Structural integrity critical when repurposing old buildings for new uses

December 11, 2019

Renovating existing buildings requires a thoughtful, tailored approach to structural engineering

 

By Stacey Brown

I have always thought of structural engineering as a sneaky profession. Structural engineers design the bones of a building, and often our best designs fade into the background. It is my responsibility as a structural engineer to analyze and design a stable structure capable of resisting all imposed loads, but my passion is creating and supporting impactful design. Sometimes, this means that I help my team breathe new life into a repurposed structure. I love the variety of structural conditions and challenges we see in existing buildings.

Building reuse can be as simple as swapping out rooftop equipment or as involved as stripping the building down until only its “bones” remain. If the owner is interested in increasing program area, we can design an addition adjacent to the renovated space. The myriad of design options reflects the complexity of the structural design.

It is important to have open discussions with clients early on when renovating existing structures. The first question a client always asks is whether it is feasible to reuse their building. If the structure is deteriorated or designed for much lower loads than envisioned for future use, it may be more cost-effective to demolish and start anew.

 

Strong Hall at Eastern Michigan University, where a 1950s academic building was renovated and expanded to meet today's educational needs.

 

If reuse is possible, we talk about the extent of repairs and strengthening. The client needs a rough estimate of the cost of bringing their structure in line with their vision. Rooftop amenities are a popular request that almost always require extensive renovations because existing roofs are designed for lighter loads. Having early discussions about the cost implications allows the client to make informed decisions before design is locked in. Once we develop scope, we can get into the “how” of renovating an existing building.

 

Design and materials have changed over the years

In order to work on any existing structure, I need to be an expert in historic as well as modern construction. Construction practices, technologies, and even the value of labor in relation to materials have changed over time. In the early 1900s, wood structures were constructed with wall studs that ran past floors from foundation to roof. My parents’ house was built like this. However, material availability and fire concerns led to modern construction.

In modern construction, wall studs span floor to floor and the floors are constructed with engineered wood products. If we are modifying a multifamily residential wood-framed building, I need to know when the building was built to get an idea of the details I need to provide and the materials I should be specifying for connecting new framing or repairing existing framing.

 

Wood-framed platform framing.

 

Material strengths have changed over time as well. When I look at steel-framed buildings that are 100 years old, I expect yield stresses to be about 70% of the stresses that I would design to today. Existing steel may be difficult to weld. Concrete and concrete-reinforcing steel have undergone similar strength increases. I need to use the right strength of material or I risk under-designing, putting occupants at risk. If I cannot find the material strength, the owner may need to perform destructive testing on the structure where a small sample of steel or small core of concrete is cut out of the structure and tested to failure.

 

Understanding restrictions on existing structures

Another part of what I do is help my team understand what restrictions the existing structure imposes on design. A one-way concrete slab is much simpler to modify and cut openings and core than a two-way concrete slab. One-way slabs span from beam to beam, making it possible to simply cut out an area of slab between beams (or even remove a whole beam width) without adversely impacting the system.

Two-way slabs rely on a complicated two-direction load path. Much like the ropes of a hammock, the load gets to the supports along multiple paths, and not all ropes take the same amount of load. For a two-way slab, the strips of slab running between columns are more heavily stressed than the center of the slab. A common structural modification like widening stairs will usually be fairly easy to achieve with a one-way slab because we can take out a beam and its associated slab, but the two-way slab is dependent on the behavior of an entire bay to carry load back to the columns. As a result, it is much harder to widen the opening unless we remove the whole bay or provide extensive strengthening.

 

The blue tape is reinforcement in the two-way concrete slab. In this situation, the small cores could not be shifted, and the floor needed to be checked for a reduced capacity.

 

_q_tweetable:When we reuse an existing building, we are working with a building that is meant to last. It is my job to be a steward of that structure._q_The differences between the two types of slabs also come up if the owner wants to add through-floor utility cores to an existing structure. When we reuse an existing building, we are working with a building that is meant to last. In many cities, we have old warehouses and industrial structures designed for heavy loads being repurposed as retail, workplace, kitchen, or residential—all of which represent a decrease in code-required loading. We want to be stewards of the structure and leave as much of that extra capacity in place as possible to make future needs easier to accommodate, which means avoiding cutting reinforcement when we can. With a one-way slab, we can limit our damage to an isolated strip. With a two-way slab, the reinforcement spacing changes between the middle and perimeter of the bay and reinforcement spans in two directions. It is densest near the columns where we often like to run vertical penetrations. All of this makes it harder to predict where exactly a bar will be and harder to thread our penetrations through openings between bars. The slab may need strengthening if the existing rebar cannot be avoided.

Strengthening can also be required if we add load to a structure. This commonly occurs when new topping slabs or equipment are added to the slab or when an office space is converted to a kitchen, laboratory, or public space. Equipment on a concrete housekeeping pad can easily exceed the capacity of an office floor. Adding a kitchen is another common occurrence that greatly increases anticipated floor loads. I can reinforce structures for these load increases, but the cost impact is going to vary widely with the type of structure and the amount of strengthening required. For large amounts of strengthening, we may even need to shore beams to remove the dead weight of the floor prior to performing repairs. Most strengthening we do will increase structural depth, so I always check in with the mechanical, electrical, and plumbing (MEP) team to avoid routing clashes.

To avoid strengthening an existing structure when constructing an adjacent new building, we need to make sure that we are not adding more load to the existing structure. At the base, we need to match the level of the existing foundations. We do this for two reasons:

  1. As load travels through the soil, it spreads out like a pyramid. If that pyramid runs into a basement wall, the basement wall is loaded by our new foundations and will likely need to be strengthened;
  2. We cannot control how the soil adjacent to an existing structure was infilled when the original building was constructed. If there are gaps in the compaction, the new building will settle and develop cracking.

Above ground, the new building must be separated enough from the existing building that it cannot push on the existing building when it moves with wind and temperature changes. Here in Michigan, we get snow. If the new building is taller than the existing building, additional snow will pile up on the existing roof near the new building. We need to strengthen the existing roof for those snowdrift loads or incorporate a low section into our addition design to collect all the drift.

 

Chipped down column requiring structural repair to restore capacity and protect exposed reinforcement.

Existing buildings can be full of surprises

And of course, I can do all the planning and measuring I want, and the conditions in the existing building can still manage to surprise me. We might find structural water damage from an undiscovered roof leak. Lintels that were assumed to be in good condition can be rusted and need replacement.

I worked on one building with concrete columns that weren’t quite plumb. During a prior renovation, a masonry wall had been built alongside the column. Rather than working with the existing concrete column, the column was chipped down to the reinforcement in some places in order to line it up with the wall. The whole assembly was then hidden behind wallboard. There was no way I could anticipate that condition!  

Building placement errors can occur when building an addition adjacent to an existing structure. When threading between two buildings, I encourage my team to leave a little extra room from the existing building to the column line to allow for unknown geometric conditions. If there is a long building with a slight discrepancy in true north or an assumed squared existing building that really has 89.9-degree corners, we can end up with clashes, structures that do not fit as they are supposed to, and costly field fixes.

The most successful reuse projects bring me to the table early in the design process to make sure that design stays feasible and cost effective. I use my experience with existing buildings to identify potential problems while the design is still being developed. Working with my colleagues and the client, I can help make sure we are presenting a strong design for the existing structure. If we are adding to an existing building, I can work with my team to mitigate the impact that the new structure has on the existing one. If you are working with an existing building, bringing in a structural engineer early on the project can inform and streamline the design.

There are so many things to think about when we inherit an existing structure. We are entrusted with a building that has already proven that it can stand the test of time. It is my job to be a steward of that structure by preserving, protecting, and repairing it so that it can be enjoyed by the next generation. And, when the renovation is done and the building is opened to the public, you won’t notice that I’ve done anything at all—if I’ve done my job right.

 

About the author

Stacey Brown is a structural engineer with more than 10 years of experience based in Detroit, Michigan. Stacey enjoys working with her team to identify and accommodate project needs—whether those needs are thermal isolation, large masonry openings, specialty equipment, or showpiece spaces.

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