in detail the requirements of the "One and One-half-times District," as they are identical with the others in prin¬ ciple and more stringent in specification. The corresponding "Area Districts," the requirements of which regulate the sizes of yards and courts, are the "A Dis¬ trict" and the "B District." The "A Dis¬ trict" is that covering the warehouse and industrial areas and is the most lenient in its requirements. The "B District" covers most of Manhattan and specifical¬ ly the sections in which we may expect the erection of future notable buildings. As, ill addition, it applies similarly to the various "Height Districts," it may be considered jointly with the "One and One-half-times District." The "One and One-half Times" (one and one-half times the street width) height limit will not impose any greater restrictions upon tenement houses than is now imposed by the Tenement House Law except upon streets exceeding one hundred feet in width. The "B District." moreover, is no more stringent than is the combination of Tenement House Law and Building Code requirements enforce¬ able in the construction of tenement houses. We may thercfbre eliminate them from further consideration and de¬ vote our attention to non-tenements. Of these, the simplest type that inay possibly be erected to a height exceeding one hundred and fifty feet is the loft building without dividing partitions and, therefore, easily lighted without the ne¬ cessity for intricate courts or yards. Aside from the set-back requirements, the only new point introduced by the Zone Law is the necessity of providing a yard space behind buildings on certain interior lots and the limiting specifica¬ tions for the length of outer courts and for the size of inner courts. Other prospective big buildings will fall into either the office building class or into the hotel class, in each of which the sim¬ plicity of the loft type will be lost. New problems will be induced by the neces¬ sity of lighting all living rooms and the necessary introduction of open spaces or By FRANCIS P. SCHIAVONE courts into the heart of the building. Thus a more complicated floor plan is required and the most economical layout of a given plot more difficult to find. In¬ stead of there being, as heretofore, per¬ haps, one typical floor plan, there will be several, as the height rule neces.sjtates set-backs of the the street w-alls and the yard and court rules require progressive increases in the width of open spaces. The theory of the height limit rules of the Zone Law is the same as that which forms the basis of the court rules of both the Building Code and the Zone Law. The requirements are designed to maintain a predetermined "angle of li,ght" or "sky angle" for all heights of building. They establish inclined planes beyond or above which no part of a structure may project. So long as these planes are not pierced, there is no height limit. In applying this theory to the limita¬ tion of the walls upon the street line, in the "One and . One-half-times District." the limiting plane (or planes) is a verti¬ cal production of the street line to a height of one and one-half times the street width and is thereafter inclined away from the center of the street in the ratio of one horizontally to three vertically. These planes arc related to the street rather than to the space be¬ tween streets and are to be maintained inviolate on all sides of blocks irrespec¬ tive of property subdivisions or the identity of buildings to the end that no building shall project beyond the limit¬ ing plane for any street whether it faces on that street or on another street or on no street at all. This principle applied by the Building Code to courts, erects opposite vertical planes forty-eight feet high and four feet apart and other planes inclined thence apart in the ratio of one hori¬ zontally to every twelve vertically. An inner court is limited by four such planes, an outer court by two. The Zone Lavv modifies this in the "B District" by specifying certain mini¬ ma for yards and outer courts and a ratio of one to six for the inclined planes of yards. It imposes also a somewhat complicated limit upon inner courts, a requirement consisting of an area mini¬ mum and a correlative width minimum. A sharp distinction should be drawn between the yard required by the Zone Law and all other open spaces. The pro¬ vision of a yard is in certain cases com¬ pulsory and in such cases it is required to be of a certain size. All other open spaces are not required to be provided, but if included in the plan must equal or exceed a certain minimum. The yard is required primarily for block lighting and ventilation irrespective of its utiliza¬ tion by the building on the same lot. Other open spaces, if provided, are de¬ signed exclusively for the building on the same lot. The yard, being required, must begin, for lots on residence streets—-not above the curb level and on business and un¬ restricted streets—not above the second tier of beams or in any case above a point twenty-three feet above curb level. Other open spaces may begin at any level they are needed. It may confidently Jbe stated that prac¬ tically all large buildings will be built, as heretofore, on corners or large plots including corners. This the Zone Law encourages by exempting corner lots from the necessity of providing a yard irrespective of the character of the block plan. Likewise a corner may be any size; may include one corner, two, three or a whole block. Interior lots will, under certain cir¬ cumstances, have to have yards. Build¬ ings running through trom street to street may have to provide open spaces on either side in lieu of yards. Each of these is required to be at least of the area of an inner court, the width of an outer court and to start not higher than twenty-three feet above the curb. As yards in the "B District" must be at the bottom at least ten per cent, of the lot length and increase above that depth in the ratio of two inches in depth for every foot of height, it is evident that the question of whether a yard is re¬ quired or not and its size are all-impor¬ tant. It must be remembered that the set¬ backs on the street line and the sizes of yards and courts are all factors of the height; that is, that the open spaces whether in street, yard, outer court or inner court must increase in width as the height increases. In the design of a building bulk, therefore, that will best suit certain requirements, the factors of net-cubage, width of pavilions, character of open spaces and height of buildin.g are all very closely interrelated. No one may be altered without in most cases drastically affecting some or all of the others. Within certain limits the height of the building must be known before the court spaces may be determined upon. The minimum and maximum w'idth of pavilions—varying with the character of the building and size of rooms—will place a limit on the dispo¬ sition of open spaces; the height of the open spaces will be as potent a factor and will be more important perhaps in determining whether these open spaces shall be inner or outer courts. Where land and Jloor space are ex¬ tremely valuable there will be set-backs in court and yard walls, but where the space thus, to be saved is not worth the cost of the set-backs, the court walls will be unbroken in their vertical sections. A selected motif of design may require that there be many small set-backs in the exterior walls or the mansard may be used, but otherwise there will usually be not over two set-backs below the tower, which, under the Zone Law, is exempt from the height limitations so long as it is a certain variable distance back from the center of the street. The requirements of design will not be so important in courts and the only reason for set-backs in the walls of such open spaces will be the utilization within the building of all available space and perhaps the' lighting of various floors by means of skylights set in the roofs or decks of the set-backs. The necessity for set-backs on the street wall, and in many cases on court walls, will mean the almost exclusive use of the skeleton frame. In tall buildings on narrow streets it is probable that though the central portion will be skele¬ ton (with walls supported at every floor) the exterior portion of the building above which the rest sets back will be curtain- wall construction. On wide streets and where the height limit is two or two and one-half times the street width, the street skeleton will most probably be used throughout.