Design 2147 CEO Sisto Martello recently had the opportunity to speak with Scott Hughes, Principal at Silman Structural Solutions, about the latest developments in office-to-residential conversions. Here are some of the important takeaways from the conversation.
Q. When a developer takes on an office-to-residential conversion, what are the structural engineering risks they need to keep in mind?
A. As with most projects involving existing buildings, I would say the greatest risk lies in unforeseeable existing conditions. Ideally a developer does their due diligence to determine what condition the existing building is in prior to starting the project – but even that doesn’t eliminate all risk. Almost invariably there will be base building repairs uncovered in the course of construction. The more that can be gleaned through probes or non-destructive testing, the better that risk can be managed, but it likely can never be completely eliminated.
Q. For mid-block office buildings, an inner court will need to be created to create light and air for habitable rooms. What obstacles does this create for structural engineers?
A. Depending on the location and size of the proposed inner court in relation to the existing lateral load-resisting system, re-establishing a viable, Code-compliant lateral system is certainly a significant challenge, though that would be true of a corner lot as well. You’re creating a large hole in several diaphragms, and you may be taking out an elevator core or braced frames in order to create the inner court. That lateral load capacity would need to be replaced.
In addition, for a mid-block building, construction logistics can be a challenge. Simply getting materials to where they need to be in a constrained site will prove challenging, and to say that’s entirely the contractor’s problem to solve is not necessarily true. Schedule and cost impacts become everyone’s problem to solve, including the design team’s.
Q. Assuming the core of the building will need to be demoed to create an inner court for light and air, how would you as the engineer suggest the removal of the building core and the bracing for the altered structure under the current codes?
So much of this would be dependent on the specifics of the project – just about every solution will be unique – but an approach to construct the new lateral load-resisting system within the building prior to tearing out the existing core would be ideal. Again, that presents logistical constraints – for example, getting materials onto floors through openings in the façade before creating the inner court could challenging. There may be other methods of sequencing the construction in concert with the removals that may also prove beneficial from a scheduling standpoint. But the specifics of any given project will likely start to define those opportunities.
Q. A developer who is currently in the middle of an office-to-residential conversion stated that when they build new buildings in Manhattan, their biggest cost exposure is when they are digging and installing the foundation. Once they’re above grade and start the super structure, structurally there are no unknowns. Do you agree with that thinking?
A. In general, yes. I’ve found that in most cases of new construction the area of greatest exposure is in the foundations, especially in New York City. However, to the point I made earlier, when dealing with existing buildings there are certainly plenty of unknowns above grade. It all depends on how much information you have about the existing building during the design process. Original documentation, an extensive probe package, in many cases non-destructive testing all contribute to a better understanding of the condition of the existing building and will mitigate risks once construction starts. But in my experience they will still exist more so than they would with new construction.
Q. When converting a Class B or Class C office building that is all steel, is there flexibility to enhance the design through structural changes?
A. There will always be opportunities for creativity, and the existence of more constraints only accentuates the need to be more creative in solving those problems. Whatever the structural system of the existing building, there will be opportunities to utilize it in a sensible, efficient way. How you approach that – the tools in your toolbelt – will be different with a steel-framed structure vs. a cast-in-place concrete building vs. a building constructed from an antiquated system like slag block. The creative approach with an existing building or with specific structural systems may differ from the creative approach for a new building, but it’s every bit as important if not more so.
Q. Structurally, what makes office-to-residential conversions worth the investment in cost and labor?
A. I think it’s probably well-known at this point that the environmental impact of a renovation is almost always smaller than that for new construction. So from a sustainability perspective, there is a ton of value in re-using these commercial buildings for residences in addition to the points made earlier regarding the risks associated with foundation construction in a congested and historic city like New York that represents bottom line value to developers. I know the numbers won’t work out in every case, but I have to think there are dozens if not hundreds of candidates across the City.
Q. Do you see office-to-residential conversion as a net-positive for New York City?
A. I think it certainly has the potential to be an enormous positive for the City. In addition to the sustainability issues noted above, it would be amazing to see these conversions take on the housing issues that have been present in New York City for a very long time. I understand that the economics need to work from the developers’ side – in addition to the regulatory hurdles that need to be overcome – in order to move forward with such a conversion. Numerous variables need to align for it to make sense, and these have to be assessed on a case-by-case basis. But there seems to be such a high, positive upside when everything falls in place.
