Determining differential movement and overcoming resulting detailing challenges.
By Anthony J. Nicastro, P.E., Robert P. Antes, R. Scott Silvester, P.E., and Niklas W. Vigener, P.E.
A renewed need for medium-density housing within and surrounding East Coast cities, and economic pressures following the recent recession, have led to an increased use of economical wood-framed construction in large, multi-story residential buildings. The preferred aesthetic of these buildings in the mid-Atlantic region includes brick masonry facades. The technical aspects of brick cladding in wood-framed low-rise residential structures are well understood by practitioners, but the desirable look of brick masonry combined with the bargain of wood framing can be problematic in multi-story construction.
In multi-story wood-framed buildings brick masonry is typically supported at the foundation level or at a concrete podium level, and brick masonry heights often exceed prescriptive code limits. The brick masonry and the wood framing will both undergo volume changes (expansion for the brick and contraction for the wood) that are not only opposite but irreversible. The significant and additive differential movements of these two materials are proportional to height and therefore more impactful on taller buildings. Without relieving angles or another form of intermediate support for the brick cladding, the wall assembly cannot incorporate horizontal brick masonry expansion joints customarily used in multi-story brick construction. Building elements that bridge from the wood-framed structure across the brick masonry, such as windows, dryer vents, etc., require special detailing to accommodate this differential movement. If the wall system is not detailed to accommodate this movement, damage can occur at these restrained elements that project into the plane of the brick masonry but are anchored to the building frame. (Figure 1).
Section 1405.6 through 1405.9 of The International Building Code (IBC) recognizes the potential problems generated by differential movement between brick masonry and wood-framed supporting structures by referencing Section 6.1 of TMS 402/ACI 530/ASCE 5.
ACI 530 includes several requirements for anchored masonry veneer, which originate from the master requirement set forth in Section 6.1.2 “Design of Anchored Veneer,” which states:
Anchored veneer shall meet the requirements of Section 6.1.6 and shall be designed rationally by Section 6.2.1 or detailed by the prescriptive requirements of Section 6.2.2.
Section 6.1.6 “General Design Requirements,” includes a requirement to “Design and detail the veneer to accommodate differential movement.” This requirement is a catch-all that puts the burden on designers to determine the amount of anticipated differential movement through analysis, and to detail the veneer accordingly, regardless of whether designed by rational analysis or using the prescriptive approach.
Section 6.2.2 “Prescriptive Requirements of Anchored Masonry Veneer” of ACI 530 includes requirements for the anchoring of brick veneer and, most relevant to the subject of differential movement, Section 184.108.40.206 “Vertical Support of Anchored Masonry Veneer” requires that the height of anchored veneer with a backing of wood framing not exceed the height above the noncombustible foundation of either 30 feet at a plate or 38 ft. at a gable. This limit sets the maximum height allowed using prescriptive detailing without conducting a rational analysis. Assuming a floor-to-floor height of 10 to 12 ft., the code practically limits the vertical height of brick masonry to only three stories without further analysis.
Many wood-framed structures are Type III-A construction that extend four or five stories above the foundation or podium base. These buildings require a rational design for the support of the brick masonry following requirements of Section 6.2.1 “Alternative Design of Anchored Masonry Veneer.” These larger wall heights also require accommodation of proportionally larger differential movement between the anchored veneer and wood-framed backup. A rational analysis should examine the potential magnitude of movement and include exterior wall details that can handle the anticipated movement. The remaining sections of this article can assist designers whether designing rationally or detailing exterior walls using prescriptive requirements.
Methods to quantify differential movement
To properly detail portions of the wall that bridge between the brick masonry and backup structure (e.g., masonry veneer anchors, windows, mechanical penetrations) to accommodate movement, designers must first quantify the anticipated differential movement.
In order to understand how much movement can occur between the brick masonry and back-up wood-framed structure, designers must examine the design of the structure, calculate estimated wood shortening, and add the effects of brick masonry growth to that of the estimated shortening of the wood framing to determine the total differential movement.
Total differential movement between the brick masonry and wood-framed backup structure can be estimated using the following simple equation:
(Δbrick moisture + Δbrick temp) + (Δwood drying shrinkage + Δwood creep) =Δdifferential
In this equation, the left parenthetical represents the contribution of brick growth and the right parenthetical represents the contribution of wood shortening.
The total estimated brick growth includes Δbrick moisture for the irreversible growth of brick over time with increase in moisture content and Δbrick temp for growth of brick with increase in temperature. Total estimated shortening of the wood framing includes Δwood drying shrinkage for shortening of the wood due to drying from the moisture content at installation to the equilibrium moisture content and Δwood creep for shortening of the wood framing due to time dependent strain under sustained long-term load. The temperature range experienced by the brickwork is the difference between low and high mean temperatures of the brickwork after construction and is based on the low and high temperatures of the exterior ambient air.
Other factors contribute to differential movement, but can be considered negligible compared to these mechanisms. For example, brick creeps under sustained load, but the amount of shortening due to brick creep is small. After framing reaches its equilibrium moisture content, thermal expansion of moist wood (moisture content greater than 5 percent) tends to be negated by drying shrinkage due to additional moisture loss. Framing also shortens from settlement of construction gaps. Articles by Alfred Cummins and Dominic Matteri estimate this settlement of the wood framed construction to be as much as 1/8-inch per floor. However, this value is difficult to predict and some gaps may disappear as the building is loaded during construction, and prior to brick installation, reducing its effect on differential movement.
Exterior sheathing and interior finishes tend to restrain and therefore reduce the amount of wood shortening, but this restraint depends on type and arrangement of these components and should not be fully relied upon. Example estimated quantities of brick growth and wood shortening for balloon and platform framing are provided below without consideration of restraints from sheathing and finishes. Note that while the calculations assume design values for both brick growth and wood shrinkage based on applicable guidelines, these design values can increase or decrease depending on actual project circumstances.
Movement of brick masonry is expected in new construction. The values in Table 1-1 reflect the movement of unrestrained brick masonry due to changes in temperature and moisture in the summer months, when brick expansion due to temperature change is the greatest.
Note: The calculations above assume a story height of 10 ft.
The calculations tabulated above consider the temperature range experienced by brickwork to be 100°F and a coefficient of moisture expansion of 5×10-4 in./in. as recommended by Brick Industry Association Technical Note 18.
The calculations tabulated above also consider coefficient of thermal expansion of 4×10-6 in./in./°F as recommended by ACI 530. These and other resources such as Masonry Designers’ Guide provide designers alternative coefficients for expansion based on industry recommendations.
To quantify wood shrinkage due to moisture content change, Δwood drying shrinkage, designers can consult the Wood Handbook published by the United States Department of Agriculture, which describes shrinkage of common wood species based on change in moisture content and dimensional change coefficients dependent on species and orientation of grain. Mid-rise residential and mixed-use buildings usually include wood framing oriented in a balloon configuration similar to traditional balloon-framing. Low-rise residential structures are frequently constructed using platform framing (Figure 3).
Approximate values of per-floor shortening due to change in moisture content and creep for typical assemblies are tabulated below. The shrinkage strain for members loaded across the wood grain such as plates and joists is significantly higher than that of studs which are loaded parallel to the wood grain. The balloon framing calculations assume two head plates and one sill plate at each floor while platform framing calculations assume two head plates, one sill plate, and 2×10 floor joists at each floor.
Note: The calculations tabulated above assume a story height of 10 ft.
The calculations above also consider southern pine in the mid-Atlantic region with moisture content at installation of 19 percent and an equilibrium moisture content of 12 percent. The designer should use the appropriate values and properties for the environmental conditions for their location. The values in Table 1-2 above do not include an allowance for settlement of construction gaps in the wood framing.
During the summer months the average temperature of the brick will be at its highest value for the year. The accumulation of moisture expansion of brick, wood drying shrinkage, and wood creep increases with time. The cumulative expansion of the brick masonry and shortening of the wood framing over time and considered during the summer results in a total maximum differential design movement between the brick and wood backup systems as follows:
Note: Values in the table above represent theoretical and probably upper-bound anticipated movement for balloon-framed structures, using the assumptions noted above for the masonry and wood. The theoretical movements for platform framing are higher than these. In our experience actual movements will be less than the computed movements. This difference is due, in part, to restraint of wood shrinkage and creep by attached interior finishes.
Effects of differential movement on penetration detailing
Differential movement between cladding and backup is straightforward to address in blank masonry walls. The brick veneer must be attached with wire type brick veneer anchors that engage a backup plate and allow vertical movement of the anchor with respect to the backup plate without compromising the anchor’s load capacity. These anchors can provide several inches of vertical adjustability that is primarily intended to facilitate brick veneer installation, but, if the anchors are deliberately installed with appropriate clearance, they can allow the wire anchor to slide upward with the brick masonry veneer, while remaining engaged in the backup plate.
Windows and doors, balconies, and mechanical penetrations such as dryer vents and PTAC unit sleeves that will bridge the backup and brick veneer are more challenging to deal with than anchoring penetration-free walls. Strategies to accommodate movement include allowing the penetrants to flex between the structure and brick cladding, or to allow the brick cladding to slide around them. After determining the required clearance between the penetrant and the brick veneer based on the expected total cumulative differential movement, the brick veneer must be installed with this clearance around each penetration or fenestration. The resulting gaps must be covered to protect vulnerable backup construction, including the building’s water-resistive barrier and window perimeter flashing, and to be aesthetically pleasing, all the while accommodating vertical movement. The large movement demand prevents the installation of reasonably-sized sealant joints, so the gaps must be filled with overlapping flashing or trim components that can slide over each other and are detailed to keep out bulk water and insects. The following arrangements cover some typical cases. These examples are not attempting to address water and air penetration resistance of these details – these are important considerations for all enclosure detailing, but beyond the scope of this article.
Window head: At window heads, the upward movement of the brick veneer will tend to open up a gap between the underside of the steel loose-lintel and the window frame. After the window is integrated into the wall water-resistive barrier, a trim piece that is fabricated from durable sheet metal flashing or synthetic wood board to match the appearance of the window frame can be installed over the window head. As the steel lintel moves upward over time, an increasing portion of this trim is exposed.
Window jamb: Along the window jamb, the upward movement of the brick veneer results in a shearing motion between the brick and the window. This movement can be managed with an L-shaped metal trim cavity closure piece that is attached to the backup along the jamb. The brick masonry is finished to the closure.
Window sill: The required gap between the window sill and brick masonry results in a skyfacing opening that has to be covered with a piece of metal cap flashing trim that keeps out bulk water, and must
be integrated with the window jamb trim. The large downturned leg of the trim piece represents a significant aesthetic compromise over stone sills or brick masonry rowlock sills that are traditionally used for this detail.
Mechanical penetrations: These typically extend beyond the face of the masonry and the required gaps at the perimeter of the penetration sleeve can be covered with a flashing collar that extends over the brick masonry.
The past years have seen an increase in wood-framed buildings that are covered with multi-story brick veneer cladding without intermediate gravity load supports. This configuration can result in substantial differential vertical movement between veneer and backup. Designers should calculate anticipated differential movement on a project-by-project basis. The building enclosure details must consider the organization of the structure to successfully accommodate movement. To avoid damage to façade components that straddle the backup and the veneer, the enclosure design for these buildings must include effective provisions to accommodate this movement.
Anthony Nicastro, P.E., senior staff II, is a member of Simpson Gumpertz & Heger Inc.’s Building Technology group. Robert Antes, staff II, has two years of experience in the Structural Engineering group at SGH. Scott Silvester, P.E., is an associate principal with SGH. Niklas Vigener, P.E., is a senior principal and division head with SGH.