Structures: Concept, Design and Analysis
The simplest perception of a structure is its ability to transmit or support loads. Structural Analysis could be defined as the determination of the effects of loads over a structure and its components. Models of different complexities are used to determine these effects. The basic objective in Structural Design is to produce a structure capable of resisting applied loads without failure during its intended life. But the purpose of a structure goes beyond the goal of just supporting loads, and it’s defined by a need to solve a given problem. To cross a river, to go under a mountain, to cover a given space, to restrain water, to keep noise away, to sustain a number of habitable spaces one on top of another, to let light in or to keep it out… the mix of many elements, each with a given priority, will determine the purpose of a structure. Therefore the Concept of a Structure should be based on the understanding of all the elements that give the structure its purpose (raison d’être). Only after the Concept is understood, can the engineer produce an optimal Structural Design. And only after the structure has been designed, it makes sense to perform a complex structural analysis. This process is iterative by nature, and must be the result of an intense dialogue between various professionals: architects, geotechnitiens, façade experts, sound engineers, highway engineers, environmentalists, etc. In too many projects the dialogue between architects and engineers is avoided at an early stage. Too many engineers want to receive drawings with columns so they can “analyze” them and give the proper dimensions. Too many engineers get their answers from manuals, without understanding their meaning. Too many rivers are crossed with bridges not meant to be there. As a result, the complex art of structural engineering has become a commodity. Many resources are wasted in badly designed structures; structures that are inefficient, expensive, ugly and sometimes unsafe. This happens as much in buildings as in infrastructure projects. In the case of buildings, there should never be a conflict between the work of an Architect and a Structural Engineer. As Peter Rice explained, architects' work is based on creativity in which a personal answer is given to a particular problem. Engineers on the other hand are inventive, giving pragmatic solution to problems, transforming them into physical material properties. The following examples try to show how an open dialogue and a deep understanding of structural purpose can produce fine results, leading to economic and attractive structures.
Example 1: Arade River 315ft span bridge, designed to achieve a cost effective solution with an attractive result. Client: CINTRA Structural Engineers: Ferrovial Agroman
This bridge is part of the A-22 Lagos-Lagoa Highway in Portugal. The highway was financed, designed and built by a consortium led by CINTRA, in which Ferrovial-Agroman had the design&build responsibilities. The Arade River flows by the stunning city of Silves and connects to the sea at Portimao, a very popular destination for summer holidays. The contractual requirement was simple: to cross the river with “limited” environmental impact. The requirement from the contractor was also simple: design the cheapest bridge to be built in the fastest way. Therefore the structural team had a concept, to cross the Arade River on a beautiful landscape, respecting the environment, with minimum cost and construction time. The bridge is on a seismic zone, and the Concessionaire expected a low maintenance bridge. This opened an interesting dialogue between experts of different specialties: highway, structures, geotechnics, environment, means&methods, construction management, etc. The environmental experts gave priority on reducing earthworks, this requirement could be even more important than the number of piers in the river. Furthermore, construction speed was key, in order to minimize the effect on the river habitat.
The highway designers found a place where the river was the narrowest, while maintaining a fairly straight alignment making the bridge as short as possible and limiting earthworks Searching for the most cost effective solution, the Construction Managers' first idea was to build short spans using precast beams. During the design process, that idea proved to be ineficient. Due to very poor geotechnical conditions, the bridge required very long piles, so longer spans would result more cost effective. Also, manmade islands were expensive, had a large environmental impact and were not stable to support heavy loads and cranes. The means&methods team proposed a balanced cantilever process, as the company owned the formwork and the launching trusses necessary.
A dozens of different designs were analyzed; all of them based in hand calculations and priced using steel and concrete ratios from the company’s data base. At the end, an 1800ft long bridge, with 315 ft spans, using precast segmental was the best option, a choice very different from everyone’s first ideas. This design process lasted over 2 months. For the next 8 months, the structural analysis of the bridge was performed, including hundreds of different models to take into account the complexity of the construction phasing. The bridge was built in 14 months. The result of understanding the purpose of the bridge and the open dialogue among a wide range of professionals was a success. A cost effective, environmentally friendly, safe and good looking structure was the result.
Example 2: Esfera Monterrey A Zaha Hadid creation on a budget. Architect: Zaha Hadid Arquitects Structural Engineers: Fhecor Ingenieros Consultores / Alonso & Asociados / micp-engineering
Zaha Hadid Architects was commissioned to design a residential complex of over 1.000 apartments in Monterrey. The development is targeted at the middle-class sector, so the construction cost was contractually limited by the developer. Zaha’s team's creative process is simply fascinating. Every shape is the result of a reasoned choice and a balance between form and function, successfully incorporating dozens of variables from natural light and ventilation, to aesthetics and space organization. The structural team was called to participate in the conceptual evolution of the project on a very early stage. This opened an interesting dialogue that included the structural design and construction process in the equation of the architectural design. The architects briefed the structural team on the concept behind each solution. This information, together with budget limitations, served as the base for the development of the structural concept of the building. We discussed in depth different structural solutions, detailing the reasoning behind each choice, with the effect they would have on the internal layout; the cost and the construction process. During this dialogue the architects showed as much interest in the management of forces and materials as the engineers in the creation of shapes and spaces. This process, led by Zaha’s team, created an atmosphere of trust where everyone understood the purpose of the building and everyone worked for the benefit of the project.
The project is divided in 3 very long buildings of over 1000ft each and around 300 apartments. The buildings main towers are made of concrete, while steel bridges “jump” from tower to tower. There are no expansion joints in the buildings. The structural system for the concrete towers was simplified to a maximum as a result of this dialogue. Many of the unconventional columns and large cantilevers were modified, in order to respect budget and simplify construction, with minimum impact on the architectural concept. The final structure is a simple array of walls and slabs, easy to build at a reasonable price.
The same was true for the steel bridges, where the position of each steel element was studied by the architectural team to balance its impact on the layouts with the efficiency of the structure. During these phases, lots of hand calculations and simplified models were used, followed by a very complex 3D analysis of the structure. The project is now on its final stage, and the construction is planned to start by the end of 2016.
Días antes de la publicación de este artículo fallece la arquitecta Zaha Hadid, llenándonos de una profunda tristeza. Pero sus proyectos e ideas permanecen y el impacto que su obra quedará con nosotros por siempre. by Manuel Contreras Pietri