A
Th e
Component Manufacturing dverti$er
Don’t Forget! You Saw it in the
Adverti$er
October 2018 #10231 Page #48
Errors in Substituting Errors in Substituting Loadfor for Live Load in Wood Design DeadDead Load Live Byin Scott Wood D. Coffman, P.E., SECB Load Design Construction Science and Engineering, Inc. Introduction
By Scott D. Coffman, P.E., SECB Construction Science and Engineering, Inc.
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 codeprescribed 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 weight1. This simplistic viewpoint fails to consider other relevant building code requirements, including differential floor deflection, longterm 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 1 2
Granite 1-1/4� thick weighs between 18 psf and 22 psf. Cabinets with a shelf weigh approximately 10 psf. 2015 International Residential Code Chapter 2 Live Load Definition Commentary.
PHONE: 800-289-5627
Read/Subscribe online at www.componentadvertiser.com
Continued next page
FAX: 800-524-4982