The Witte Museum, founded in the northern outskirts of San Antonio in 1926, recently received a $100 million upgrade. The Witte campus rests in a part of Texas where evidence of human habitation dates back longer than 11,000 years, making it a fitting site for the city’s museum of natural history. San Antonio has grown from 160,000 to 1.2 million people since the 1920s and the museum is no longer on the outskirts. With its now-prominent location between Broadway Street and the San Antonio River in historic Brackenridge Park, the Witte serves a constant and integral cultural role.
Led by Museum President Marise McDermott, designed by Lake|Flato Architects, and built by Linbeck, the campus makeover met the museum’s three most critical needs. It included the addition of a new building to host special events and changing exhibits, provided reconfigured galleries and interior spaces, and added a large entry and exhibit hall.
Mays Family Center: Whales and ballrooms
The first of the three components of the project to be competed was the 16,500-square-foot, single-story Mays Family Center building, located at the north end of the campus. This facility required an incredibly flexible space, and the variety of uses posed a host of interesting design challenges. The Mays Center was to serve as a home for traveling museum exhibits for a portion of the year and as a ballroom for special events and indoor receptions between exhibits. Lake|Flato’s solution to this programing challenge resulted in a multipurpose room with a 70-foot by 131-foot column-free space, flanked on the north and west by trellised exterior circulation galleries.
To aid in the flexibility of the space, Datum Engineers recommended designing the ceiling for an array of potential hanging loads to accommodate diverse and changing exhibits, in addition to a uniform 10-psf ceiling load. The coordinated array consists of small, 360-pound concentrated loads at a frequent spacing, coupled with an 1,800-pound point load, which permitted a wider spacing. This hanging load allowance was soon utilized with the hosting one of the first changing exhibits — Whales, which called for several small whale skeletons and a full-grown male Sperm whale to be partially suspended from the roof structure.
The roof over the multipurpose room was also designed to support a 33,000-pound mechanical unit and an associated concrete pad on steel form deck. The concrete pad provides both structural support and noise dampening. The final criteria for the roof was a building height limitation that set an available structural depth at 6 feet. The obvious solution for this clear-span space was a series of roof trusses.
Early in the design phase, the team discussed a common architectural language for the campus additions and decided to integrate wood into the key architecturally exposed portions of the structure. Datum’s response for the Mays Center roof was to include vertical wood compression members in an otherwise steel Pratt truss. Datum also recommended pairing the chord members to accommodate and conceal electrical conduit for lighting fixtures integrated in the truss.
The final roof solution included a 1-1/2-inch steel roof deck supported on 8-inch-wide-flange purlins, spanning 15 feet between trusses. The trusses consist of 10-inch by10-inch wood timber verticals, paired with 8-inch steel channel chords and steel rods for both the truss diagonals and the truss bridging. The truss configuration allowed the design team to open up the space and make room for the suspended wood ceiling panels, lighting, and ductwork.
Consistently throughout the project, the roofing system transitions from steel metal of roof deck over interior spaces to exposed wood tongue-and-groove roof decking outside. Thermal breaks are provided in all the steel members crossing through the building envelope to address thermal bridging. In the case of the Mays building, this included the wide flange roof purlins and the top chord extensions of the truss.
The ground floor for the changing exhibits proved to be an equal challenge. Near the San Antonio River, most of the building’s footprint lies in the 100-year floodplain. The team decided to elevate the Mays Center’s ground floor above the floodplain, which will help reduce the potential for flooding of future campus structures. Datum worked to coordinate the floodplain cross section with the project’s civil engineer, Pape Dawson. In this process, pier sizes and shapes as well as those portions of the building that would have to remain below the floodplain were minimized.
The program also required lateral circulation between the Mays Center and an adjacent existing building, setting an invariable finished floor elevation. Subtracting the floodplain limitation and required under-floor insulation and finish left only 10 inches for the ground floor structural system. Datum proposed a 10-inch, two-way concrete slab supported on 24-inch and 30-inch drilled concrete shafts. Linbeck cleverly integrated the under-floor insulation into the flat-slab formwork to reduce post-construction labor.
The ground floor slab was designed for 150 psf to accommodate the most intense uses required by the changing exhibits. Designs also incorporated locations for electrical floor boxes arrayed throughout the changing exhibits room, as well as recesses and drains for bathroom, kitchen, and lobby flooring.
The floor for the exterior open porch was subject to the same overall depth limitation. As the porch was elevated significantly above adjacent grades, the team transitioned to a hardwood (Massaranduba) floor deck supported on wide flange steel beams and paired-channel girders for a visually appealing exposed structure.
As the Mays Center progressed, work began on the renovations and additions to the existing facilities.
The 37,000 square feet of renovations required seamless integration of the architectural spaces and mechanical services across multiple generations of building additions, including demolition of the northernmost wing of the original 1926 building and removal or strengthening of existing structural components.
The initial hurdle for the renovation work was gaining a reasonable understanding of the existing conditions. Early in the schematic design phase, arrangements were made for after-hours investigations into the Witte structures. Datum staff crept through the length of the building’s accessible crawl space, explored the attic, and chipped into the plaster soffits of floors through ports cut in the plaster ceilings.
The key finding was a structural system (clay-tile formed ribbed concrete floors) that Datum’s staff recognized from renovations to a local circa 1930s San Antonio high school. The team even found a concrete slab cast on draped, wire-reinforced synthetic fabric. Further research into the ribbed slabs led to recovery of a portion of the existing construction documents from a structural firm doing work from the 1920s through the 1960s on the Witte campus. With the help of these drawings, it was possible to illustrate (and date) most of the previous phases of work and to understand the extent of expansion joints and load paths within the building.
The proposed exhibit designs included a large space on the ground floor to showcase the museum’s Texas Wildlife collections. Ideally, this space would be column-free, but its location within the existing plan put it at the intersection of three phases of prior building construction, which required outside corner columns to stay in place. Datum took on the task of removing as many of the other columns as structurally feasible. Working with the architects and exhibit designers, the team identified six (non-corner) removable columns.
Logically, elimination of these columns resulted in additional loads to the remaining foundations. Unfortunately, the existing building drawing set did not include foundation details. From experience with this era of construction in San Antonio, the Datum team expected the foundations to consist of deep spread footings supporting cast-in-place columns — an early version of a belled pier. Linbeck helped to expose a corner foundation on the museum once the adjacent building was demolished, confirming this foundation type; but the existing conditions complicated the discovery of reinforcing patterns in the foundation and further study was abandoned. Understanding the geometry was enough, and Datum recommended supplementing the remaining column footings to account for the increased loading, and provided detailing for new battered helical piers and pier caps to be installed through small holes in the existing ground floor framing.
Renovation to the second floor and roof were considerably less complicated, though not without challenges. In addition to bridging the abandoned columns with steel reinforcing, directions were provided to trim as much as 12 inches from the bottoms of the second-floor concrete beams to increase space for lighting and ductwork. Reinforcing was also provided around a new, large circular hole in the second floor.
One of the most memorable tasks was repurposing an existing riveted roof truss. This truss spanned a single-story wing of the museum in the initial 1926 construction and was removed and reused when a second story was added in the 1960s. Although moved only a few feet north of its prior position, this was the third use of the same truss in the building.
Jim Michel of Project Control and the Witte’s project manager said, “The rehabilitation of the [existing] space was an enormous task. Datum’s engineers proved their worth from the forensic investigation through the final structural reinforcement of the multiple construction types presented by the composite building types. The removal of columns — reducing the beam depths — creating an Oculus hole in the concrete slab are worthy of their own exhibit.”
Susan Naylor Center: Dinosaurs and dresses
Following demolition of the original north wing and during interior renovations, Linbeck began work on the main building addition, the Susan Naylor Center. This addition provides a 26,000-square-foot lateral expansion to the existing facilities and adds classrooms, shops, display galleries, and two separate two-story volumes — an indoor/outdoor exhibit area and the Valero Great Hall.
The primary space of this addition is the dinosaur-filled Valero Hall. The architect wanted to introduce significant, albeit controlled, daylight into this space. Their designs included a south-facing monitor at the roof level and faceted walls on the north side. As mentioned, the team wanted to incorporate wood into the exposed structural elements, resulting in glue-laminated heavy timber roof decking over steel beams and tapered glue-laminated wood king-post roof trusses. Also, similar to the changing exhibits building, the top chord member is a pair of steel channels to conceal electrical and fire sprinkler conduit, with steel rod tension chord and bridging. The final truss connection detailing may look simple, but it is the result of a number of cycles of suggested and tested arrangements.
Additional second-floor framing along the east façade was necessary, in part to help the Witte showcase a larger portion of its collection of several hundred donated Fiesta royalty gowns from San Antonio’s annual Battle of Flowers parade. That floor framing was executed in composite steel with varying specifications of architecturally exposed structural steel. To match the existing construction and reduce the impact of expansive soils, the ground floor system was designed as a two-way concrete slab, cast on cardboard forms and resting on belled piers.
The addition wraps around and into the west courtyard, where it transitions into a two-story screened porch enveloping an ancient Cypress tree. The team crafted designs for a wood-decked steel roof, a composite steel second floor, and a ground floor framing of heavy timber decking on steel beams around the tree’s trunk. At the foundation, the team’s precautionary measures for the tree included exposing the near-surface roots during construction and obtaining an arborist’s opinion of acceptable excavations. Following those guidelines, the drawings were revised, moving a new pier further from the tree, shallowing up the perimeter concrete and steel floor beams, and cantilevering over larger roots.
For the approach to the building from the south, and also for the entry into the great hall from Broadway Street and from the Witte’s west courtyard, something special was in order. For these three entries, the design team wanted to introduce V-shaped pairs of wood columns with deep trellised wood-decked overhangs. Lake|Flato’s concept designs show the columns terminating at the bottom of wide-flange girders. Options were reviewed, including one making the column and beam centroids coincident, but the team determined that the visual effect was not as dramatic.
In keeping with the exposed structures elsewhere in the museum, Datum recommended introducing offset steel tie rods to take the thrust from the columns. Further study on the wood columns proved a glued-laminated, tapered member to be ideal. Due to the proximity of museum visitors, the connection detailing here was even more critical than the Great Hall trusses, and Datum’s engineers worked closely with Lake|Flato to provide clean knife-plate reinforced pin connections.
The Witte Museum has always been a cultural anchor for San Antonio, but the reimagined spaces have put it on par with the top-caliber natural history museums in the country. Kim Biffle, chief of engagement at the Witte, said that the city and local residents have shown a new level of pride and ownership in their museum. This is evidenced in the increased attendance and consistent flow of requests for the banquet spaces and courtyards.
Furthermore, the new spaces are allowing the Witte to host the premier event of a changing exhibit for the first time in the museum’s history, and to be the centerpiece of exhibits for “Confluence in Culture,” an exhibit showcasing 300 years of history coinciding with San Antonio’s upcoming tricentennial.
Larry Rickels, P.E., served as principal in charge of the structural team and managed the renovations to the existing structures for Datum Engineers Inc. (www.datumengineers.com); Datum’s Tim Stocks, P.E., managed the enhancements to the Susan Naylor Center; and Craig Rios, P.E., president of Datum Rios, managed the structural designs for the Mays Family Center.