Reimagining Structures: Adaptive Re-Use in Practice
The Dynamics of Balancing Preservation with Innovation, from Structural Retrofits and Innovative Re-Use Strategies to Adaptive Design
We attended a session by David Farnsworth from Arup, where he described the structural process and challenges of converting an old (1950s) building into a residential building. More about the carbon savings by doing these conversions can be read in the ARUP report: https://www.arup.com/globalassets/downloads/insights/office-to-residential-conversions-the-carbon-story.pdf
One key aspect explained during the session was that the shape of those buildings that resemble a square wedding cake has big floor plates (postwar designs, when AC was invented), which, for residential layout, is not useful as the developers would like to maximize the area that has a window.
So, the solution Arup found in this case was to notch a portion of the “cake” and increase the windows' availability, thereby increasing the number of units on every level. Then the notched volume can be repurposed at the top or side of the building. You can read more about this topic in the following article: https://www.nytimes.com/interactive/2023/03/11/upshot/office-conversions.html
With these changes and additions, the structural process kicks off. There are several challenges during this process: old code used and re-assessment of the structure using current codes, new loading, stability, Wind connections, structure (beams, columns, and connections) strengthening, cinder slabs/pipe risers.
In this article, we will focus on the steel connections and steel structure retrofit.
“Wind connections”
The “wind” connections (Type 2 with wind) allowed in the AISC ASD 1989 provisions were a simplified solution: the connection is rigid enough to resist lateral loads but flexible enough to allow the beam to behave as simply supported under gravity loads, AKA partially restrained.
During the analysis of existing buildings, structural engineers found this typical detail constructed with WTs or angle sections in the top and bottom flanges of the beam and a simple shear connection in the web connected with rivets.
To make structural changes, the structural engineer needs to assess the structure's current capacity. An investigation of the building is performed, and using modern tools, companies are modeling the existing conditions in global structural analysis software and running analyses on them to learn the capacity.
However, because the connections are partially restrained, the beam ends (beam end releases) cannot be modeled as pinned or fixed in the structural software. The solution is to input the real rotational stiffness of the existing connection.
In the current AISC Specification, the commentary notes that partially restrained connections' moment-rotation curves can be found in several databases (Goverdhan, 1983; Ang and Morris, 1984; Nethercot, 1985; Kishi and Chen, 1986). The main issue is finding the database that matches the exact connection sizes or conditions you have on site.
The alternative in this case is either to calculate the stiffness using component modeling (based in the Eurocode hand calculations) or use finite element analysis software.
IDEA StatiCa has the ability to model and design steel connections using CBFEM based on international codes like AISC. Apart from structural design, the software includes advanced analysis like: Capacity design for seismic conditions, linear buckling analysis, joint design resistance to learn the maximum capacity of the connection, and stiffness analysis which classifies the connection based on design codes and calculates the moment-rotation diagram.
The software can provide a multi-linear or linear spring value to characterize the end condition of the beam in commercial global analysis software.
From some user experiences, the software have been used for structural analysis assessment of existing structures like the process is followed to convert an old office building to a modern residential building in New York City.
Also, it has been used to confirm the assumption of the selected rigidity either pinned or restrained. For instance, a common case is when HSS connections are modeled, even if we are sure (based in the designer experience) that we have model a rigid connection, the stiffness of the support member is always debatable.
The existing condition of the connections are important to calculate the stiffness of the connections, to do that in IDEA StatiCa you can model rivets, historic cross section and materials, modify steel sections with openings and trims. After all the analysis the user can conclude that strengthening is required then the user can add weld reinforcement, add seated connections (WT’s reinforcement), and plate reinforcement. Read more here: https://www.ideastatica.com/blog/connection-design-rfis-retrofitting-existing-steel-connections
Wind and gravity beams reinforcement
As explained before, the stiffness analysis of the connection is part of the structural process, but once they obtain this data from software like IDEA StatiCa, the engineers input the values in their global analysis software so they can run a comprehensive nonlinear analysis on the structure to get the current capacity of the entire structure.
Later, they need to modify the structural model to represent what will be the new structure: remove part of the structure due to the notches or volume extraction, increase of loads due to the structural code changes, addition of levels, etc. Once they modify the model using most of the current structure, the engineers can find that some of the beams and columns needs to be reinforced.
That can be adding reinforcement at the end/start of the beams for moment frames, or reinforcement in the middle of the beam for gravity frames.
One solution that IDEA StatiCa provides to their steel users, is the ability to analyze and design beam Members using FEA. Instead of using 1D beam elements as most of the software uses, the Member application models the beams as shell elements. That allow the user to model openings in the member plates and reinforcement like welded/bolted plates or cross sections. Learn more about: https://www.ideastatica.com/case-studies/26-story-office-tower-transformation-project
Additionally, in the Member application, instead of modeling the end condition of the beam as pinned or rigid, the connection is modeled explicitly. No back and forth need, the calculation of the moment and shear diagrams will be accurate considering the real stiffness of the frame.