Article No: 224

2007-12-10 16:45:38
Tech Talk: Concrete Homes: Designing with Modularity
By: Jim Baty, CFA Technical Director


 
Part V: This six-part series focuses on details of today’s concrete homes. Over the past two years, this column has provided considerable information on the general benefits of concrete homes, the various methods used for construction and the performance characteristics. These series provides a focus on strengthening the understanding of the decisions, details and results that can affect the quality achieved in the above-grade concrete home industry delivered by removable concrete forms (RCFs). Since strongly entering the market nearly a decade ago, this method of construction offers an ever-expanding variety of architectural and practical construction solutions for today’s home owner and designer. This month, we introduce the concept of designing these homes to embrace the technology that is available to construct them. 
Look at any home construction site and you will see materials in very specific sizes that constrain the design. Plywood, drywall and other sheathing products are manufactured 4 feet in width and either 8, 10, or 12 feet in length. Studs and joists that frame the structure of the “traditional” home are also found in common lengths measuring an even 8, 10, 12 feet, and so on, as well as accommodating the practical construction with standard precut lengths like 92 5/8 inches. Over the last hundred years or so, construction systems have evolved around other basic materials including concrete masonry units (8-inch foundations) and modular brick (window openings). It will not come as a surprise that when designing the concrete home, the need to understand the modularity of the systems used to form the concrete will impact the design of the structure.


The modular form
We’ve established in previous articles that the concrete home is constructed by placing concrete in its near fluid state within the confines of a forming system. These systems are assembled on the construction site with pins and wedges securing adjacent form panels. When finished, an entire wall or level of walls is prepared for the placement of the concrete. For removable concrete forms, these are manufactured from steel, aluminum or wood in standard 2- or 3-foot panels. There are, however, many “filler” sizes that are also manufactured to provide flexibility to the contractor based on the design layout for the home.

It is not uncommon to see a formed wall comprised of many “fillers.” Depending on the structure that is being formed, the wall may incorporate a bow or curve, the room dimensions may be irregular or the transition between rooms may occur at an increment that would otherwise be in the middle of a standard form. The ability to provide flexibility in formed wall length with these fillers is a distinct advantage to the system without the need to cut or waste a pre-manufactured material. The result is a smooth, finished concrete surface that matches that of the areas formed with the standard modularity with little or no variation.

There should be no surprise that to maximize the economy of any construction, optimizing the design for the module of the construction material is advantageous. Although this can be said of RCF construction, most contractors encourage designers to break free from the stigmatism for module constraint.

“I don’t think the concrete designer should be constrained by any type of modularity. The basic principles of structural design are the same regardless of the building system. Concrete building systems deliver the structure more efficiently because we are not concerned with standard lengths or unit dimensions. If a 7-inch slab is required, we can do this with no problem.” —David Pfanmiller of Security Building Group, Sneads Ferry, North Carolina.

The problem often comes when a designer thinks of an RCF home with the same modularity constraints as another method of construction. Pfanmiller continues:

“Actually, I think many times there is a backlash. We unnecessarily modify our methodology so it conforms to the traditional modules. For instance, nailers on 16-inch or 24-inch centers so the plywood panels break properly, even heights and widths. This decreases our efficiency and increases cost.”

Form height
Select a home today and you are usually buying a pre-determined ceiling height that is either 8, 9, or 10 feet (actual dimensions are less due to finish thickness). These heights have been engrained into our expectations largely due to the efficient use of manufactured materials, like drywall, to finish the spaces. This constraint of dimension does help to improve an efficient use of resources, however, the constraint of wall heights to otherwise arbitrary dimensions may not provide the optimum solution for the client’s needs.

The concrete form for an RCF home similarly embraces wall heights based on the history of the construction method. Since basements have been 8 or 9 feet tall for many years, the panel lengths used to form these walls have become standard. Add in the 10-foot panels for better lower-level living and concrete walls match up easily with the more traditional forms of construction. However, the RCF forming systems also benefit from modularity in their widths. Forms are easily turned on their side and added along the top of a vertical form set to expand wall heights quickly to 12-feet or more. This flexibility comes at no cost to the efficient use of materials since no raw material has been delivered to the jobsite yet (i.e. the ready mix concrete). Conversely, if you require a wall that is 8 feet, 4 inches in height, you use a 9-foot form module and hold the top of pour down 8 inches.

Wall thickness
Another area where housing traditionally follows constrained modularity is in the thickness of walls. Consider the framing of a wall with a 2-by-4 or a 2-by-6, or perhaps the layout of 6-inch or 8-inch concrete blocks. These are prescribed wall thicknesses that are more about the material used and less about the desired aesthetic.

An RCF system establishes a new sense of modularity, but one that allows a greater array of choices. The forms are separated by wall ties that secure the forms against the fluid pressure of the concrete during placement. These wall ties (or form ties) are manufactured in dimensional increments that deliver wall thickness variations from 4 inches to 12 inches or more without requiring alterations in any of the forms. This flexibility responds to greater variation in wall height, façade massing and change of direction.

The design
Most of today’s homes consist of a configuration of open, semi-public spaces that develop a greater sense of interaction and facilitate better entertaining. The openness of the floor plan that allows the hosts to freely interact with guests or parents to participate in a movie night while preparing a meal is a primary goal for these spaces. As such, the constraint of spatial dimensions to a particular module of material is not as important. It is the ability to deliver consistency and quality in larger expanses that is of greater importance. This has pushed RCF systems even further to capitalize on the structural efficiency of concrete.

“The ability to deliver maximum span flexibility for our concrete floors has become one of our greatest advantages. Spans of 28 feet have contributed greatly to the openness of these homes without sacrificing the stability of the floor above. These floors are rock solid with no spring. If you want greater spans, the concrete floor adapts quickly to a simple column and beam to double the open span with little change to the structural design.” —Van Smith, Smith Bros. Concrete Contractors, Wallkill, New York

Technology in system design is also something that continues to advance the state-of-the-art. For instance, where modular forming systems in the last decade have largely been of the traditional 2-foot and 3-foot variety, today’s contractors employ both gang form and large panel systems as well as whole-house and whole-wall tunnel forming technology to create many of the mass housing designs that provide quick solutions to the need for storm-resistant housing.

The resulting envelope of the concrete home today considers many common shapes and angles. The traditional “box” appearance that prescribes so many framed homes can be explored through a wider array of form shifts, corners, angles and occasional curves. The modularity of the forming system produces a monolithic structure with greater stability and less movement. This offers the design and construction team the ability to think less about the ability to support a design element and more the freedom to individualize the form. The most economical solution is always one with fewer corners and fillers but the same can be said for a framed wall or one where blocks must be repeatedly cut. True economy comes from the combination of structural freedom with the ability to embrace design aesthetics.

Still wondering …


“The options are limitless in a “liquid rock wall.” We fill the walls with a material that is nearly liquid that will assume the shape of the form we set up, so the only limits are those of imagination.” —Mike Hancock, Basement Systems, Edmund, Oklahoma

Often the greatest temptation when embarking on the design of a new home is to remain in a comfort zone of the known. With a modern aesthetic being recreated worldwide through the more efficient delivery of the concrete home, homeowners, designers and builders should plan with more freedom rather than “fitting the mold.” Despite the appearance of modularity in a forming system, RCF housing proves the ability to alter the structure of housing to such a degree that efficiency becomes an absolute. What will be the first step you make towards your next home?

Established in 1974 for the purpose of improving the quality and acceptance of cast-in-place concrete foundations, the CFA has a variety of resources on this topic including materials available from the Concrete Homes Council established at the CFA to help contractors transition into this market. In addition to providing promotional materials, educational seminars, opportunities for networking and a telephone network that places members in one-on-one contact with an experienced contractor for assistance in resolving a variety of issues, the CFA and CHC represents the interests of its members and the industry on several code and regulatory bodies, such as the American Concrete Institute’s committee responsible for the creation of the “Residential Concrete Standard.” The CFA has several of its members on the ACI committee responsible for this document and will endeavor to ensure that the interests of the foundation contractor are considered. For more information about CFA, see cfawalls.org or call (319) 895-6940. For more information about CHC, see concretehomescouncil.org or call (319) 895-0761.

Jim Baty, jbaty@cfawalls.org, is Technical Director of the Concrete Foundations Association and the Concrete Homes Council.