2001 Planning Commission Packets

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�� I�� a�� h� Maximum allowa6le he[ghts for street walls form an important dimension influcncing the urban experience because none of us, as pedestrians, can relate much to those parts of buildings rhat exceed a certain human scale, Above perhaps 50 feet, it is not only hard to notice what is happening in a building, but the building itseif may produce a feeling of alienation from the built environment. For this reason alone, cities commonly have assumed that the heig'nt of street walls should not be greater than I.5 times the width of the street. Tn order to maintain a sense of proportion, the step-back should continue che direcr line 6etween the opposiee side of the street and the top of the street wall to the 6uilding's upper levels. For instance, a 1989 Doauntown Hefght Study for Portland, Maine, suggested stepping back 15 feet above the seteet wall for downtown buildings that mostly were ranging in height from 45 to 9Q feet, a[though a few along the core business district rose higher than 100 feet. Ideal Street Wall Proportions � � engineer's assessment that testing wind impacts on building cladding and structure is essential when a building's •13eight exceeds its width by a ratio oF5 ra 1. On the orher hand, bu'cldings up to four stories high along streets about 66 feet wide tend to produce shelter from wirzds, as noted in an acticle orz Toronto's efEorts to znod'zfy wind velocities {see "Urban Form and Climate," journad oftheAmeriean PlanningAssociution, Spring 1995). Of course, such a pattern stays well within the I.5 to 1 design proportion of height to sereet ��vidth cited earlier. While weather patterns vary among cities, the fact rema€ns that when the temperature is freezing and the wind is guscing, urban wind tunnels can more than increase thcse speeds by half. Wind speeds above about 40 miles per hour become a serious hazard for pedestrians. Thus, mitigating wind patterns through effective design regufation becomes a signiFicant prioeity. At the same time, stepping bacEc upper levels of the building reduces the size and volume of the resulting building shadow, thus mitigating another k�actox e}zat can make winter streets uncom- Tower Envefope `, �\ � � �� .,,,owFn _.��, � � � `, ` � � ,' ��, ' � � P' � .�:' \\\�. \ � � \' . y�,' \��\ � g � � �� ` - x� `�'�1�� \��\\ �;� � �� \��\ � � �� \\\�\\ \\�\��\ � � �� \� �� `�: � , \ \ �\ , . � �� \ `� �\� s= �� \V�� \ � / 1� .��;;\\��`��\�\,�� � ., �� \\.��\\��\��. i At street level, of course, blank wails of any size can be alienating. Probabiy about half of the wall space should consist of openings in the fotm of doors ar windows; and this also can be created through building design regulations, including various types of incentives. Stepping bacic the upper floots instead of allowiug a sheer wali serves another purpose that aids pedestrians and thus increases street traff'ic. The scep-bacic at highet levels breaks wind patterns and reduces the propensiry for tall buildings to pour powerful downdraEts to srreet 1eve1, creating the infamous wind tunnels that plague some high-rise districts. In fact, the wedding-cake design is one way to blunt the impact of swirling winds. New Yorlc City, however, for a variery of reasons completely revised its zoning in 1961 to incorporate a"slcy exposure plane" concept that replaced height districrs as a means of goveming building setbacks, This device became a means oF measuring the sunlight penetration to street 1eve1 allowed by a proposed building design. The same zoning revision also introcluced fioor area raxios, which have since gained wide acceptance etsewhere, as well as providing floor area bonuses for plazas and other open space around new buildings. Currendy, New Yoriz is again undertaking a revision of its height and bullc regulations. Scheduled far January 31 is a third City Planning Commission on a proposed Unified Butic Ordinance (see "The Big Apple Gets a i,ittle Smaller," March 2000). The wind issue, hocvever, is iinpartant to building owners as � well as planners and pedestrians because of legat issues invoiving ]rabiliry for wind-related injuries induced by questionable building design. In an article in Urb�n Lancl ("Testing the Winds," October 2987), Terry Jill Lassar notes a Colorado wind Alternative Tower Envelo fortable, Needless to say, corzzbining long shadows, which reduce microclimate temperature, with how[ing winds can make some streetscapes almost completely inhospicable. ��i��a �� a�� L��t� Context matters in regulating building heights. Achieviag some degree of consistency in building height is important not only wit(un a district, but between neighboring districts. The rule oE thumb seen�s to be that a wise gradatian of allowable building heights between adjacent zoning districts is about 50 percent. In other words, a distt'rct that allows a maxirnum height of 80 feec could safely adjoia another, presumably closet to the center, that allows I20 Feet, w6ich in turn could yield to another that allowed up to 18Q feet. Cieatly, there is room foz compromise uader special circumstances, but rhe ovecall idea is to ratchet up the allowable height gradua[1y. Lilce step-backs above saeet waI(s, this affects wind patterns because a 300-foot bui(ding ne�cr to chree-story apattment buildings is bound to introduce disruprions ia wind patterns that a more fine[y graded urban pattern might mitigate. In addition, the shadowing effect on neighboring tall buildings is Iess than t[Ie effecr produced when tk�ere is a sharp diffexential. Cities often use more elaborate means of regulating height and bulk than simply imposing height limits. F1oor area ratios (FAR) are commonly used co permit greater flexibility in build'tng design and to encourage both the trimming of bnik at higher levels and the greater preservation of ground-level open space. These measures also can increase real estate vafues by �roviding more views at higher elevations. Although the use of FAR regulations intraduces grearer pocential variation in actual building height, the zoning ordinance can still apply much the