Comment> History Lesson.
Dana M. Cohen and Kevin L. Poulin. The Architect’s Newspaper.
Two structural engineers explain what they learned while preserving historic masonry.
Although New York City is continually evolving, its architecture remains a testament to its rich history. For example, townhouses built from the Gilded Age to the Roaring Twenties evoke lavish lifestyles of luxury. Moreover, ornate terra-cotta façades are a tribute to the Renaissance Revival style, inspired by the Italian Renaissance and characterized by classical details; these façades are the by-products of advancements in travel and technology that allowed architects to see Italian architecture first-hand and document it using photography to replicate later.
As structural engineers, we are often asked to upgrade or repair historic buildings while minimizing alterations to the façade. Balancing these competing interests, we recently completed retrofits of several historic masonry buildings in New York City. This article presents four lessons learned from our experiences that can help you with similar projects. Although it is not a comprehensive checklist for masonry façade renovation work, this article provides guidance based on our experiences.
Make Conservative Assumptions
Material testing allows us to quantify the strength of existing masonry. However, often this testing is too costly, too time-consuming, or too invasive. If testing is not feasible, design professionals should assume that façade masonry has limited strength based on age, type, and condition, as determined from a visual survey.
As a point of reference, standard practice assumes that early-twentieth-century brick masonry in New York City in fair-to-good condition has an allowable compressive strength of 225 psi to 250 psi, and an allowable flexural tensile strength of 1 psi to 5 psi (normal to bed joints). The allowable compressive strength roughly corresponds to a relatively low compressive strength for the masonry assembly (brick and mortar) of 1,000 psi. The allowable flexural tensile strength is a nominal value that reflects the minimal tensile capacity of the existing assembly. These values contrast with the corresponding minimum values for new brick masonry, which has an allowable compressive strength of 625 psi and an allowable flexural tensile strength of 20 psi to 30 psi, as specified in the 2011 Building Code Requirements and Specification for Masonry Structures.
Existing building codes, including the New York City building code, typically provide little-to-no guidance on allowable stresses for old masonry. Usually, design assumptions are based on the engineer’s experience, and therefore it is important to engage an engineer that has a good working knowledge of the subject of masonry. The properties of masonry are dependent on the locally available materials and, therefore, can vary by region. Thus, the engineer should have experience not only with the type of masonry used (brick, terra-cotta, etc.), but also experience with the local varieties of the specific masonry type used. Furthermore, the engineer should have experience assessing the condition of existing masonry construction.
Field-Test Masonry Anchors
Anchor strengths in the field can vary significantly from the manufacturer’s published values. Whereas the manufacturer’s values are based on installations into new masonry in a laboratory setting, field strengths are highly dependent on the quality of the installation and the substrate.
A discrepancy between published and field-strength values occurred on one of our recent townhouse projects, in which field-testing showed tensile strengths into 80-year-old brick at approximately 25 percent of the published ultimate values. Although allowable values typically include a safety factor of four to five, on our project this safety factor was not enough to offset the effects of poor installation and a poor substrate. Thus, field-testing of anchors is critical, as emphasized by the recent New York City mandate to test a representative sampling of each size and type of post-installed masonry anchor installed on a project.
The current industry standard is to only perform pull tests in the field. This is because shear testing is logistically more difficult to perform. The New York City mandate, referenced above, requires that post-installed masonry anchors be pull-tested to twice the allowable load listed in the applicable evaluation reports, such as those by the International Code Council Evaluation Service (ICC-ES). While shear strength cannot be correlated with tensile strength, good tensile strength is typically an indicator of quality workmanship and a sound substrate, and by extension good shear strength.
Field-testing of masonry anchors is neither time-consuming nor expensive; it can be completed in a day and is often performed free of charge by the manufacturer. However, testing must be discussed early on with the contractor so that he can accommodate it in the schedule and can complete it before the anchor design is finalized. Lastly, a statistically significant sample of each size and type of anchor must be tested to ensure that the data is meaningful.
Replacement in Kind?
Replacement in kind is common when existing masonry is damaged to a degree that it cannot be repaired. Even with extensive mock-ups, it is difficult to create a seamless transition between new and existing masonry. This is especially true for old brick when the original brick is no longer produced, either because the manufacturer no longer exists or because the clay source is gone. The design team must be cognizant of the potential for variations in color and texture, even when reusing old brick or creating terra-cotta replicas. The aesthetic impact of these potential variations must be conveyed to the owner, and ownership and the design team must collectively define permissible tolerances.
On one of our recent façade-restoration projects, the owner was willing to accept larger discrepancies in the color and texture of the terra-cotta blocks at locations higher on the building where the blocks were less visible. We worked with the owner to identify these locations, and we used all new masonry at the most visible areas to ensure consistency. In areas where salvaged terra-cotta blocks were to be reinstalled, we specified an extensive cleaning procedure for the salvaged blocks. By cleaning the blocks, we were able to mute some of the existing staining and make the block coloring more even. Matching the color of the terra-cotta replicas to the even coloring of the cleaned blocks proved to be much easier than matching the original, splotchy coloring of the weathered blocks.
Verify in Field
Field verification is crucial when working with existing buildings. Multiple renovations can render the original design drawings ineffectual. Moreover, original drawings are often not available. Unfortunately, contractors often submit shop drawings without field-verified dimensions, with the intent to finalize dimensions during construction. However, for a successful project the design team, the contractor, and ownership must be active participants. The contractor must verify dimensions during the shop drawing process, and the design team must enforce this requirement. In addition, the design team must react to unforeseen field conditions quickly so as not to affect the project schedule.
Sometimes there are bumps in the road. We have had projects in which the contractor did not field-verify critical façade dimensions when developing shop drawings. These oversights resulted in misaligned structural elements.
On one recent project, we designed a new moment frame to provide additional lateral support for the rear masonry façade of an existing townhouse building. Because existing obstructions were not investigated during the layout of the moment frame, the locations of the frame and the supporting concrete piers/foundations were offset by several inches, requiring a rapid redesign during construction. On another project, we designed new steel supports for a terra-cotta cornice consisting of flat channels hung from tube steel outriggers; the cornice was to be hung by threaded rods from the channels. The cornice was not properly dimensioned by the contractor prior to fabrication and installation of the support steel. Consequently the installed channels were too short to support the last cornice block. Moreover, some of the threaded rods were not fabricated long enough to extend through the channels.
Resolving these issues required the respective contractors to remove previously-installed work and necessitated some redesign effort, which was costly and time-consuming in all cases. The impacts of unverified dimensions are not always this severe, but it is vital to emphasize to ownership that the contractor must field-verify dimensions prior to fabrication and construction.
A masonry façade is long-lasting and durable, and is therefore an excellent medium for preserving a piece of the local history from the time the façade was constructed. But like any building component exposed to weather, these façades deteriorate over time. It is our responsibility to restore them in order to continue preserving the history that they embody. With the guidance provided above, we hope to make this a little less of a daunting task.
Dana M. Cohen is a Senior Staff II engineer at Simpson Gumpertz & Heger.
Kevin C. Poulin is an associate principal at Simpson Gumpertz & Heger.