By Michael D. Zajac, PE, LEED AP
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Designed by the firm of Cope and Stewardson in the American Beaux Arts style and dedicated as the District Building in 1908, the John A. Wilson Building currently serves as the city hall of Washington, D.C. The building is prominently situated along the south side of D.C.’s Pennsylvania Avenue corridor between the U.S. Capitol Building and the White House.
The structure of the Wilson Building is among early examples of so-called “fireproof” construction in Washington, which became standard practice for large commercial and important buildings following several disastrous fires in other cities around the turn of the 20th century. The structural floor systems were designed and constructed utilizing structural flat-tile-arch construction, which consisted of open-cell clay tile masonry unit blocks placed on temporary formwork and mortared into place. A lightweight cinder concrete topping slab was then placed, followed by sand-set terrazzo flooring, and a plaster ceiling finish along the underside. Although considered archaic compared to today’s construction materials and practices, such construction was common during the late 19th and early 20th century for commercial structures due to its fire resistance, lightweight, and shallow depth. At the Wilson Building, wrought iron beams were specified to support the clay tile structure of the office suites at each floor; the flat tile arches of the east and west corridors of the building bears upon the brick masonry corridor walls.
Problem Discovered
While visiting the building to investigate an unrelated structural scope item, engineers from Simpson Gumpertz & Heger (SGH) noted an uncharacteristic sag of several corridor floors, most notably on the 5th floor of the building. SGH noted subtle hairline cracking patterns in the plaster ceiling finish at the underside of the sagging slabs running parallel with and located approximately at the centerline of the corridor. At the south ends of each corridor, above an acoustic dropped ceiling, investigators found a prominent crack approximately 1-inch wide extending nearly the full length of the suspended ceiling area.
Cause of Damage
In order to determine the cause of the noted damage, SGH began researching various structural clay tile reference sources. Discoveries indicated that structural clay tile floor systems were specified by first taking into consideration the overall span of the floor system followed by determining the required load capacity, then a minimum clay tile depth was selected to satisfy the load and span requirement. Although significant redundancy and safety factors (sometimes as high as 7) are associated with structural clay tile floor assemblies, the team discovered that structural clay tile design guides and specifications at the time generally did not recommend 12-inch deep clay tile for spans exceeding 8 feet regardless of required load capacity. The corridor widths at the Wilson Building are on the order of 10 feet.
Another factor which likely contributed to the damage relates to the August 23, 2011, earthquake experienced in the region. Although the characteristic corridor floor damages were observed at five of the six corridor floors framed with structural clay tiles, the most pronounced floor sag was observed at the top (5th) floor corridor slabs, where 1-1/2 inches of slab deflection was measured. Similar to other heavy stone masonry structures in the region, including the Washington Monument and Washington National Cathedral, the Wilson Building may have experienced significant movement and racking during the earthquake, disrupting the compressive force load path of the structural flat-tile-arch corridor floor systems. No information was available regarding the condition of the corridor floors prior to the August 2011 earthquake event.
Conventional Repair Solutions Considered
To address the structural damage to the clay tile floor structure, the following conventional structural repair options were considered.
Option 1: Removal and Replacement. This option included full demolition of structural clay tile floor slabs followed by replacement with either new cast-in-place reinforced concrete floor slab construction, or new composite slab on metal deck construction. This solution would have required vacating large areas of the building for an extended period. In addition, the historic plaster crown molding would have been lost.
Option 2: Reinforcement. This option was characterized by installing a new supplementary structure beneath the existing structural clay tile floor system. Such a structural system could have consisted of a steel beam framework supported on the masonry bearing walls of the corridor and shimmed tight to the underside of the existing clay tile floor assembly above, or installation of a new concrete slab directly beneath the clay tile floor system. The new concrete construction could have been engineered to carry the dead loads of the original floor slab, as well as the code-prescribed superimposed live loads. Similar to Option 1, this repair had the convenience of straightforward and conventional engineering, detailing and construction, but would not necessarily have required vacating various floors of the building during construction. However, the historic plaster crown molding would be lost.
The Modern-Day Solution
Given the cost, disruption, and loss of historic fabric associated with the conventional repair options, the design team elected to consider a carbon fiber reinforced polymer (CFRP) reinforcement solution. CFRP has been commonly associated with successful repair and strengthening of concrete structures for many years, but little information exists with respect to reinforcing structural clay tile flat arch floor systems. Whereas the original structural clay tile relied upon compressive forces offered by flat tile arch construction to carry dead and live loads, a CFRP system would change existing floor structure into a one-way reinforced clay tile and concrete slab, with tension forces carried by the CFRP bonded along the underside of the structural clay tile, and the concrete topping serving as the compression block. Since there was no indication of shear distress or shear failure of the slab-to-wall bearing, the CFRP could stop short of the bearing points, preserving the historic plaster crown molding.
The existing structural clay tile flat arch floors consisted of multiple parallel rows of 12-inch square by 12-inch deep clay tile units. The repair specified a continuous strip of 6-inch-wide CFRP bonded along the centerline of each 12-inch-wide row of clay tiles.
The CFRP system offered the following benefits:
- Occupancy: Vacating the spaces of the building served by the corridors would not be required.
- Cost savings: Compared to the conventional demolish-and-replace repair options, CFRP would offer significant savings.
- Retention of historic fabric: The existing structural clay tile floor system, original terrazzo floor finish, and historic plaster crown molding could all remain.
Although the CFRP-reinforced terra cotta floor slab construction is not part of any fire-rated UL assembly, the design assumed that the existing condition of the damaged floor assembly was suitable to support the self-weight of the existing floor system in addition to nominal live loads.
Execution
The repair work began with installation of temporary wood planking work platforms integrated with conventional metal shoring frames and supplemental shore posts extending down multiple floors to the basement slab on grade. The generous floor-to-ceiling heights of the building permitted the shoring frame system to incorporate the elevated work platforms above each corridor so the workspace could be fully enclosed to allow the corridors to remain open for daily use and egress. In addition, installation of the shoring system utilized screw jacks with wood blocking to engage the vertical shore posts into the underside of each slab above, installed in “snug tight” manner. This installation technique was critical to avoid heaving of the slab and to maintain the compression load paths of the structural clay tile arch floor system. Temporary corridor lighting was installed along the underside of the work platform, along with protective barriers to prevent building occupants from accessing the work areas.
With access to the work area completed, the repair team began executing plaster removal by mechanical tools with careful means and methods to prevent significant damage and abrasions to the base clay tiles. The substrate was then cleaned and prepared to receive the initial epoxy resin base coat followed by continuous CFRP reinforcement strips. The project included testing mockups and pull tests to confirm that the bond between the epoxy resin and clay tile substrate was sufficient to develop the CFRP. All mockups successfully satisfied the project specification.
Where existing features such as hanging fixtures and electrical items prevented centering of the CFRP reinforcement along the centerline of the clay tile row, the CFRP was offset as field conditions permitted. At areas where the clay tile was extensively damaged, CFRP strips were placed side by side to provide full coverage of the underside of the clay tile floor slab. No attempts were made to lift the slab back into a more level profile as the structural engineering team anticipated difficulty achieving success and the risk of opening tension cracking in the concrete topping and disrupting the compression block load path, in addition to damaging the original historic terrazzo floor finish, was high. In addition, no leveling compounds were placed on top of the slab. The original terrazzo floor finish remained present and unaltered under the existing carpet finish.
As is often common with historic structural clay tile floor systems, several unforeseen conditions arose during execution of the work, including discovery of broken tiles and miscellaneous voids along the underside of the floor slab. Since the CFRP system required a continuous substrate along the underside of the floor slab, the the voids of broken clay tiles were infilled with a cementitious overhead patching mortar prior to application of the epoxy coating and CFRP reinforcement.
Final Finish
Two final finishes were specified as part of the work.
Type 1: At areas of exposed plaster ceiling and crown molding, a new plaster finish followed by a finish coat of paint was installed. In order to ensure proper mechanical bond of the plaster coating to the repaired substrate, the CFRP reinforcement was coated with a secondary layer of epoxy embedded with sand. Adhesion testing of the new plaster coat on the cured epoxy repair material indicated a bond even better than the original plaster coating on the underside of the original structural clay tile floor system.
Type 2: At the suspended ceiling areas, much of the original plaster ceiling had already been penetrated with various mechanical and electrical connections prior to execution of the work. Following the completion of the work, these areas would remain concealed above the suspended ceiling bulkhead below. Therefore, the owner elected not to reapply plaster finish at the concealed condition. Instead, a water-based intumescent coating was applied to provide a layer of fire protection for the CFRP. In order to protect the CFRP strips from future damage during future mechanical/electrical/plumbing work, “DO NOT CUT” stenciling was applied.
Closing
Applying a strengthening technique usually associated with concrete structures ultimately resulted in a clever and successful solution that saved time, money, and historic building fabric while allowing the building to remain occupied during construction, with no obvious visible changes to the spaces at the completion of the project. ■
About the Author
Michael D. Zajac, PE, LEED AP, is a Senior Project Manager in the Washington, D.C., office of Simpson Gumpertz & Heger. (mdzajac@sgh.com)