By Robert Brunetti
In “Bones of a Mansion,” my article in the Spring 2007 issue of The Modern Estate, I addressed the trend toward using commercial-grade construction technologies in some of today’s large homes. This is a trend driven by the ever- increasing size of today’s estates, as well as homeowners’ design demands and preferences for more advanced living environments. The advantages these technologies offer, however, come with some additional costs and complications. This article explores the pros and cons of specific building technologies to help you, the homeowner, choose the right approach for your home.
Steel framing greatly expands the design options available to today’s homeowner: Steel framing can accommodate wider floor spans between walls, extra- tall interior free-spans and foyers, and heavier exterior finishes like thick stone veneer and expansive slate roofs. Indeed, your builder may need to use steel superstructures to address extreme loading conditions in specific locations throughout the house (extra-heavy furnishings, heavy sculptures, large safes) and wide expanses of exterior glass curtain walls. Using steel, which has greater rigidity that conventional wood framing, means that your house will undergo minimal house settlement and movement, and there can be extra strength provided in strategic places to help support special conditions like freestanding staircases, cantilevered slabs. and other unique design elements.
Though modern mansions typically require at least some steel framing, timber framing can still be used in the majority of most homes, especially if the builder uses today’s engineered lumber. Therefore, homeowners and architects are faced with three options:
• Avoid using steel altogether by minimizing the scope of design elements
• Use steel only in discrete areas where the design absolutely requires it
• Design the entire structure with a steel superstructure
This decision is not to be taken lightly: A full steel superstructure is more expensive than conventional framing, requires a totally different design and construction process, and usually takes longer to construct. With conventional wood framing, the builder can immediately start framing the house once the design is complete, the permits are in place, and the foundation is finished. As the house is framed, local building officials can inspect the building as part of the homeowner’s fee for their permit.
This conventional three-step process of foundation, framing, and inspection is greatly complicated with a steel superstructure. When steel framing is to be used, the process starts once the architect’s structural engineer completes the framing design. The design is then given to the builder’s steel subcontractor, who produces a full set of design drawings, with calculations, member sizes, exact dimensions (often-times verified in the field), and fully detailed structural connections (Step 1). In a perfect world, these “shop drawings” would be completed and approved prior to the start of the foundation, but more often they are produced as the foundation is being built and then sent to the original design engineer for review and approval (Step 2).
Frequently, a second round of drawings is required from the steel subcontractor to address comments provided by the reviewing engineer (Step 3). Once the shop drawings are resubmitted to the engineer and eventually approved (Step 4), the steel subcontractor can start fabricating each separate piece of steel—a process that can take a number of months (Step 5). To ensure a perfect fit on the foundation, sometimes a portion of the steel fabrication is not started until after the foundation is fully completed and final as-built dimensions and elevations can be measured. The final dimensions of the steel superstructure are then adjusted to accommodate the natural variation in the foundation, and the remainder of the steel is produced and erected on-site (Step 6).
Once erected, a steel structure requires a different inspection process than a wood-framed structure would. Local building inspectors will not inspect such a complex structure; thus, the homeowner must hire an independent engineer to perform a special inspection and submit a report to the local building official (Step 7).
And note that the foundation design for a steel structure is itself much more complicated than that for a wood-framed structure. Steel superstructures focus all of the house’s vertical and horizontal forces to specific foundation locations directly under each of the superstructure’s support columns. The foundation is specifically designed to accommodate these loads; typically, specific steel and anchor bolt details are provided below each column location.
All of these complications add about two months to the construction schedule (if all goes as planned) and a 20% to 40% cost premium to the framing budget.
Warning! In addition to the added complexity and cost of using steel, here’s a crucial factor homeowners need to know about: A steel structure’s design is much less accommodating to design changes than that of a wood-framed building. Any change to a steel structure must be designed by the original engineer, detailed by the steel subcontractor, and implemented in the field by steel erectors (usually requiring prefabrication of steel members in the fabricator’s shop and then cutting, welding, and bolting in the field). Minor changes that could be implemented on a wood-framed structure in a matter of days with coordination between the architect and a carpenter could take weeks to implement on a steel structure and cost as much as five times that of the same change made to a wood-framed structure. Furthermore, if major changes are contemplated, the compatibility of the steel framing and the foundation could be compromised.
Thus, if you, the homeowner, decide to build with a steel superstructure, you and your architect should arrive at agreement on a final design as early in the design process as possible. At the very least, the full house layout and exterior elevations should be finalized prior to starting construction—and never revisited. More specifically, the configuration of the floor plan, stair locations and configurations, any special loading criteria, design of the exterior envelope, the architectural treatment, window sizes, and window locations should not change once the contractor puts a shovel into the ground. The homeowner and architect should target a final structural design, ready for the steel subcontractor, on or before the start of excavation. Once the foundation is started, any substantive change to the steel superstructure can bring the job to a halt and easily add months to the project duration.
Structural Concrete Floor Slabs
If a homeowner and architect decide to use a steel superstructure, they can use a wood-floor framing system (supported on the steel frame) or a structural concrete floor slab.
The structural slab is typically found in multi-story light commercial buildings. It is created by installing a series of steel pans between the superstructure beams and then placing approximately four inches of structural concrete on top of the pans to create the floor system. The result is a very quiet, strong, and rigid floor system.
The technical and design benefits achieved by using a concrete floor system include:
• A fully rigid structure with no flex in the floor system and minimal vibration and noise created by people walking, playing, or exercising in the house (many homeowners like this sort of floor)
• Excellent sub-floors for supporting inflexible finishes such as floor tile and marble
• Minimal thickness for the floor system: This helps create an open gallery for the house utility infrastructure between the ceilings of lower floors and the floor systems of upper floors.
• The minimizing of post-construction settlement and house movement
• The ability to take full advantage of the rigidity of the steel superstructure. The concrete floor system can easily be made stronger in specific locations, where needed ,by adding reinforcing steel within the slab, or making the slab thicker.
There are, however, some negative aspects to concrete floor slabs. Steel-pan and concrete-slab structures do not have the same thermal properties as wood, and, through their direct connection with the steel superstructure and the exterior façade, tend to be colder in the winter than a wood flooring system. In some cases this impacts the energy calculations for the house and requires additional insulation measures.
Another disadvantage: Some HVAC engineers recommend installing radiant heat for floor warming when a concrete floor slab system is used. This adds cost to the project, since most houses install radiant heat only in select locations and not throughout the entire house (tall foyers and bathrooms are most common). Furthermore, the radiant heat needs to be isolated from the floor structure so energy is not wasted trying to heat the entire structural floor and framing system. Most often, a heat deflector or an insulation layer is placed between the slab and the radiant heat to direct the heat upward into the living space.
Concrete floor systems also create complications when hardwood flooring is to be installed. In a conventional wood flooring system, the hardwood floors are nailed directly into the wood sub-floor system. But when hardwood floors are installed over a concrete floor system, wood sleepers or thick plywood underlayment must be affixed to the slab to provide a system for nailing down the wood flooring.
The inflexibility factor. A concrete floor slab system, like a steel superstructure, requires early decisionmaking. Before constructing concrete floor slabs, the builder must identify all openings in the floors. This includes openings for stairs, shower and toilet drains, plumbing risers, all electrical wire paths (power, communication, data, audio, video, etc.), chimneys and vent stacks, and all HVAC ductwork, vents, and piping. Each penetration size and location will be evaluated by the engineer to determine if box-outs or sleeves are sufficient for creating the necessary voids in the concrete slabs, or if special braces or thickened slab areas are warranted at some of the larger openings.
A concrete slab floor system is not always more expensive than conventional wood flooring. When wood prices were driven up by the reconstruction efforts in Afghanistan and Iraq and by the rebuilding efforts following the Florida and Gulf of Mexico hurricanes, a concrete slab system was actually more cost-effective. However, the complications discussed above, including energy calculations, wood flooring installation, and radiant heating (if desired by the homeowner), can have a significant cost impact. If a homeowner truly values the benefits provided by the concrete slab system, these additional costs are usually a reasonable tradeoff.
The concrete slab system’s biggest potential complication is the difficulty of making changes to penetrations in the field, after the installation of the slabs. With a wood floor framing system, all modifications can be made by a carpenter, and most can be designed on site between the carpenter and the architect, using direction provided by the building code about headers, joist hangers, and acceptable framing details.
In contrast, every slab penetration made or adjusted after the slab is in place needs to be reviewed and approved by the engineer. Changes often require additional bracing and special infill details to patch old penetrations that no longer will be used. Implementing a change frequently requires the services of a concrete cutter, a steel erector or welder, and a mason (if you are relocating a penetration and need to patch the old hole). The most common culprit is the HVAC design and determining the exact locations of air supply and return ducts. To properly locate HVAC vents, the homeowner, decorator, and architect need to locate all draperies, area rugs, recessed light fixtures, floor borders and patterns, and furniture. Since the structural floor slabs are part of the very early stages of construction, it is often a tall order for homeowners and their design professionals to create a final interior design so early in the process.
If a house is going to employ a steel superstructure and concrete floor slabs, the best way to control costs is for the homeowner and architect to complete the full design of the house, including the design of the interiors, prior to the start of construction. Otherwise, the cost of rework and delay can be staggering.
Metal Wall Studs
As a natural extension of the steel superstructure and the concrete floor slabs, metal wall studs are showing up with more frequency in some of today’s advanced homes.
The major benefits of metal studs are that they are perfectly straight, extremely strong. and do not twist, bend, or bow, as wood studs do as they age and dry out. Furthermore, in climates where extended heating seasons are countered by hot and humid summers, a natural product like wood is constantly in a state of flux.
All of these features become especially beneficial when wall heights start to exceed 10 feet or the design calls for extremely delicate finishes. In addition, metal studs are lighter than wood and have predrilled/punched penetrations that allow easier access for plumbers and electricians to run their lines.
On the downside, metal studs do need wood blocking and infill in places where typical wood framing would have been sufficient for installing trim, shelves, small doors, and other light installations. Internal headers and box-outs for windows, doors, utility penetrations, and other framing details with steel studs are essentially done as an assembly in place. As with the other commercial technologies, metal stud framing is less tolerant to changes than is wood framing. A wood-framed wall can always be adjusted by cutting out certain portions or nailing additional members to the existing members. Adjustments to a metal-framed wall (especially a complicated wall with multiple openings or alignment changes) usually require the disassembly and full rebuild of a wall section.
Many parts of the country are routinely experiencing power outages due to rolling blackouts (caused by our aging electrical distribution system) and downed power lines (due to the advancing age of trees growing too close to power lines). Fairfield and Westchester counties have been particularly hard hit in the past couple of years. It is becoming almost commonplace to install a generator of some size when a large home is constructed.
Many homeowners are putting refrigerators, garage-door openers, well and sump pumps, security systems, wine cellars, critical heating systems, minimal interior house path lights, and critical bathroom lights on backup generator power.
It is even becoming common for homeowners to put full HVAC systems (air conditioning requires the most power), a select number of power outlets, and all the house lighting on backup generator power.
A few homeowners building large homes are putting their entire house on backup power—including guest rooms, media rooms, all outlets, all kitchen appliances (including microwaves), etc. This may not sound particularly difficult, but some of today’s mega-mansions (houses 15,000 square ft. or larger) are installing 1,200 –1,400-amp electrical services (or even larger systems). To put this in perspective, a new 4,000-square-foot home, fully air conditioned, with an in-ground pool, is typically built with 400-amp service. There are more than a few custom houses in the Greenwich, Connecticut, area that are actually installing 1,800 amp (or larger) services.
Here’s an indication of how powerful such a system is: A 300-kilowatt (kW) generator is needed to satisfy the power needs of a complete 1,200-amp electrical service. In fact. the largest available residential-grade generator from Onan and Generac (two popular manufacturers) is a 150-kW model. These companies classify any generator in excess of this size as a commercial-grade generator. Even the largest residential-grade generators are sizable (10 feet long, 3.5 feet tall, and 5 feet wide). But commercial-grade generators can be massive: A 300-kW generator can be nearly 15 feet long, 7 feet high, and 7 feet wide. Homeowners (and their landscape architects) have a tough time finding an appropriate location on their property even for a large residential generator, let alone a monstrosity like a 300- or 400- kW generator.
Generators that are 150 and larger are difficult to permit and difficult to support with a suitable fuel source. Most residential generators prefer to run off of liquid natural gas (through the public gas company) or propane. Once generators begin exceeding 150 kW, the public gas supply is usually insufficient in residential areas, and installing an appropriately sized propane tank also becomes a challenge. Generators of excessive size usually rely on diesel fuel only, and can require tanks in excess of 500 gallons.
To meet permitting requirements, all generators must meet strict property setback restrictions and noise ordinances. Most towns require that generators be located within building setbacks, so the options for placing generators away from the main house and out of sight are often limited. Furthermore, most towns require that the noise of a generator be no more than 50 decibels at the property line. This becomes extremely challenging when excessively large generators are used—and even more difficult with diesel generators, since they produce more noise per kW than gas or propane fueled generators of the same capacity.
Nevertheless, if a homeowner truly wants full backup power, arranging for it is possible, in most cases. However, using backup power can be extremely challenging for homes with electrical services in excess of 700 amps (the maximum output from a 150-kW generator is approximately 625 amps). Since most houses that are 10,000 square feet or larger are built with 800-amp services, homeowners building a large houses often decide to prioritize the power functions in their house and put only a portion of the full house electrical loads on back-up generator power.
Choosing partial backup
Working with an electrical engineer, a homeowner can determine the tradeoffs between various sizes of generators and what household items can be supported on backup power. Here are the issues to be considered:
1. What do you want covered if you choose minimal (emergency) backup? Minimal backup should include pumps, refrigerators, security system, and critical heating zones to avoid frozen pipes in the winter. Determine with your electrical engineer the size of the generator required to support these items. For most large houses, minimal backup can usually be satisfied with a 35- to 50-kilowatt generator (even for the largest houses).
2. Evaluate the available public gas service you have at your home, and consider how big a propane tank your space could accommodate. Most homeowners prefer an underground tank, if it’s allowed. Determine how large a generator can be supported by the gas service or the appropriate-size propane tank. The generator most likely will have some additional capacity than the power needed to support the minimal emergency-backup items listed in item 1 above.
3. Identify the dimensions of the generator in item 2 above. Locate the generator on your site plan and consider how much noise it produces (the manufacturer will supply this information). If it produces less than 50 decibels, you can locate the generator anywhere within your building setback. If it produces more than 50 decibels but less than 55 decibels, you may need to enclose the generator within a fence to mitigate the noise. It also may be a good idea to avoid locating it directly against a structure that can deflect noise toward the property line. In most cases you can enclose the generator within a standard fence, as long as the fence is tight to the ground, has no gaps or spaces, and is taller than the unit. This should allow the noise to be at or below 50 decibels at the property line. If the unit is louder than 55 decibels, you need to engage the services of a sound engineer and most likely implement more complicated noise control measures.
4. Once you can satisfy items 2 and 3, your electrical engineer should be able to give you guidance on what else you can operate on back-up power. This should be more than the absolute minimum, but don’t be surprised if it does not approach the amount of power required to run the entire house.
Three Phase Electrical Systems
This topic that is beyond the expertise of most architects and builders—including me, and I am a construction project manager. Therefore, I will keep this simple (which is easy to do, since even the following explanation exhausts my knowledge of the subject). Still, you, the homeowner, should be be armed with a few key questions so you can at least make sure your design professionals are considering all options.
Simply put, most homes are on single phase electrical service. This is a standard double “hot” line that brings power to the home. With three phase electrical service, a third “hot” line is provided so the electrical system has three hot lines for the dwelling to draw from. The third hot wire provides more power and reduces the amperage requirement. A three phase electrical system can reduce the size of the electrical service by approximately 30 percent. For instance, a 1,600-amp single phase service can be reduced to 1,100 amps with a three phase service.
The implications are threefold:
• Equipment runs more efficiently
• The size of equipment (mechanical and HVAC equipment, in particular) can be reduced measurably.
• The electrical usage and operating costs are reduced.
In urban areas, most commercial buildings are operated on three-phase electrical power. It is becoming increasingly common to have three-phase power extend from the urban portions of a town to certain rural portions within the same town or an adjacent town. This is most common along the routes of the main feeder lines running to and from the urban sections of town.
Most homes do not have the option to tap into a three- phase electrical system. However, if a new home has access to three-phase service and the size of the projected single phase service is approaching 800 amps or greater, it is worth a closer evaluation of the potential to use three phase service by the homeowner's electrical engineer.
Many other residential design elements have become more complex in recent years. This article has tried to touch on the major elements that are encountered by most homeowners who are planning to build large houses in the near future. In “Bones of a Mansion,” in the Spring 2007 issue of The Modern Estate, I stressed the importance of finding the right set of professionals to help you with the design and construction of your project. There are many other design decisions that can have a great impact on the ultimate quality of your new home, as well as the ultimate cost and schedule for implementation. Homeowners should also ask their design professionals about central chiller plants, smart house technology (HVAC control, pool controls, AV, lighting control, shade control, etc.) and various security options.
The bottom line: If you choose to implement a project with a steel superstructure, concrete floor slabs, and metal stud wall framing, you must be scrupulous about completing the full set of design documents, including interiors, before construction starts. Only if this prudent step is taken will you realize the considerable benefits of these three technologies without risking major cost increases and months of delay.
Robert G. Brunetti, PE, is Director of Construction Services for Pecora Brothers, Inc., a custom-home builder and project management firm located at 57 Holly Hill Lane, Suite 300, Greenwich, Connecticut. 203-863-9555 x 110 firstname.lastname@example.org