Errors in Substituting Dead Load for Live Load in Wood Design

Introduction

Custom kitchens, with high-end appliances, granite countertops, and central islands, are becoming more and more prevalent in the home market today. These products and finishes are heavy and the dead loads from the central islands and/or cabinets positioned along interior partition walls are often overlooked or neglected by design professionals. This oversight often trickles down to truss/I-joist technicians in the design of the floor framing components and becomes particularly problematic when granite countertops are present.

The most common assumption is that the area covered by the island and cabinet will not exceed the code-prescribed live loads (i.e., occupants and furnishings) limits. For example, a typical cabinet with a granite countertop weighs approximately 30 pounds per square foot (psf); therefore, the code-prescribed 40 psf live load is deemed sufficient during design for the island and cabinet weight^[1]. This simplistic viewpoint fails to consider other relevant building code requirements, including differential floor deflection, long-term creep, unbalanced load, and duration of load. This oversight may result in problematic floor performance issues, as we have documented in recent cases investigated by Construction Science and Engineering, Inc.

Code-Prescribed Live Loads

Our firm has studied numerous instances of the condition described above. Our investigations have included a brief overview of the design loads specified in the 2015 International Residential Code (IRC). The floor live load is defined as a load produced by the use and occupancy of the building where use and occupancy are generally interpreted to mean people and furnishing^[2]. The IRC floor design live load typically used is 40 psf. Floor design dead load is attributed to the permanent weight of materials, finishes, and fixed equipment of the as-built construction. The floor dead loads should be calculated.

Historically, the residential building codes have included tables that provide joist spans for dead loads of 10 psf or 20 psf. The 10 psf dead load value is the common default loading used to size residential floor framing, I-joist components, and to design metal-plate connected wood truss top chords. However, homes with the dead loads and geometry/architecture described above are often finished with wood and/or tile floor finishes that exceed the 10 psf dead load. Although wood and tile floor finishes exceed 10 psf design load, the examples in this article will use the 10 psf code table minimum dead load because it is the common default value and it will demonstrate the error of substituting dead load for live load.

Building Code Violations

One duty of a design professional, truss technician, and I-joist specialist is to rely on the applicable building code to dictate minimum design loads. The compelling reason not to substitute dead load for all or a portion of the design live load is that the building code does not permit it. IRC Table R301.5 clearly specifies the minimum (emphasis added) uniformly distributed live load for design. There is no provision to replace the design live load with some amount of dead load. Specifically, Chapter 2 in the IRC defines live load in part as “Those loads produced by the use and occupancy of the building…and do not include…dead load.” By the IRC definition, the structural live load does not include dead load and the dead load cannot replace any or all of the live load prescribed by the building code.

This author is aware of the common position taken by several design professionals, technicians, and specialists that occupants and furnishings cannot occupy the same area as a cabinet. However, the following conditions have been reported and/or observed during our field investigations that challenge this position:

• People sitting or standing on a cabinet top;
• Boxes with assorted items of various weights stacked on counter cabinet tops;
• Students in college housing that load an area (including cabinet space) to create an “over-load” condition; and,
• Cabinets filled with bulk cooking ingredients, stacked with “heavy” cookware/dinnerware, and/or that serve as bookcases.

In other similar cases, the home occupants could be considered as “hoarders,” where the cabinet content easily approached and perhaps exceeded the code specified 40 psf minimum design live load. In an attempt to address this issue, one alternative suggested by the wood truss industry is to add 20 psf to the design dead load to account for cabinets with up to 2” of granite.^[3] In light of our experience, it is essential that cabinets with granite countertops be included as part of the dead load design and not infringe upon the code-prescribed live load.

Floor Deflection – Differential Deflection

In my experience, localized deflection of the floor system is the primary serviceability complaint of homeowners. Specifically, floors in close proximity to or adjacent to a cabinet and/or partition wall display a “dip” or unevenness relative to the remaining floor area. One common cause of these conditions is that the floor designers fail to consider the differential deflection between two adjacent structural members.

The National Design Standard for Metal Plate Connected Wood Truss Construction (ANSI/TPI 1) specifically identifies differential deflection as a serviceability concern that should be part of floor component design^[4]. When investigated, one method is to limit differential movement between adjacent trusses to twice the truss spacing divided by the appropriate code deflection limit (e.g., 2L/240)^[5]. For example, floor framing spaced at 24 inches on-center designed to a total load deflection limit of L/240 would yield a differential deflection limit of 0.2 inches.

The effects of differential deflection between floor framing adjacent to and under cabinets with a granite countertop can be understood from a typical floor example. A floor joist is designed to a maximum live load deflection of 0.40 inches and a total load deflection of 0.50 inches for a typical 40 psf design live load and 10 psf design dead load. Substituting a 40 psf cabinet and floor weight (30 psf cabinet + 10 psf floor) immediately adjacent to a 10 psf floor weight creates a relative movement of framing members that may be “seen” and/or “felt” when walking over the floor. Specifically, the floor framing below the cabinet would deflect 0.40 inches (i.e., 0.50” x 40 psf/50 psf) compared to 0.10 inches (i.e., 0.50 ” x 10 psf/50 psf) for adjacent floor framing without the cabinet load. The 0.30 inch difference exceeds the L/240 deflection ratio of 0.20 inch for floor joist 24 inches on-center. This relative movement is both felt by occupants and can result in damage to attached flooring and trim materials due to the differential movement.

Floor Deflection – Long Term Creep

Localized dead load deflections are progressive from the cabinet and/or partition weight over time. This time-dependent deformation under long-term dead load is known as creep deformation in wood. Creep deflection for wood members is typically taken to be one-half the initial dead load deflection^[6],[7]. Therefore, the previously calculated 0.40 inches initial (or short-term) dead load deflection from the example above would increase by another 50% (or 0.20”) due to creep. This condition creates two primary serviceability issues:

• The joist deflection below the cabinet, including creep, increases to 0.60 inches (i.e., 0.30” + 0.10” + 0.20”). The floor dead load deflection adjacent to the cabinet increases to 0.15 inches (i.e., 0.10” + 0.05”) when creep is considered. The differential deflection increased for creep becomes 0.45 inches (i.e., 0.60” - 0.15”) which is significantly more than the permitted 0.20 inches. The relative deflection as typically reported by homeowners is an “observed” floor unevenness which is “felt” when walking across the surface.Wood floor-framing members optimized for cost often approach code-prescribed deflection limits. When dead load is substituted for live load, the resulting total load deflection is susceptible to exceeding the code specified ratio. The following comparison demonstrates the effects of substituting dead load for live load that often creates a code violation. Setting the initial 0.55 inch maximum deflection value to represent the L/240 total load deflection ratio, creep deformation increases approximately 30% to 0.70 inches and results in total load floor deflection approaching the L/180 ratio.

• Wood floor-framing members optimized for cost often approach code-prescribed deflection limits. When dead load is substituted for live load, the resulting total load deflection is susceptible to exceeding the code specified ratio. The following comparison demonstrates the effects of substituting dead load for live load that often creates a code violation. Setting the initial 0.55 inch maximum deflection value to represent the L/240 total load deflection ratio, creep deformation increases approximately 30% to 0.70 inches and results in total load floor deflection approaching the L/180 ratio.

This author is aware that the IRC limits deflection checks to the live load component. Additionally, the 2015 International Building Code (IBC) limits the deflection check to the live load component and creep component for a wood member. For the IRC and IBC, the initial dead load deflection component is neglected. For wood, this is a bit silly. Historically, design professionals and truss/I-joist technicians have investigated and compared calculated live load and total load deflections to building code and/or industry accepted ratios. Relaxing the more stringent historical interpretation of live load and total load deflection checks to the current code-permitted criteria is likely to increase serviceability complaints from the end-user.

Floor Deflection – Unbalanced Cabinet Loads

A countertop is not always limited to the dimensions of the base cabinet. Granite countertops often cantilever between 12” and 14” at the island or a cabinet profile to accommodate bar stools/chairs. The granite cantilever adds approximately 20 psf of weight and redistributes a majority of the cabinet weight to the cantilevered side. Floor framing members located below the cabinet edge receiving the cantilevered granite weight deflect more to increase localized deflection and floor unevenness (Sketch 1). Previously shown differential and creep deflection would increase for cabinets with a cantilevered countertop for bar stools. Framing member design should account for the unbalanced cabinet weight.

Wood Member Stress Duration

Substituting dead load for live load introduces a strength concern that is rarely investigated by the individual making the substitution. Specifically, the lumber load duration factor (included in the building codes) associated with dead load is 0.9 compared to a 1.0 factor for live load. Engineered wood products are produced to efficiently use wood material to create a structurally sufficient product at an optimal cost. For example, the wood truss industry typically optimizes lumber grades to obtain a combined stress index (CSI) that ideally approaches or equals 1.0. A wood truss chord optimally designed to a CSI between 0.9 and 1.0 may be insufficient for a design live load replaced by a dead load when investigated for the 0.9 factor.

Additionally, the I-joist industry optimizes flange material to resist maximum flange bending and compression stresses associated with the design bending moment. For example, an I-joist originally designed to within ten percent (10%) of the published allowable moment value may be insufficient when dead load is substituted for live load when investigated for the 0.9 factor included in the codes. The same load duration principle applies to any wood-joist product and should be analyzed for the correct load combinations and associated load duration factor. Therefore, wood framing members may become over-stresses when dead load is substituted for the live load.

Conclusion

Wood framing members are an economical, proven system to support conventional framed floors in accordance with applicable building code requirements and industry standards. However, in the presence of custom kitchen islands, cabinets with granite countertops, and/or partition walls, additional design and construction concerns must be considered. Specifically, the wood floor components should be designed for the code-prescribed live load and the dead load associated with the overlying floor system, including islands, cabinets, and partitions. In essence, the floor live load cannot be arbitrarily replaced with dead load.

When properly addressed, floor serviceability should be the foremost design consideration to include checks of relative deflection, long-term creep, and unbalanced loads caused by cantilever countertops for bar stools/chairs. Experience, coordination, and input from engineered component manufacturer technicians, specialists, and/or engineers should be provided to and evaluated by the building designer. In the absence of a specific analysis, additional wood floor framing members should be installed below kitchen islands, cabinets, and/or partition walls to support the additional weight and to mitigate localized deflections. As an alternative, an additional 20 psf dead load to account for cabinets with granite countertops up to 2” thick should be added to the floor design load as suggested by the wood truss industry in Structural Building Components Association Research Report SRR No. 1601-04. Properly considering all of the immediate and long-term effects will facilitate the design and installation of products that perform as intended.

Scott Coffman, P.E., SECB has over 35 years in structural wood design experience, predominately in engineered wood building components. You may contact him by phone (864-647-1065) or email Scott at Construction Science and Engineering, Inc. in Westminster, SC.