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.■

We’re very excited to have had the following article published in the November issue of Structure magazine. It goes without saying that we’re super proud of the M+K staff members who co-authored this piece: Bert Coelho, P.E. [Principal + Dir. Business Development], Kevin Quan, P.E. [Sr. Project Engineer], and Micah Milner, P.E. [Project Engineer].

Steel Stud Bearing Walls

An Alternative to Traditional Podium Levels

Developers are always looking for new and innovative ways to make buildings more profitable and financially efficient. Traditionally, a multi-story bearing wall building over one to two levels of steel or concrete, also known as a podium structure, has filled this need by offering high-density residential units at the upper levels and parking or retail spaces at the lowest floors. For multi-story residential buildings, wood construction is generally considered the most cost-efficient form of construction, followed by cold-formed steel (CFS) bearing walls and then heavy steel or concrete framing. Because of this, developers have sought to maximize the number of wood-framed floors over the podium level before switching to more costly materials.

Wood bearing wall systems are limited to either 4-stories using Type V-A construction or 5-stories using Type III-A construction. Both types of construction can be supported on grade or a podium structure. Podium structures have been covered extensively in previous articles by STRUCTURE magazine. They are proven methods of meeting not only financial requirements but zoning and regulatory obligations as well. This article focuses on an alternate hybrid system that maximizes the number of stacking levels of residential units by introducing multiple levels of CFS bearing walls to support the wood-framed levels before transitioning to the transfer level or foundation.

The Challenge

Recently, some developers have opted to add additional residential units below the Type V-A and III-A constructed units. This means the stacking residential units from above are carried down below the podium level by minimizing or removing the retail and parking areas altogether. This change increases unit density, thus maximizing more profitable areas by limiting the less profitable retail and parking areas.

The challenge with incorporating stacking units from above with a typical steel or concrete podium superstructure is the interference of regularly spaced columns with residential unit layouts. While the podium column scheme lends itself well to open spaces required for retail and parking, the standard grid pattern presents architectural challenges at residential levels in accommodating columns within the unit layouts. Additionally, non-stacking loads from the residential bearing wall system above inevitably require deep steel or concrete beams or heavily reinforced concrete slabs and may even require drop panels. The ideal column grid may also disrupt stacked window layouts along the exterior of the building below the podium level.

Architecture is not the only discipline to face challenges with this system. Plumbing, electrical, and HVAC systems are typically accommodated by having stacking mechanical penetrations for the full height of the building. In a wood-framed building, these are commonly located within the wall stud cavity. Therefore, stacked penetrations running down through the steel or concrete podium structure invariably conflict with beam and column locations that directly support the walls above.

To resolve these issues, architects frequently need to create new unit types and floor layouts to work around the traditional podium structural elements. Unfortunately, the new layouts can create spaces that may be inefficient or undesirable to developers or prospective tenants. Continue reading “M+K Article Published in Structure Magazine”