M+K Article Published in STRUCTURE Magazine

Our article in Structure Magazine outlines our holistic approach to designing 5-story wood-framed buildings utilizing Type III construction. Congrats to co-authors Jared Hudson, P.E., and Shaun Kreidel, S.E. Great job!

A Practicing Engineer’s Approach to Wood-Framed Type III Construction

Design considerations and common detailing strategies for Wood-Framed Type III construction are discussed.

Light frame wood construction is often a desired construction method for low-rise multifamily structures due to readily available labor and materials, speed of construction, sustainability, and relatively low construction costs. A Type V construction classification as defined by the International Building Code (IBC) is commonplace for these structures; however, this construction type is limited to four stories of stacking wood construction. A Type III construction classification allows conventional wood-framed structures to include an additional level, bringing the allowable height to five stories above grade; see Figure 1 for an example of this type of construction. This construction type may be attractive to developers looking to maximize the occupiable square footage of a defined footprint while taking advantage of the many benefits that come with light-frame wood construction. To facilitate a Type III classification, unique structural and architectural detailing is needed to maintain the strength, stability, and serviceability of the wood-framed structure, as well as to address the applicable fire design requirements. These details are multidisciplinary in nature and require a high level of collaboration between the structural engineer, architect, and builder/developer to ensure that the project meets the owner’s expectations and the building code requirements of the Authority Having Jurisdiction (AHJ).

Depending on the requirements of a given project, practicing engineers may need to investigate certain design aspects that become critical when meeting the requirements of Type III construction. These design considerations include material requirements, fire-resistance rating requirements, the importance of designing for wood shrinkage, and structural detailing strategies to accommodate fire-resistance ratings at the intersection of the floor/roof assemblies and exterior wall assemblies.

Figure 1: This building in Nashville, Tennessee, is an example of Type III construction. Photo courtesy of Jared S. Hudson.

Material Requirements

While construction Types I, II and III all require the use of non-combustible materials at exterior walls, the IBC recognizes the use of fire-retardant-treated (FRT) sawn lumber and FRT wood structural panel (WSP) sheathing as acceptable materials to satisfy the requirement under Type III construction. Practicing engineers should account for FRT lumber and FRT sheathing strength reduction factors due to the treatment process. The strength reduction factors are manufacturer-specific, thus coordination with the architect and builder/developer is recommended if the intended product is unknown.

FRT treatment process results in sheathing strength reduction factors which can decrease both the allowable spans and the lateral strength/stiffness of diaphragms or shear walls. FRT lumber treatment process also affects the structural properties of sawn lumber; the designer may need to augment the wall/header designs to mitigate these effects. Table 1 illustrates the strength reduction factors from two manufacturers of FRT sawn lumber. Assumed in-service temperature of the lumber is an important consideration that may cause variation in structural property values between manufacturers. High in-service temperatures of more than 100 degrees Fahrenheit will correspond to a greater reduction in strength and stiffness when coupled with fire retardant treatment. The engineer should also account for any wood incising reduction factors that might be needed to treat the lumber and consider using lumber that does not require incising to mitigate the amount of strength reduction. All minimum assumed FRT properties should be listed as design assumptions in the contract drawings to ensure that suitable lumber and WSP products are utilized.

Table 1. FRT Strength Reduction Factor Comparison

A designer may encounter situations where spans or loads require structural properties beyond what FRT lumber alone can provide. At this time, there are no fire-treated engineered wood products on the market (e.g., LVL, PSL, LSL) known to the author. One strategy available to designers is to utilize a flitch beam; a composite beam that consists of FRT wood laminations bolted to a continuous steel plate. The FRT laminations of the composite assembly will maintain the non-combustibility requirement; however, special attention to detailing to adequately conceal the heads of the bolts of the flitch beam assembly will be required. The designer should also consider the expansion of the longitudinal steel due to elevated service temperatures for longer-spanning flitch beams. Another strategy that the designer can employ is the use of rolled steel framing members within the exterior wall. These members may require additional fire protection in addition to meeting the noncombustible requirements of the code; the project architect should be consulted for additional fire protection requirements of these members.

Fire Rating Requirements

Type III construction requires that exterior loadbearing walls satisfy a 2-hour fire-resistance rating (FRR). If exterior walls can be classified as non-load bearing, the FRR can be reduced to 1-hour for certain occupancies. A 2-hour FRR is usually accomplished by having two interior layers of gypsum board. Over the full perimeter of the structure, the added cost of an additional layer of gypsum board can be substantial. A common industry interpretation of a non-load bearing exterior wall is one that does not support anything but its self-weight and the self-weight of the walls above. The structural designer can strategically run the framing parallel or introduce girder members parallel with the exterior wall to avoid a load bearing situation. In doing so, a FRR of 1 hour can be utilized and thus an extra layer of interior gypsum board can be avoided. This approach and interpretation should be discussed with the project architect and the AHJ during design to ensure compliance with the local building code.

The vertical continuity requirements of the rated exterior wall assembly have been a hotly debated topic between jurisdictions and design professionals, but the requirements have finally been clarified in the 2024 IBC. According to Section 705.6 of the 2024 IBC, the exterior wall FRR shall extend continuously from the top of the foundation/floor system below to the underside of the roof/floor sheathing above. However, if the fire separation distance (as defined in the IBC) is greater than 10 feet, the exterior wall FRR is permitted to terminate at the underside of a ceiling (floor or roof) assembly having an equal or greater FRR than the exterior wall. Detailing at the floor levels and the roof level will need to conform to these requirements. Some commonly used detailing strategies that meet these criteria are presented later in this article. Continue reading “M+K Article Published in STRUCTURE Magazine”

M+K Ranks 4th Nationally in Zweig Competition

Déjà vu? Hell Yeah! We ranked 4th place nationally for Structural Engineering Firms in Zweig Group’s ‘Best Firms to Work For’ awards competition this year.

Our team is proud and honored to have placed in the top 10 structural firms for each of the past 13 years – and in the top 5 for 10 of those years!

This sustained record would not have been possible without full buy-in from our great employees. Since we opened our doors in 2000, we have been committed to establishing a workplace culture that is welcoming, giving, supportive and fun.

Creating an award-winning culture is a core tenet of our Mission, and it has been critical to both attracting and retaining the best employees in the business. Why? It’s simple…you need great people to provide great client service. M+K employees are, without exception, the BEST in the business!

M+K Article Published in STRUCTURE Magazine

Our article in this month’s issue of Structure Magazine examines how different lateral restraint system classifications can impact the bottom line. Kudos to the author – M+K employee Timothy R. Donahue, P.E.. Great job Tim!

Seismic Response Factor in Cold-Formed Steel Flat Strapped Lateral Systems

Two seismic designs are compared in a hypothetical 10-story building.

Generally speaking, for a given structure, the higher the seismic response factor (R) value is, the more ductile the structure is and the lower the total seismic load acting on the building. With this concept in mind, one might conclude that reducing the seismic load acting on a building by selecting a more ductile seismic force restraint system (SFRS) with a higher R value would ultimately lower the material cost; however, when evaluating the overall cost implications for what it takes to achieve the increased ductility, the cost savings may not always be realized. This is primarily due to the requirements of overstrength factors in the design and detailing of the straps and other protected components of the SFRS with an R greater than 3.

For cold-formed steel framed buildings utilizing flat strap bracing in Seismic Design Categories (SDC) B or C, the structural engineer can design for either an R of 3 by classifying the SFRS as “Steel Systems Not Specifically Detailed For Seismic Resistance”, or for an R of 4 by classifying the SFRS as “Light-frame (cold-formed steel) wall systems using flat strap bracing” (per AISI Section A1.2.3 and Table 12.2-1 of ASCE 7-16).

Due to increased ductility, buildings designed with an R of 4 have story forces that are 25% less compared to a building designed using an R of 3. To achieve the higher ductility however, more in depth design of the protected components is required in accordance with AISI-400, Section E3.2. These protected components include collectors, connections of strap bracing, chord studs, vertical boundary elements, and floor to floor connections of the boundary elements (see figure 1). This is to ensure that the energy dissipating elements (the straps) yield while the boundary members and connections remain elastic.

To better understand the potential cost implications of the two different designs, let’s look at a typical end post connection of a hypothetical ten story building, in SDC B, that uses a light gauge strapped wall bracing SFRS. Figure 1 shows the connection design for the R=3 case, while Figure 2 shows the connection design for R=4.

As expected, the strap sizes are slightly smaller (or lighter) for the design utilizing an R of 4, as the lower base shear values result in lower strap forces. However, the end post sizes are larger for the R=4 design in order to provide enough weld length for the end post tensile and shear forces. While there are other potential solutions to resolving the increased connection forces beyond increasing the post size, such as adding angles or gussets, each solution ultimately adds cost to the assembly which offsets or negates the savings found in the straps.

Another element of the SFRS that is impacted by the overstrength requirements is the anchorage of the assembly to the concrete foundation. For the aforementioned building, the embed plate with welded studs used to transfer the loads into the foundation would need to accommodate 324 kips of total load for the R=4 design once overstrength is applied but would only need to account for 215 kips in total for the R=3 design. The larger forces, once the overstrength factor is applied, for the R=4 condition results in a more expensive base connection.

In this specific test case, the lower base shear obtained by using the higher R factor did not correlate to a more economical structure. The more ductile design (with R = 4) is in fact quite a bit more costly than the simplified design (with R = 3) despite having to resist lower seismic loads. While this result will not always be the case and there often is potential to economize a building’s lateral design by taking advantage of the reduced story shears resulting from an R of 4, the cost impact of overstrength requirements needs to be considered when selecting the SFRS.■

Women in Engineering at M+K

We’re very proud to showcase Mulhern+Kulp’s ‘Women in Engineering’ group, comprised of 12 members across our Philadelphia, Atlanta, and San Diego offices. WIE launched over two years ago in support of the diversity initiative from our 2020 strategic plan.

The Mission of WIE is “Retention, Community, Volunteering” with the goal to positively impact the work experience, careers, recruiting, and retention of female engineers at M+K. At its core, the group provides a community of support and mentorship for members to discuss topics and challenges unique to their experiences as women engineers.

Working in collaboration with the Society of Women Engineers (SWE), our WIE members presented ‘Introduce a Girl to Engineering’ at a high school in Marietta, GA, attended a SWE networking dinner at Villanova University, and participated in Habitat for Humanity’s ‘Women’s Build Day’ in San Diego.

In keeping with the spirit of Giving Back at M+K, the WIE group created a team to raise funds, promote, and participate in the nationwide Susan G. Komen ‘Walk for the Cure’ event on behalf of Breast Cancer Awareness month. A ‘Wear Pink Day’ fundraiser was staged at each of our offices which contributed to the overall achievement of raising $4,000+ in support of research and advocacy to combat breast cancer!

We’re looking forward to WIE’s continued positive contributions to M+K’s staff and culture, as well as their charitable efforts and their promotion of engineering as a great career opportunity for young women.

The Boardwalk | Minno & Wasko

The city of Long Beach is part of the NYC metro area and home to a $369M oceanfront luxury residential development right on the iconic boardwalk. Aptly named “The Boardwalk”, the project takes the place of a long-abandoned stretch of land known to locals as the “Superblock”. With over 400 condominium and rental units served by 40,000 SF of amenities and 6,500 SF of retail options, it’s clear to see their mission is to create a new coastal living experience.

A single 10-story building constructed of COMSLAB floors and light gage stud walls alongside a pair of 9-story reinforced concrete buildings are all supported off a 2-story reinforced concrete podium and parking structure. This base structure and its foundations were designed to support the heavy loads standard for this type of high-rise construction (the buildings above, pools and heavy vehicle traffic, and high coastal wind loads to name a few) in addition to the breaking wave and debris impact loads required in VE flood zones, all while ensuring the structure could access (but not disturb) the boardwalk structure after which it was named.

While always proud to work alongside Minno & Wasko Architects and B2K Development, we were particularly enthused to be a part of a project that would have such a transformative impact on the city of Long Beach and the greater NYC region.

Suncadia Resort | Architects Northwest

Suncadia is a resort community at the base of the Cascade Mountains in Washington State, known for its luxury stays, amenities, and year-round outdoor activities throughout its mountains and meadows. This high-end custom home was designed and built in collaboration with Architects Northwest and KBM Construction to fit seamlessly into this landscape, with the complex elevations and sweeping roof profiles expected of any modern mountain lodge escape. Heavy timbers were used frequently in exposed roof and porch framing to complete the aesthetic and pair well with the structures of the nearby resorts.

As seen in any of the picturesque snow-capped photos of this community, they experience some serious snowfall. With that in mind, the structures in this region must be designed to support snow loads 5-10 times greater than much of the rest of the country. Supporting these heavy loads while maintaining a slender roof profile is challenging for wood framing, but thankfully fits right in our wheelhouse.

A Marketing Director’s Testimonial

My name is Rob Taylor, Marketing Director here at Mulhern+Kulp.

Initially, the bulk of my career was focused on strategic brand identity development and packaging design. I worked with multi-national corporations such as Procter & Gamble, Mattel, and Disney, among many others. I then started a consulting business, with the intent of bringing my consumer goods marketing experience to the business-to-business sector. This ultimately led me to M+K.

M+K was the first engineering firm I’d ever worked with. When I transitioned from consultant to full time employee about 7 years ago, I began the process of getting to know everyone. I must admit that, coming in, my view of engineers was shaped by stereotypes from TV, movies, and books – basically, introverted math nerds.

As time passed, I realized that M+K was anything but a monolith. Among our great employees is a published author, several musicians [some have published songs], a world traveler who has climbed Kilimanjaro [among other feats], talented golfers, basketball & soccer players, skiers, cyclists, a yoga teacher…I could go on!

Speaking of musicians, I don’t think there’s a better example of how interesting, diverse, and creative our staff is than the fact that we have an M+K Band! Members include multiple singers, guitarists, keyboardists, bass players, and a drummer who plays in jam bands [also a rabid Phish fan]!

The lesson for me is best summed up by the idiom ‘You can’t judge a book by its cover’. This truly is a great place to work, and we have 10+ years’ worth of workplace culture awards to back up that claim!

Soundwater Apartments | Studio Architects

Headquartered in Atlanta, GA, Studio Architects provides thoughtful designs over a large swath of the construction industry with projects in education, hospitality, sporting venues, and, where our paths intersect, residential. Our collaborations on multi-family residential buildings include several in New York’s Hudson River Valley and in various locations across the southeast. The latter of these regions is home to our largest project together to date, along with builder/developer Southeastern.

Soundwater Apartments offers luxury rental units with all manner of modern amenities in the Metro Center district of Nashville, TN, right on the Cumberland River. In addition to eight independent apartment buildings ranging from two to five stories, the complex also includes a dog park, two pools, and a three-story clubhouse with a fully stocked community room, fitness center, and rooftop lounge. Reaching the five-story limit of wood-framed construction with three of these buildings balances grandeur and material cost, but requires intimate knowledge of the additional structural requirements to keep the construction safe and buildable at these heights, particularly in a region with high seismic loads. These high lateral loads also provided design challenges at the steel-framed clubhouse, which ultimately required the hefty connections and careful detailing that come with Intermediate Moment Frames.

Water’s Edge | OMNIA Group Architects

Designing the structure for custom shore houses is often challenging. Cost-effective wood framing alone will sometimes not cut it when trying to resist high wind forces while leaving room for the ocean view. However, of the variety of wall and floor framing systems used to achieve these designs in the northeast region, reinforced concrete is rarely one of them.

Water’s Edge is a three-story custom single-family home on the beachfront in Long Beach Township, New Jersey. To ensure this shore home would be as robust as possible, the owners chose to build it almost entirely out of reinforced concrete. Both the walls and floors were framed with an Insulated Concrete Form (ICF) system, which uses proprietary wall blocks and light gage floor joists to speed up the concrete installation and skip the insulation step afterward. To maximize visibility, concrete was subbed-out for more slender steel columns in a few key areas, namely the two-story bright-red steel x-brace supporting both levels of outdoor deck in the center of the main elevation.

The architectural design was provided by The OMNIA Group Architects, a full-service architectural firm performing single-family, multi-family, and commercial work in the Philadelphia region, among others. While we have collaborated on some production style homes, the majority of our work together includes the more unique and challenging custom projects that Water’s Edge exemplifies.

Harrison Urby | Minno & Wasko

As one of our oldest collaborators (20+ years), we’ve had no shortage of successful projects alongside Minno & Wasko Architects and Planners. Though we have designed many multifamily and mixed-use buildings together in northern New Jersey and the surrounding region, the series of expansive urban redevelopment projects in Harrison, New Jersey with Ironstate Properties stands out as one of our first and longest running multifamily partnerships. Since 2006, we’ve designed a total of 12 mid-rise buildings and various auxiliary structures – most of which occupy a full city block.

Section 4 of Harrison Urby, the latest and largest of these projects, is no exception. With five mid-rise residential buildings (ranging from five to seven stories) and a single-story café, there was a wide variety of structural systems required to ensure an efficient use of materials. While the taller buildings are exclusively light gage metal stud bearing wall systems, the five story structures are predominantly wood framed with a single story of light gage at the first floor. The two-story basketball court built inside one of the residential buildings created an interesting challenge, requiring a steel frame to support the three levels of units above and fit between the units on either side. The café structure rounds out another varied project with its glued laminated timber framing supporting a cross-laminated timber roof deck.