Architects and engineers are constantly faced with decisions that will impact the overall outcome of a project. When it comes to overall project success, one of the most important decisions is selecting the right structural system. With nearly 40 years of experience designing virtually every type of structure, The Harman Group has conducted thousands of Structural System Studies in an effort to ensure the best outcome based on the client’s goals. This blog is the first in a series that will highlight the structural system selection process, along with specific factors to consider for each system based on The Harman Group’s extensive experience in the industry.
There are many factors and considerations that go into selecting a structural system. To ensure an effective Structural System Study, it is important to start the process early, ideally during the schematic design phase, and enter the process without any assumptions. Often, what seems like the “correct” structural system turns out to be inefficient or cost prohibitive based on market conditions, design considerations or constructability issues. We typically analyze up to eight systems before making a final selection.
In addition to the various systems such as cast-in-place concrete, structural steel, cold formed steel, and wood, it is also important to consider the layout and intended use for the project. We work closely with the owner and architect throughout the process to understand their needs and intended design to provide the best recommendation. Important considerations when selecting a structural system include:
Knowing the subcontractors and labor conditions in each market is critical. If the project labor cost will be union, non-union or prevailing wage will influence cost. Subcontractors in every market vary and considerations of talent and pricing are always a factor. Different systems are preferred in different markets and some subcontractors may be busy at the proposed bid time, causing increased cost.
One example is East Market in Philadelphia, a major mixed-use project with two residential towers, five-stories of retail and underground parking. The Harman Group, alongside the project partners, had to find creative structural engineering solutions for the project to address the competing vertical change in uses, each of which had differing clear span demands and sat on a challenging urban site in Center City, Philadelphia. Understanding the subcontractor market in Philadelphia and how busy concrete subcontractors were at the time with high-rise residential buildings, driving up the cost of concrete construction, the team opted for a steel framed structure with a precast concrete “plank” floor system to optimize the design, while remaining within budget.
Integration of the structural system into the architecture and interiors is critical. Early involvement allows us to offer options and adjustments to achieve project goals at a reasonable cost. Minor adjustments can sometimes achieve significant cost savings by avoiding major structural transfers and offset lateral systems. This early cooperative approach can save costs associated with “fitting the structure into” an already set floor plan. At the same time, early coordination with mechanical systems, accounting for locations of ducts and equipment and headroom needs, allows the tuning of the structural system to the project goals at a reduced cost.
Different materials along with different variations in structural bays, spans and column locations must all be studied. We factor constructability into the design process, as this is an important part of structural engineering. The structure we recommend will be efficient and practical to build.
The goal is to balance cost and integration with the project as a whole to achieve the optimum structural solution.
There are two components when discussing the cost of a project, materials and labor. Often, the average installed materials cost is used as a measure of efficiency. (Weight of steel or volume of concrete per square foot of floor area). However, with constructability considered, this approach is generally inaccurate. The labor component must be considered. More complicated layout of reinforcing steel or minor variations in steel sizes in place of repetitive sizes can increase labor demands significantly. Often a slight increase in steel weight or concrete material to achieve simplified installation labor allows faster construction with fewer delays. The reduction in labor cost outweighs the small increase in materials.
Working closely with a construction manager to establish the cost difference for one system over another is key. Additionally, the cost of one system compared to another changes over time as the relative cost for each material changes. The cost of concrete, reinforcing steel, structural steel and wood can vary over time so the projected bid date is important too for accurate pricing.
In the case of office and life sciences buildings, the design load for the floor requires careful consideration. Major tenants in the market may have more restrictive requirements than those set forth in the Building Code. The structural system must be selected that meets the needs of potential tenants for these buildings with potentially heavy loads. Options must be considered with the owner to select this criteria.
Sometimes owners and occupants overestimate the design-load requirements. It is our job to ask questions and ultimately come up with the most cost-effective solution while helping the owner meet the needs of tenants.
The Harman Group’s philosophy as it relates to structural systems is, “extra time spent in schematic design saves time down the line.” The key to success is to challenge all assumptions and provide recommendations that ensure the owner comes out with the best system for the project, the optimum system.