Historical Renovation at Fort Leavenworth
At the “Intellectual Center of the Army,” a first-of-its-kind approach was employed to reinforce Building 427, a nearly 100-year-old structure selected for adaptive re-use as office space.
By David Nordin
BEFORE: Building 427 at Fort Leavenworth, Kansas, is a nearly 100-year-old structure that required significant renovations and structural reinforcement.
Photo courtesy Satterfield & Pontikes Construction
At Fort Leavenworth, Kan., the renovation of a nearly 100-year-old structure led to development of an ambitious, first-of-its-kind process to reinforce the structure of historic buildings—a method that expands the parameters of traditional coring and reinforcement methods in both size and depth.
Building 427 at Fort Leavenworth spans 22,500-ft², and was built in the 1920s as housing for military families. Now listed on the National Register of Historic Places, the building had been recently chosen to be renovated for adaptive re-use as office space. The scope of work included whole building renovations and modifications. This required incorporating strict preservation guidelines for historical buildings as well as Department of Defense (DOD) antiterrorism/force protection and progressive collapse design requirements.
Building 427 is a classic load-bearing masonry structure. It is four stories high with a sub-basement. The walls at the parapet and upper floors are approximately three bricks in width. On the first and second floors, wall thickness is 12-in to 18-in. In the basement, the solid masonry walls are 2-ft to 3-ft in width. As a building rises progressively higher, the loads get higher and the brick walls beneath have to be wider. In addition, the brick walls had no tie-back. The veneer was simply attached to the core with mortar.
The work at Fort Leavenworth presented two primary design components that offered significant challenges:
- Military base requirement for progressive collapse mitigation.
- Limited reinforcement options due to historical preservation requirements.
Significant challenges were defined by the coring method and materials used as well as risk to subcontractors. The difficulty was compounded further by several factors: the likely presence of unknown materials in the walls; the fact that there were no similar previous solutions from which to draw ideas; and most importantly, because subcontractors were reluctant to tackle the job.
Modifying a nearly 100-year-old building presents numerous risks to the structure and to those who perform the procedure. Because the reinforcement process was beyond typical coring and tensioning parameters, subcontractors were reluctant to participate because they would be responsible for damage the process might cause.
In addition, no information existed to describe what might be found inside the walls. The drawings showed only a thick, black line denoting ‘”wall.” Subcontractors had no way of knowing what they might be liable for. Coupled with the building’s age and some walls listing up to 2-in, nobody wanted to assume the risk.
AFTER: Building 427 received extensive reinforcement to meet Department of Defense requirements, leaving no visible signs of the work performed.
Photo courtesy Satterfield & Pontikes Construction
Engineering specifications required dry coring. Typically, dry coring is performed only to a depth of about 20-ft with a maximum 2½-in-diameter core. However, this project specified a 4-in core of 35-ft to 40-ft. In researching possible solutions, the project team discovered that a 4-in dry core apparently had never been performed before.
Adding to the complexity was the requirement that post-tensioning rods were to be made of stainless steel. Ferrous metal inside the masonry assemblies was not allowed because of the possibility of corrosion. But stainless steel reinforcement rods are not only unusual for this type of procedure (stainless steel is generally not used because it is somewhat brittle and can break), they are also very difficult to procure. And Building 427 required stainless steel with ductility, which means it has the ability to bend without breaking. This type of material has an annealing step in the manufacturing process to give it ductile characteristics that are not normal for the material.
The problem to be solved required successfully integrating strict preservation guidelines (those requiring that a historic building retain its original appearance) with DOD regulations (those requiring progressive collapse protection). In order to satisfy both seismic and DOD requirements, installation of a structural post-tensioning system was necessary to reinforce the building.
To successfully meet historic preservation guidelines, the reinforcement was performed completely inside the building’s masonry load-bearing walls so there would be no visible signs of alteration. Pouring concrete backup walls for reinforcement, as is sometimes done, was simply not an option. The solution included the installation of 28 post-tensioning (strengthening) rods to satisfy DOD specifications. The process involved drilling 28 holes in the building’s perimeter walls, from the roof down to the basement, and installing 20-mm stainless steel rods and large, 3-in-thick stainless steel plates to compress the masonry. The cores were 4-in in diameter and extended 35-ft to 40-ft in depth.
Hydraulic power produces the required tensioning force, which is transferred from the jack to the end anchor plate.
Photo courtesy David Nordin
The Macalloy S1030 post-tensioning rods were placed in grout socks and fitted with grout tubes. Grout socks minimize grout leakage into cavities in the masonry assembly. A hydraulic jack applied the required force in the rod, which was then transferred from the jack to the end anchor plate. Grout was injected into the cores, and grout tubes were removed through holes in modified end plates.
In conjunction with this procedure, helical wall ties were used to anchor the building facade to structural members. This procedure tied wall assemblies to the concrete slab while diaphragm strips hold the concrete slab together. All the components act to compress the structure and tighten it together.
PLAN IN ACTION
Satterfield & Pontikes Construction (S&P) served as the general contractor for the renovation of Building 427, and was instrumental in developing the process to reinforce the structure.
Throughout the work, S&P reduced subcontractor risk by dividing work into many separate smaller components—coring, materials and labor—to make bidding palatable to subcontractors. S&P took responsibility for coring and hired a specialist to perform that single task. Still, not a single subcontractor was willing to take on the remainder of the process. S&P next took on the separate task of locating and purchasing the stainless steel reinforcing rods. In addition, the firm had to hire specialized post-tensioning labor to install components.
S&P, the U.S. Army Corps of Engineers (USACE), and several consulting firms discussed various ways to tackle the coring issue. It was determined that wet coring was the best solution. USACE accepted the wet coring alternative, which resolved a significant challenge.
Coring bits, however, use a great deal of water. A plan to protect the building from water damage during the coring process had to be designed. After significant research of how to resolve the issue, experts were brought in to develop a process for adding weep holes and a vacuum system with negative pressure to draw the injected water from the wet coring to the outside of the building, thereby protecting the historic interior finishes.
Pressure is applied and measured for proper specifications for one of the 28 stainless steel vertical tensioning rods.
Photo courtesy David Nordin
Procuring stainless steel that performs to the specifications of this application proved difficult. For one, the material needed to have the ability to be tensioned tightly enough, but not so much that it breaks. Second, the stainless steel that would carry those specifications was not available in the United States. While the team was able to locate a manufacturer overseas, a long lead time impacted the project schedule.
S&P even purchased separate components of the post-tensioning system for better logistics and control. The stainless steel end plates connecting to the post-tensioning bars are large—2-ft long, 6-in wide and 3-in thick. These were purchased separately because they had to be modified to make the system work.
UNIQUE AND INNOVATIVE
The post-tensioning process employed at Fort Leavenworth is significant because the system is designed for reinforcement completely inside the walls.
This is a first-of-its-kind solution in several ways:
- Reinforcement was performed completely inside existing walls, with no visible signs of any work being performed.
- 4-in cores were wet-drilled to a depth of 35-ft to 40-ft. Typically, the normal approach would be dry coring with a maximum 2½-in diameter and 20-ft maximum depth.
- Reinforcement material had to be stainless steel. It was difficult to find the required material because stainless steel is somewhat brittle, and the material for this project had to have a ductile characteristic.
This innovative approach expands the parameters of traditional coring and reinforcement methods in both size and depth, while setting a new standard for construction methods. The process is applicable for preserving, reinforcing and extending the life of old or historical structures without damaging the facades or interior. It reinforces the structure with no major interior renovations and no visible signs of the post-tensioning system.
Although in this instance DOD required the use of stainless steel, similar results can be achieved with standard galvanized carbon steel encased in a greased sleeve to obtain the same Class 1 corrosion rating.
- S&P served as the general contractor for the project.
- Design professionals DWA Structural Engineers wrote the specifications along with other contracting consultants who work with DWA.
- Structural Preservation provided the experienced, skilled labor for installation.
- Williams Form Engineering provided plates and engineering observation of installation.
- Macalloy in the UK provided the stainless tensioning steel rods.
- KC Coring performed coring services for the post-tensioning system.
- Restoration Waterproofing provided the prep work for the system.
- USACE provided access to the design professionals and structural engineering and worked with S&P to approve substitute coring methods.