Highlights from this Month's e-Newsletter
Skip the Lightweight Concrete Topping with a Thicker All-Wood Floor System
Meet acoustic and fire performance standards, avoid delays
Lightweight concrete and gypcrete toppings are routinely used as a component of fire-rated floor assemblies to provide both fire and acoustic performance in multifamily and commercial construction. This extra step can be eliminated, however: All-wood floor systems can meet fire and acoustic performance requirements and speed up construction by eliminating delays associated with the installation of lightweight concrete or gypcrete topping.
In supporting the development of all-wood floor systems without lightweight gypcrete or concrete topping, APA – The Engineered Wood Association conducted a series of acoustic tests for code compliance based on sound transmission class (STC) and impact insulation class (IIC) ratings. The 2015 International Building Code (IBC) specifies a minimum STC rating of 50 for walls, partitions, and floor-ceiling assemblies between adjacent dwelling units, sleeping units, and adjacent public areas and a minimum IIC rating of 50. The floor assemblies tested by APA had STC and IIC ratings that met or exceeded IBC standards for multifamily residential and nonresidential buildings.
These thicker all-wood floors are stiffer than conventional light-frame floors. While typical light-frame floor construction employs a single layer of 23/32 Performance Category floor sheathing, the thicker all-wood floor systems use thick subfloor sheathing, such as a single layer of 1-1/8 Performance Category panel, or double layers, with a 19/32 Performance Category top layer over a 23/32 Performance Category base layer. The resulting floor is an enhanced assembly that performs above the code requirements satisfied by the typical light-frame assembly. Multiple assemblies were tested with different finish flooring, including cushioned vinyl, vinyl tile, quarry tile, hardwood flooring, and carpet and pad.
“In my region, the apartment market is very strong. I am also seeing more and more wood-framed hotels,” said Warren Hamrick, Engineered Wood Specialist. Hamrick has shared the all-wood systems with multiple large developers and design firms and notes that feedback has been largely positive, with this floor assembly making a good option for certain projects. "Having documented testing results showing the acoustic performance of this assembly with multiple floor coverings as well as the assembly’s fire rating has been key when presenting the information to the interested parties."
For more details on the floor assemblies and their test performance, download Technical Note: Acoustic Performance of All-Wood Floor Systems, Form T230
Warren Hamrick is an Engineered Wood Specialist for APA – The Engineered Wood Association, serving the southeastern United States. Warren conducts workshops and consults with designers, code officials, and other building professionals on best practices for specification, selection, and application of engineered wood products. Warren graduated from North Carolina State University with a Bachelor of Science in civil engineering. Prior to joining APA, he worked as a project manager for a structural engineering firm that specialized in commercial construction.
The Moorehead Apartments
Crystal Beach, Texas, is a gorgeous location, but there are very strict parameters for building there. Buildings are designed for extra resilience because they must withstand high coastal winds and deal with a corrosive marine environment that includes seawater floods. This means that all of the buildings in Crystal Beach must be raised at or above base flood elevations, which translates to a minimum of 17 feet above sea level.
Designers of the Moorehead Apartments, a 32-unit multifamily midrise structure, had their work cut out for them. The project engineer, Robert Dinjar and Chandra Franklin Womack, P.E. and President of Aran + Franklin, not only had to meet the stringent building code of Crystal Beach, but also plan for a few special requests from the building owner: leaving space below the building to accommodate a pool at ground level and designing an attic that can easily be converted into a third floor with leasable units at a later date. And money was an object: construction costs for the building needed to be as economical as possible.
To meet the combined goals of resiliency, durability, and cost-effectiveness for the 60- by 120-foot building, Dinjar selected wood products. The building stands on treated 12x12-inch southern pine timber pilings in combination with proprietary floor joists and preservative-treated stock glulam beams.
The first floor was raised an additional 4 feet above the required minimum elevation for two reasons: to give more headroom at the pool below and to give the owner a nice break on future flood insurance premium payments.
The entire exterior, as well as most of the interior, was sheathed with wood structural panels, creating strong, resilient diaphragm walls. To resist the corrosive nature of the marine environment, all exterior fasteners are either hot-dip galvanized or stainless steel.
One cost-saving measure Dinjar implemented was standardizing the sizing of the stock glulam beams as much as possible. Deeper 3-1/2-inch-wide treated glulam beams from APA member Anthony Forest Products made a strong yet economical substitute for shallower 5-1/4-inch-wide beams where appropriate. Easy to build with and readily available, using the deeper, more narrow glulam beams saved the owner an estimated 10 percent.
The roof design presented a challenge. Because the owner plans for future top-story expansion, the attic needed to be as open as possible, so the structure was designed with a tall gable roof—not a typical choice in a high-wind zone, but the project engineer made it work by beefing up the structure, using a double 2-x12 ridge tied with 18-gauge strapping, collar tie, and above-code rafter connections.
Creative problem-solving made it possible to achieve a clever design that met the client’s expectations while working within the constraints of strict parameters. Planning for long-term cost savings up front, such as the decision to add elevation height to mitigate insurance premiums, coupled with savvy material choices, such as the substitution of deeper glulam beams, satisfied both building code and budgetary requirements—not to mention the client.
Mid-Atlantic Wood Solutions Fair—Washington DC, August 29, 2017
Wood Solutions Fairs are free multi-faceted, daylong educational events on the use of wood in nonresidential and multifamily buildings. Attendees can earn up to 6 AIA/CES LUs (HSW) or PDH credits. Bob Kuserk, PE, will be presenting “Connection Design Solutions for Wood-Frame Structures” at 8:00 and 1:20. This session will feature a discussion of wood connection design and specification, including common fastener types and where design values can be found for each. Other topics will include the orthotropic nature of wood and its role in connection design, commodity and specialty connectors, and the use of steel connectors in wood-frame construction. Discussion will also include techniques for designing efficient, durable and code-compliant connections, examples of best practice connection details, and additional resources.
Outside the Circle