Pedestrian Bridge as Public Art: Architecturally Exposed Structural Steel

October 16, 2017 | Autor: Terri Boake | Categoria: Architecture, Steel Structure, Steel Structures, Bridges, Architecturally Exposed Structural Steel
Share Embed


Descrição do Produto

Pedestrian Bridge as Public Art: Architecturally Exposed Structural Steel Terri Meyer BOAKE Professor of Architecture University of Waterloo Cambridge, ON, Canada [email protected] www.tboake.com

Terri Meyer Boake teaches and researches in the area of construction, environment and steel design. She has published three important books with Birkhäuser on architectural steel design, diagrids and AESS. She works with the Canadian Institute of Steel Construction to develop learning modules. She is an active board member of the Council on Tall Buildings and Urban Habitat.

Summary The construction of pedestrian bridges has seen a surge in recent years. Utilitarian structures have given way to the creation of inspirational, aesthetically driven elegant structures that can be seen to assume a dual role of “bridge” and “public art piece”. This duality of purpose has resulted in the allocation of higher than normal funding for these projects, allowing for some spectacular results. This paper examines the use of detailing in Architecturally Exposed Structural Steel as the means to elevate the function of the bridge to the status of “art”. Such detailing in AESS has its origins in the early development of historic bridges, with more recent modifications as a result of enhanced detailing arising from the High Tech Movement in the 20th century. The AESS design, fabrication and specification methodology presented will reflect work by the author in the creation of the AESS Standards for the Canadian Institute of Steel Construction (also adapted to Australasia) and in process for use in the United States.[1] The paper draws heavily on a book published by the author titled “Architecturally Exposed Structural Steel: Specifications, Connections, Details” by Birkhäuser, 2014.[2] Keywords: pedestrian bridges, architecturally exposed structural steel, AESS, detailing, steel design.

1.

Introduction

Since the early days of cast and wrought iron, the bridge has been able to serve simultaneously as a means of crossing and an expression of a significant spanning structure. Unlike structures that are hidden inside of buildings, the long span solution of the bridge is iconic in the public realm. Early iron bridges often took on truss or arch shapes, their exposed steel detailing highlighting the joinery of the period. The material limitations of iron often resulted in the need for multiple piers at the point of crossing. The invention of higher strength steel and cables facilitated the creation of new

Fig. 1: Seattle Museum of Flight Bridge, USA, by SRG Partnership

Fig. 2: La Roche sur Yon, France, by Bernard Tschumi and Hugh Dutton [3]

Fig 3: Puente de Luz, Toronto, Canada by Francisco Gazitua

bridge typologies and permitted much longer spans. The car dominated society of the later 20th century and its massive road and highway networks tended to downplay the role of the expressive bridge structure. Crossings were numerous and came to be treated in a routine manner. High strength materials resulted in spans that were achievable with beams, the use of suspension cables and more elaborate steel truss structures being reserved for more significant vehicular bridges over major crossings. The 21st century has seen a resurgence of the pedestrian and bicycle. Major roadways and waterways are now seen as barriers that divide neighbourhoods and separate parts of cities, making it difficult for pedestrians and bicycles to navigate or cross without having to go many kilometers out of their way. A new genre of bridge type has evolved to address the need to reconnect the divided parts of the city and make the path of the pedestrian or cyclist a delightful one to traverse. Many of these bridges are located in significant urban areas, encouraging designers to be more innovative and expressive with their bridge solutions. As the pedestrian and cyclist loads are much lighter than vehicular bridges, the bridges can also be lighter which permits a wide range of design expression.

Fig. 5: Peace Bridge, Calgary, Canada, by Santiago Calatrava

Fig. 4: Arganzuela Bridge, Madrid, Spain by Dominique Perrault

This paper will look at the application and detailing of Architecturally Exposed Structural Steel (AESS) as it has been effectively used to create five major pedestrian bridges. Each is characterized by a significant span and each adopting a fairly circular cross section arrangement. The specific and deliberate nature of their AESS detailing will be shown to feed quite directly into their architectural character and artistic qualities as they simultaneously serve as Public Art.

2.

The Basis of AESS Detailing

2.1 The Development of a Graduated System for AESS Decision Making The Canadian Institute of Steel Construction (CISC) spent several years in the well considered development of graduated standards for design in AESS. This system of tiered Categories and related Characteristics is consolidated into a decision making matrix. The method is based on work intitated by the American Institute of Steel Construction (AISC) in 2003.[1] The method has been adopted by Australia and New Zealand. The development of these new standards by a team comprised of architects, engineers and fabricators, was a reaction to a state of detailing in AESS where higher than required standards were being applied to all exposed steel projects, regardless of building use, distance of view or exposure conditions. Communication had also been confusing, given the lack of clarity or specifications to reference. The CISC system set as the basis for the Categories that in order to design and detail AESS in an appropriate way a differentiated approach must therefore recognize the following realities:

 Distance and Visibility: If you cannot see it up close or touch it, the finish requirements can be softened.  Bolted versus welded connections: Different connection types result in different aesthetics, requiring differing levels of finish.  Tolerances at fabrication and erection: Different tolerances are needed as a function of the scope and complexity of the project but typically tighter tolerances are needed for AESS than for standard structural steel framing.  Access to detail to perform required finish: The greater concern for workmanship may mean altering the detail or its location to allow access for different types of tools. Highly articulated connections or details need to be constructible and maintainable.  Degree of expression: The complexity of structure and connections will impact the detailing and cost.  Size and shape of structural elements: W (Universal) and HSS shapes have different connection requirements and their use infers a different approach to detailing and finish.  Interior or exterior setting: This impacts weathering issues, cleaning and maintenance access, the need for fire protection and the potential for impact damage.  Paint finish, corrosion resistance, fire protection: Depending on the relative thickness of the finish material, more or less care may be required when preparing the surface, edges and welding of the steel.

Fig. 6: The CISC AESS Category Matrix sets out categories of increasing fabrication requirements according to the distance to view and architectural intentions of the elements.[4] A view distance of 6m/20ft was established to determine whether or not the steel was close enough to view or even touch, or remote enough to allow for less precise detailing. Pedestrian bridges will necessarily fall into the close view range, placing them in AESS 3 or AESS 4 categories. It will be seen in the examination of the five bridges in this paper that this type of structure can adopt a wide

range of detailing given that it is also serving as Public Art. The detailing of these bridges feeds into significant textural and fabrication variations as a result of the artistic intentions that will tend to override decisions made in other types of AESS building applications such as airports, galleries, commercial and institutional projects. 2.2

The AESS Categories

A brief description of the AESS Categories and Characteristics is required as background for this discussion. The basis of all is Standard Structural Steel design and detailing as the structural sufficiency of steel necessarily must first be served. Above this are four primary categories to which are applied increasingly fastidious fabrication requirements, as set out on the Matrix, opposite. AESS 1, Basic Elements is used for distant elements such as roof trusses, simple canopies or more industrial applications. Its characteristics include: – The surface preparation of the steel must meet the standard of the Society for Protective Coatings SSPC-SP6.[5] This assumes commercial blast cleaning to ensure good coating adhesion. – All of the sharp edges are to be ground smooth. – There should be a consistent weld appearance for all welds. – It is assumed that bolted connections will use standard structural bolts (including tension control bolts, TC, if available in your region). – Weld spatters, slivers and surface discontinuities are to be removed. AESS 2, Feature Elements with a view distance >6m/20ft is suggested for high level architectural elements that are part of buildings with greater architectural aspirations than AESS 1. Its characteristics additionally include: – Visual samples may or may not be required depending on the scope of the project. – Standard fabrication tolerances of no more than one half as compared to standard structural steel constructions are required to ensure good fit and better appearance. – Fabrication marks – number markings put on the members during the fabrication and erection process – should not be apparent. – The welds should be uniform and smooth, but not remediated. AESS 3, Feature Elements with a view distance
Lihat lebih banyak...

Comentários

Copyright © 2017 DADOSPDF Inc.