In the western gateway to downtown Des Moines, Iowa, resides the Des Moines Public Library. The David Chipperfield creation replaced the existing 100-year-old facility with more than a shelter for books. This $32 million, 140,000-square-foot project acts as a community center with a wide range of educational services, and houses a conference centre, extensive youth and children’s facilities, and commercial space.
The architect envisioned a structure with neat, clean lines. The vision became reality with concrete. Due to its inherent fire resistance, additional fireproofing was not required and allowed for the aesthetically pleasing exposed flat plates and round columns to support the unique and complex layout. With the public space and quiet environment of a library, the structural design team favored the thick slabs as they made it “easier to meet the vibration requirements because of their increased dampening characteristics over steel construction.”
With the utilization of exposed concrete slabs came the strict architectural criterion to keep the soffit flat. This became a great challenge for the structural engineer. The library live loads were significant and the raised floor construction, creating an access space for mechanical and electrical equipment, demanded a thicker flat slab which adds to the self weight of the structure. To safely account for the high punching shear at the support locations, the typical floor framing is comprised of a conventionally reinforced 18-inch thick flat plate (14-inches thick at roof), supported by 27-inch diameter interior columns and 14-inch diameter exterior columns. These floor slabs and columns are designed with 6,000 psi concrete strength to span the general 30-by-30-foot bay spacings. In order to resist the high punching shears, a cruciform-shaped arrangement of shear stirrups was designed and cast in the slab at each interior column intersection for all levels.
Constructability was another factor favoring concrete as the material of choice. The staging area was extremely limited within the congestion of downtown Des Moines traffic. Concrete’s local availability allows construction to proceed without stocking significant amounts of materials on site. To meet the stricter demands of an exposed concrete structure, pre-pour coordination meetings allowed the construction manager to make adjustments without delay to the schedule. These precautions helped ensure all form joints and embed plates were located correctly, since they have a visual impact on the structure.
Sustainability is featured in this signature structure with the implementation of green roof construction, the first of its kind for Des Moines. The function of the Green Roof Infrastructure provides many benefits to the building and the surrounding community. In addition to beautifying the built environment, there are intangible benefits as well. The manufactured microclimates have a noticeable impact on the heat gain/loss of a building, as well as the air quality, humidity and reflected heat in the surrounding neighborhood. With the evaporative cooling effects, air conditioning demands of the building are reduced, which in turn reduces the “Urban Heat Island Effect”. The plantings of the microclimate also capture particulate matter to slow the smog production rates in urban areas. Another global feature of Green Roofs is the stormwater runoff reduction. The water is retained by the substrate and taken up in the plants to return to the atmosphere in transpiration and evaporation, in lieu of taxing the community’s infrastructure.
To have a durable and low maintenance facility, the design team incorporated sustainability into the interior of the structure as well. The building’s cladding is a unique glazing system that uses a copper screen mesh sandwiched between panes of glass. This controls light intensity, reduces energy costs, and provides a warm copper glow in the evenings. Considering all of the project criteria, the design team determined since this is a public facility, we needed to use durable materials; concrete was an easy choice.”
Des Moines Library Foundation
David Chipperfield, London, England
HLKB, Des Moines, Iowa
Jane Wernick, London, England
Shuck-Britson, Inc., Des Moines, Iowa
Weitz Company, Des Moines, Iowa
Miron Construction, Cedar Rapids, Iowa
A1 Ready Mix, Des Moines, Iowa
Ambassador Steel, Newton, Iowa