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ReEngineering the Engineer: Making It Buildable

Douglas Fitzpatrick on May 5, 2023 - in Articles, Column

Through the years, I’ve noticed my firm does a lot of additions to existing buildings. In the early days, much of it was driven by our hospital work. Hospitals are constantly growing—expanding emergency departments, adding beds, increasing diagnostic capabilities, offering new services, etc. Most campuses we work on still have enough real estate that allows them to expand horizontally.

But even as our breadth of project types expanded, we still had to deal with expansions. Private schools add new wings for upper-level grades or growing populations. Churches add larger sanctuaries to replace or augment existing spaces, expand their education wings or add gathering spaces. Even warehouses add space for more storage or manufacturing area.

Tricky Connections

Although the new work typically is cut and dry, dealing with connections to and building against existing buildings can be tricky. They also can be great learning experiences for younger engineers. Wrapping their heads around typical greenfield design generally is easy, but throw in an existing building, and the challenges pop up quickly, especially if available drawings aren’t very good (or non-existent).

Although it’s tempting to focus on upper portions of the buildings, because we like to design from the top down, the foundations typically are the first place to start looking for problems and often can be a driver for the column grid. The architect is going to want columns right against the face of the existing building, but that’s typically not possible due to existing foundations.

Historically, we try to locate the first column grid 9-10 feet off the existing building. You can usually avoid conflicts between new and existing footings at that distance, and the cantilevers still are manageable. This usually allows new footings to be totally clear of the existing foundations.

If the existing footings are dropped for some reason, you may be able to get closer by cantilevering grade beams over/through new footings and above existing ones. Depending on previous conditions, shear and moment in the cantilevered grade beam can get away from you in a hurry, adding significant dollars to the foundation.

The new addition also may be a slab on grade, but the existing building might be a basement. Location and elevation of the new footings is a concern, so the new footings don’t surcharge the existing basement walls.

As Always, Be Prepared

Regardless, studying the available drawings early in design is necessary to get the column grid off on the right foot. There’s nothing worse than getting well into design for the upper portions of the building and then figuring out you have major problems at the foundations. It doesn’t look good to your client when you must backpedal, and project profitability is going to take a big hit.

Erection and formwork typically are the issues on the framed levels above. It’s easy to draw new framing close to the existing building for steel and concrete members. However, if the new building is steel framed, can someone physically get a hand in behind a bolted connection to install the bolts up against the existing face of the building? Are any welds called out against the existing building actually accessible? It’s easy to draw the arrow on the plan or detail, but someone in the field still has to be able to make it happen.

If it’s a concrete building, can they get formwork in place to form concrete beams and columns against the face of the existing building? We’ve used some one-sided forms before, but that’s typically not our go-to solution.

If the addition is against an existing flat exterior wall, dealing with the interface can be straightforward. However, if the exterior wall has ins and outs, it’s challenging to avoid problems in the field, especially if the owner isn’t willing to pony up for an accurate survey of the exterior wall.

Lessons From Experience

We found cantilevering the slab edge over to the existing wall generally is easier to accommodate variations in the field than having framing tight against the wall. If they run into fit-up issues in the field for the steel scheme, the bent plate typically can be modified much easier than a bunch of steel beams. Similarly, for the concrete scheme, the beams could be pulled off the building to allow a slab to cantilever to the existing building. The concrete guys can simply adjust the formwork for the slab portion.

Similar to the foundation early work, understanding the existing conditions on the upper levels could inform how the floor is framed. There are several ways a floor can be framed out, and one way may provide more flexibility to adapt to conditions in the field.

These things may all seem trivial to a senior engineer, but to a young engineer, they’re serious land mines. Dealing with the conflicts during construction is time consuming at best, but also may translate into real dollars due to delays or refabrication and could result in an unhappy client.

We all have to learn and expand our understanding of construction. Let the young engineers work through these existing conditions for a little bit on their own, but sit down with them and go through the challenges together. Help them understand the reasoning behind economical ways to deal with existing conditions. Giving them the answers isn’t particularly helpful. Explaining the thought process provides tangible insight that doesn’t just help with today’s problem, but arms them with some reasoning power that will be helpful for other challenges. 

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About Douglas Fitzpatrick

Douglas G. Fitzpatrick, P.E., is the founder, president and practicing engineer of Fitzpatrick Engineering Group, a 14-year-old structural engineering firm specializing in commercial and healthcare building design.

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