Continuous Load Path Overview
An Accurate Review White Paper, Spring 2018
By: Kevin Walsh, AIA
Continuous Load Path Overview:
Light Frame Wood Construction
Light frame wood construction, or Type V construction per International Code Council’s International Building Code, is commonly used by architects and builders as an efficient construction method for residential and light commercial applications. Despite various other emerging construction methods, wood frame construction remains a popular method due to the ease, familiarity of the construction and project budget. General framing techniques have not changed drastically over the last century; however, the science behind lateral and uplift resistance has significantly improved how connections, fasteners, bracing, and diaphragm action work together to develop a continuous load path. As architects, builders, and building code officials, understanding load resistance is crucial for the safety and reliability of new construction.
Every structure experiences various loads, or forces, at varying magnitudes and directions, such as gravity, wind or seismic, and uplift loads. Gravity load is the accumulation of dead loads (a constant load from the permanent components such as the supported structure) and live loads (fluctuating loads from occupants, snow, furniture, etc.) that has a vertical direction, simply resisted by properly sized structural members. Lateral loads, such as wind and seismic forces, do not act vertically, or at least they are not calculated to act vertically; instead, lateral forces act horizontally on the structure causing base shear, torsion, and overturning. Uplift forces occur due to excessive wind loads, negative air pressures, and flood events; which cause structural members to be pulled away, vertically (upward), from the structure. The anticipated degree to which each of these forces, except gravity, affects the structure is partially dependent on the location and geography of the site.
Building codes such as the International Building Code and International Residential Code provide charts and tables indicating wind loads and seismic regions. Along with wind speed, the site exposure category must also be considered; this reflects the ground surface characteristics, such as open terrain, urban or wooded areas, or flat unobstructed areas such as waterfront sites. It is critical to review these sections of the code to verify the appropriate wind speed and seismic forces are accounted for in the structural design of the building.
Coastal regions, such as the southeast coast of the United States (US), has much greater risk of high winds than most regions; therefore, uplift resistance requirements in these areas are much stricter to reduce the threat of loss of life during a hurricane or similar event. Similarly on the west coast, seismic activity requires additional consideration to resist lateral forces from earthquakes. Buildings or structures outside of these regions can still be susceptible to severe weather events capable of significant property damage or loss of life. For example, the Midwest US experiences numerous severe storms annually, many of which have tornadic activity, and some unexpected regions in the US have seismic risks, such as the New Madrid Seismic Zone from Southeast Missouri to western Tennessee. Additionally, significant flood events are common throughout the US and especially in the Mississippi River Valley.
By understanding the risks and potential forces that may act on a structure, one can understand vulnerabilities in a building and where reinforcement or better connections need to be incorporated in the structural design. A continuous load path is a structural condition where all of the previously described loads acting on a structure or building are effectively resisted. An advantage of light frame wood construction is that there are numerous structural members, creating equally as many load paths; however, numerous members generally require numerous connections. In seismic regions, multiple connections play a vital role in the integrity of the structure by allowing energy to dissipate through the ductility of the structure. On the other hand, connections are typically the point of failure in a structure due to lateral and uplift forces; therefore, it is critical to understand the loads on specific building components to specify adequate connections and fasteners to reinforce the connections between structural members. In light frame wood construction, these are typically rafter-to-rafter or rafter-to-ridge beam, rafter-to-stud ties, stud-to-floor ties, stud-to-foundation hold-downs, and so on. Essentially, a continuous load path should have uniform and consistent load paths from the roof ridge to the foundation.
A number of reputable manufacturers, such as Simpson Strong-Tie and USP Connectors, supply various straps, ties, hold-downs, anchors, fasteners, hangers, etc. to provide a continuous load path, capable of resisting the loads previous described if properly designed and specified. Each product has fastening requirements in order to achieve the desired strength or load resistance. Specific connectors should be specified on the construction drawings with the appropriate fastening schedule. From an inspection or site observations stand-point, visually identifying any fastening issues such as split or end grain connections, incorrect hardware, or too few fasteners could mean avoiding a compromised structural system. Additionally, multiple level wood structures (3 levels or more) may experience shrinkage over time, which could reduce the effectiveness of traditional connections; verify shrinkage calculations have been completed and accounted for with specified connections.
Fasteners and connectors alone cannot resist base shear and overturning forces a typical structure experiences; therefore, shear walls are developed to brace the structure and resist the lateral loads acting on a building or home. Shear walls can simply be plywood sheathed walls that meet specific fastening requirements typically identified in the building codes; an example may be Structural Grade I plywood fastened at 4 inches on-center with 8d 2-1/2” nails. Manufactured shear walls are prefabricated braced panels that are framed into the wall of the building. These typically have specific foundation and anchoring requirements, determined by the manufacturer, to achieve the desired load capacity. Shear walls are commonly installed adjacent to large openings, as such as garage doors; corners; and within and perpendicular to long span walls to prevent racking or overturning. Moment resisting frames can also be used at larger openings to achieve similar results. Proper floor and roof construction is necessary to develop diaphragm action to allow lateral and torsional loads to transfer to the shear walls and through the continuous load path to the foundation.
Without ties, hold-downs, shear walls, properly developed floor and roof diaphragms, and straps, a light frame wood structure is susceptible to significant damage or collapse when excess or extreme loads are acting on the structure, which could result in serious injury or loss of life. Homes and other buildings construction years ago without modern connections, anchors, or bracing have stood the test of time under normal conditions; however, when faced with a severe event, the structures ability to resist the excessive loads is unpredictable.
This has been a brief overview of typical loads and basic strategies to resist the loads acting on a structure. In the International Building Code, Chapter 23, if applicable, has specific requirements for connections, fastening, member sizes and grades, etc. to resist lateral and uplift loads. It is critical that the design professional, builder, and building official are all familiar with these code provisions to ensure the structural integrity of new homes, buildings, and other structures. If a municipality does not have an adopted building code that has specific wood fastening, bracing, etc. requirements, the design professional has an obligation to ensure the structure is adequately designed to resist all of the potential loads, within reason.
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