Having projects in the urban environment representing a large proportion of my career experience, I am always a little surprised when I encounter design professionals and contractors who do not fully appreciate the challenges and constraints associated with building on urban sites. While a lot of design professionals, contractors and other stakeholders have urban project horror stories, they do not necessarily associate those adversities with choices that were made or not made during the project. It is almost as if they believe that nothing can be done.
Perhaps I should not be surprised. The fact is that most of the Architecture, Engineering and Construction (A/E/C) industry is focused outside of the urban cores. In a lot of major metropolitan areas, development has focused on low-density sprawl with large parking lots and generous setbacks. For these projects, consideration of the outside world may be limited to curb cuts and utility connections. Is it any wonder then that designers and constructors underestimate what it takes to build on a constrained urban lot.
The confluence of millennials, empty-nesters and white-collar jobs into certain prosperous and now safe urban centers in the past two decades have changed development strategies, perhaps permanently. While the influx may have slowed down, demand for affordable housing, walkable communities, amenities convenient to home and work and access to major urban centers are planning considerations for suburbs and smaller cities. Urban construction is not going away; instead, as smaller cities try to duplicate the successes of major metros it may become more common. Understanding how to manage performance, cost and risk associated with urban construction will be increasingly necessary to support this change in land use planning.
Construction in the urban environment is characterized by higher cost and higher risk compared to building in undeveloped areas, due to logistical constraints and the presence of existing facilities on and adjacent to the site. These challenges are compounded when dense urban developments require that new facilities be constructed below ground. Much of the increased costs and risks are associated with interfacing a proposed structure to its site. The selection and design of foundation systems must account for how construction methods and new building loads may cause movement of the foundations of adjacent facilities. Underpinning and excavation support may be required to maintain the stability of adjacent properties during below-ground construction. Shoring and other protective measures may be required to protect people and property just outside the site limits for the duration of work. Party walls and unanticipated site conditions may add to the potential construction hazards requiring engineering solutions.
Together, these additional considerations represent a significant increase in the engineering, construction planning and risk management required to build a project. However, unlike a lot of construction challenges, they cannot be so neatly separated between design and construction. The design of permanent work directly influences construction means and methods, and construction means and methods expose an unusually large group of stakeholders, including the project owner, to a variety of risks. Unfortunately, outside of a few major cities, relatively few in the real estate development and A/E/C industries seem to fully appreciate this fact. Consequently, construction on an urban site is often approached analogously to construction on open sites. This approach is characterized by fragmented responsibility and information sharing, increasing the risk for all involved.
I sometimes find that the A/E/C industry lacks logical, geographically-consistent definitions that facilitate communication between professionals in different market sectors and locales. Perhaps the practice of engineering and risk management for construction on urban sites would be better appreciated if it had a name. I like the term “site-structural engineering” for this practice area. It is analogous to the more familiar “site-civil” engineering required for suburban and rural sites. For those sites, the engineering required to interface new construction to the site typically falls into the domain of civil engineers, including grading, drainage and utilities. For urban sites, more of the engineering is structural in nature.
I am guessing that my selection of nomenclature might not seem obvious to a lot of other engineers. Most building structural engineers that I know would not want clients (or others) to assume that what I call site-structural engineering is part of the ordinary and customary scope of services that they provide. Structure engineers design the primary structural systems of the building and are not granted the scope or budget for these other tasks. Many of them are ill-equipped to take on site-structural problems because they are so fundamentally different from the design of the base building. They may view these services to be included in the geotechnical engineering or perhaps the construction engineering performed for the contractor.
In some cities, the early geotechnical firms came from a foundation engineering pedigree. Their successors have traditionally provided much of what I call site-structural engineering. However, these firms are not representative of the majority of geotechnical firms. While there are clearly geotechnical considerations to below-ground site-structural problems, they are not exclusively geotechnical problems. An understanding of structural behavior is necessary, as many of the potential failures modes are problems of structural mechanics and design, such as instabilities or inadequate connections. Further, foundation engineering on urban sites requires some level of comprehension of the structure to be built.
In some respects, it is accurate to say that site-structural engineering is simply construction engineering for an urban site, especially when foundation design is delegated to the contractor. However, that misses the point. Delegation to the contractor can lead to site-structural considerations not being addressed until the construction phase. At that stage in the process, nearly all of the fundamental decisions about the project have been made. Consequently, opportunities to optimize the design or reduce cost or risk by holistically considering the subsurface conditions, the proposed construction, adjacent structures and likely construction methods together will likely be missed.
If it is thought of as a field of practice, site-structural engineering is an interdisciplinary specialty concerned with interfacing new construction to the existing conditions of an urban site so as to proactively reduce construction risk. It includes a lot of old-fashioned foundation engineering, structural construction engineering as it applies to excavation and protective work, and condition assessment of existing structures. In addition to assessment and design of “hard” systems, site-structural services include devising risk management and control strategies, such as allocating risk and monitoring impacted structures and temporary works.
Site-structural engineering is not part of base building structural services. Too much of it is outside the expertise of typical building design firms. The need to understand structural mechanics and the behavior of existing structures is beyond what most geotechnical engineers can provide. While site-structural engineering is construction-oriented, construction engineers work for the contractor and are typically allowed to rely on the construction documents to limit their risk. Someone needs to holistically help the owner and design team to manage site-related uncertainty, allocate risk and reduce construction hazards through design, as well as provide performance standards and baseline assumptions for the construction engineer. The need to integrate design and construction and manage risk for the owner and other stakeholders means that the site-structural engineer should be a consultant to the design team. You may not like the name that I have coined, but having this specialty available to the design team is necessary for successful projects in the urban environment.
The information and statements in this document are for information purposes only and do not comprise the professional advice of the author or create a professional relationship between reader and author.