With the use of engineering software tools, structural engineers can design buildings faster and more efficiently than ever before. In this blog post, Clifton Melcher, P.E., a senior project manager for cold-formed steel connectors, discusses the various enhancements included in version 2.5 of Simpson Strong-Tie® CFS Designer™ software. Continue Reading
The SE Blog is taking some time off for the 4th of July holiday this week. However, we’ve just released the 2017 edition of our Connectors for Cold-Formed SteelConstruction catalog – order a hard copy to be mailed to your office or download a PDF copy and start using it today!
Connectors For Cold-Formed Steel Construction
The C-CF-2017 is a 308-page catalog including specifications, load tables and installation illustrations for our cold-formed steel connectors and clips, helping you easily specify and install in commercial curtain-wall, mid-rise and residential construction.
This week’s post comes from Jeff Kreinke, PE, SE, a structural Project Manager with Excel Engineering in Fond du Lac, WI. Jeff earned his Bachelor’s degree in Civil & Environmental Engineering from the University of Wisconsin-Madison. He has worked in Excel Engineering’s structural department for 15 years and specializes in cold-formed steel systems design. This specialization has included the design of dozens of “blast” resistant structures per the Unified Facilities Criteria standards. Jeff is licensed in 22 states as a Professional or Structural Engineer.
Back in the year 2000, the U.S. Department of Defense (DoD) was charged with incorporating antiterrorism protective features into the planning, design and execution of its facilities. The main document developed to meet this requirement is the Unified Facilities Criteria “DoD Minimum Antiterrorism Standards for Buildings” (UFC 4-010-01). The current version was published in October 2013. This document covers what is most commonly referred to as “blast” design.
All DoD inhabited buildings, billeting (military housing) and high-occupancy family housing projects are required to comply with this standard. The most common projects incorporating cold-formed steel framing are for the military branches (including National Guard and Reserve components). Notable exceptions are: low-occupancy buildings (11 occupants or fewer), transitional structures (with intended life cycles of five years or less), standalone retail establishments and parking structures. A complete list of excluded buildings types is located in chapter 1-9 of UFC 4-010-01.
General structural requirements for DoD projects are provided in the UFC 1-200-01 standard. The current model building code referenced is the 2012 IBC. Specific structural design criteria are also listed for all major military bases. UFC 4-010-01 specifically states that its requirements do not supersede the structural requirements of UFC 1-200-01. Basically, three lateral designs are required for all DoD projects: wind, seismic and blast.
The main design strategies employed by UFC 4-010-01 are to maximize standoff distance, prevent progressive collapse and minimize hazardous flying debris. Progressive collapse avoidance is addressed by UFC 4-023-03. The main criterion to note is that it is only required in structures of three or more stories in height.
UFC 4-010-01 tables B-1 and B-2 provide the conventional standoff distances and minimum standoff distances for a building based on its construction type and building category. The conventional standoff distance is where conventional construction may be used for building components other than windows and doors without a specific analysis of blast effects. The minimum standoff distance is the smallest permissible distance allowed regardless of any analysis or construction hardening.
To minimize hazardous flying debris, two basic components are addressed: typical wall framing and opening support framing. UFC 4-010-01 Table 2-3 provides the allowable height for cold-formed stud typical wall framing based on material, spacing, support conditions and supported weight.
For both brick veneer and EIFS cladding, the maximum allowable height is 12″-0″. Note that 50 ksi stud material is required to meet these requirements. There is no similar strength requirement for connections.
For framing that meets both the conventional standoff distance and allowable member height, NO further blast analysis is required. For framing that does not meet either the conventional standoff distance or allowable member height, additional blast analysis IS required. The support framing (head/sill/jambs) for windows and skylights ALWAYS requires additional blast analysis. Per section B-3.3.3 of UFC 4-010-01, the support framing for doors, glazed or solid, is not required to be analyzed for blast. Doors are designed to remain in their frames and not become hazards to building occupants. Sidelights and transoms that are included within the door assembly are also not required to be analyzed for blast. There are exclusions provided for unoccupied areas of buildings, such as exterior stairwells and exterior walkways. Personal experience with the U.S. Army Corps of Engineers Protective Design Center (USACoE PDC) has shown that unoccupied attic areas are also allowed to be excluded from the provisions.
Two “levels of protection” are provided for in the UFC 4-010-01. Only Low Level of Protection (LLOP) and Very Low Level of Protection (VLLOP) buildings are addressed. Low Level of Protection allows for moderate damage with collapse being unlikely and having the potential for serious but not fatal human injury. Very Low Level of Protection allows for heavy structural damage with progressive collapse unlikely and having serious human injury likely with potential fatalities. Any projects that require a higher Level of Protection designation must be referenced elsewhere. The project appropriate “Level of Protection” is provided in UFC 4-01-01 Tables B-1 and B-2.
Static analysis of “punched” openings (framed with head and sill members and supported by jamb studs) is only allowed to be performed if the conventional standoff distance is met. Ribbon windows, aluminum curtainwalls, storefronts, etc., are required to be designed by the dynamic analysis method. The “punched” window supporting structure is to be designed to account for the increased tributary area representing the area of the window and the walls above and below it. These supporting elements must have moment and shear capacities greater than the calculated conventional wall capacities multiplied by the applicable tributary area increase.
For example, if the tributary area of the “punched” opening is three times the typical full-height stud spacing, three members are required for the jamb stud assembly. An alternate member with moment and shear capacities greater than or equal to three times the typical full-height members’ capacity can also be used. UFC 4-01-01 section B-126.96.36.199 states that connection loads shall be determined based on the increase in member shear capacity. It makes a difference which members are chosen for the jamb stud assembly, as the connection must be designed for the shear capacity of that specific assembly. Per UFC 4-010-01 section B-3.1, the connectors themselves are designed using LRFD methods with a Load Factor of 1.0. The resistance factor for bending is allowed to be 1.0, while the resistance factors for other failure modes remain per the AISI code (Shear = 0.95). Per UFC 3-340-02 section 5-47, when LRFD values are not published for connectors, a value of 1.7 times the allowable strength is permitted. Published LRFD strengths already have the appropriate resistance factors incorporated. Simpson Strong-Tie publishes LRFD values for all of its connectors.
Dynamic Analysis Method
As mentioned above, dynamic analysis is performed with the SBEDS spreadsheet tool provided free of charge by the USACoE PDC. There is a simple approval process to undergo in order to receive a software key for the program. The general inputs for the program are fairly straightforward. There is a list of standard preset stud shapes that can be selected. The list is not comprehensive, but shape property inputs are provided for “user defined” members.
The SBEDS spreadsheet is formatted to analyze single-span wall framing. For analyzing opening support framing, the stud spacing and supported weight will have to be adjusted. Per PDC TR-10-02, the stud spacing is to be figured the same way as in the static analysis method. The spreadsheet analyzes the entire wall at one supported weight, so this will also have to be adjusted to account for the differing weights of the building cladding and glazing, providing an average support weight. For EIFS cladding, all weights are typically assumed to be the same (6 psf actual). For brick veneer (46 psf actual), the supported weight must be reduced to account for the lower weight of the glazing (6 psf actual). The wall span will need to be adjusted to the actual length of the head and sill members.
The blast parameters are input as “Charge Weight & Standoff.” The appropriate project explosive weight is referenced from the UFC 4-010-02 standard. This publication is authorized only to U.S. Government agencies and their contractors. Approval must be obtained from the USACoE PDC to obtain this document. The standoff distance is entered as the project-specific standoff distance. The Incidence angle can be calculated as the arc tangent (ATAN) of the height to the center of the opening divided by the actual standoff distance. The remaining typical blast parameter inputs are shown below.
The typical response criteria inputs are shown below. Typically stud framing is required to be “connected top and bottom.” There is an option to select “Top Slip Track,” but personal experience has shown that this option does not pass the analysis. There are two options provided with the “Level of Protection” type. “Primary” would be selected for load-bearing projects, while “Secondary-NS (Non-Structural)” would be selected for curtainwall-type projects.
For the Solution Control inputs, the main check is to verify that the inputted time step matches the calculated recommended time step. The calculated value will change when various other inputs are changed. The “% of Critical Damping” can be entered as 5% for CFS framing, per the spreadsheet cell comment. The “Initial Velocity” is entered as zero, also per the spreadsheet cell comment.
Connections are designed to meet or exceed the “Peak Reactions Based on Ultimate Flexural Resistance” value given in the SBEDS output. Per UFC 4-010-01 section B-3.1, design is done per LRFD methodology with Load Factors equal to 1.0. The Resistance Factor for bending is allowed to be 1.0, while the Resistance Factors for other failure modes are per the AISI code (Shear = 0.95). Published LRFD strengths already have the appropriate resistance factors incorporated. The conservative assumption would be that shear controls the failure, and an increase in the LRFD Resistance Factor is not appropriate.
Per UFC 3-340-02, there are additional increases in connection strength that can be taken for dynamic blast design. The Strength Increase Factor (SIF, a.k.a. Static Increase Factor or Average Strength Factor) considers that yield stresses for all CFS materials provided are typically higher than the minimum yield stresses required by ASTM A446 (replaced by ASTM A653) steel. Per section 5-12.1, the SIF is listed as 1.21 for all CFS framing but only applies to the yield stress (Fy). Any calculations involving the ultimate stress (Fu) value are not allowed to be increased. The SIF also does not apply to fasteners (screws, bolts, PAFs, welds, etc.). The Dynamic Increase Factor (DIF) considers the strain-rate effects from rapid blast loading. Per section 5-34.2, this increase is listed as 1.10 for all CFS framing and per UFC 4-0-010-01 section 4-7, 1.05 for welds. Conservatively, it is assumed that the DIF equals 1.0 for all other fasteners (screws, bolts, PAFs, etc.).
While consideration of bracing is important for any structural element, this is especially true for thin, singly symmetric cold-formed steel (CFS) framing members such as wall studs. Without proper consideration of bracing, excessive buckling or even failure could occur. Bracing is required to resist buckling due to axial or out-of-plane lateral loads or a combination of the two.
There are two methods for bracing CFS studs as prescribed by the American Iron and Steel Institute (AISI) Committee on Framing Standards (COFS) S211 “North American Standard for Cold-Formed Steel Framing – Wall Stud Design” Section B1. One is sheathing braced design and the other is steel braced design.
Sheathing braced design has limitations, but it is a cost effective method of bracing studs since sheathing is typically attached to wall studs. This design method is based on an assumption that the sheathing connections to the stud are the bracing points and so it’s limited by the strength of the sheathing fastener to stud connection. Due to this limitation, the Designer has to use a steel braced design for most practical situations. AISI S211 prescribes a maximum nominal stud axial load for gypsum board sheathing with fasteners spaced no more than 12 inches on center. AISI S211 Section B1 and the Commentary discuss the design method and assumptions and demonstrate how to determine the sheathing bracing strength.
Sheathing braced design requires that identical sheathing is used on each side of the wall stud, except the new AISI S240 standard Section B188.8.131.52 clarifies that for curtain wall studs it is permissible to have sheathing on one side and discrete bracing for the other flange not spaced further than 8 feet on center. The wall stud is connected to the top and bottom tracks or supporting members to provide lateral and torsional support and the construction drawings should note that the sheathing is a structural element. When the sheathing on either side is not identical, the Designer must assume the weaker of the two sheathings is attached to each side. In addition, the Designer is required to design the wall studs without the sheathing for the load combination 1.2D + (0.5L or 0.2S) + 0.2W as a consideration for construction loads of removed or ineffective sheathing. The Designer should neglect the rotational restraint of the sheathing when determining the wall stud flexural strength and is limited by the AISI S100 Section C5.1 interaction equations for designing a wall stud under combined axial and flexural loading.
Steel braced design may use the design methodology shown in AISI S211 or in AISI Committee on Specifications (COS) S100 “North American Specification for the Design of Cold-Formed Steel Structural Members.”
Steel braced design is typically either non-proprietary or proprietary “clip and bridging” bracing, or “flat strap and blocking” bracing periodically spaced along the height of the wall stud.
Steel braced design is a more practical bracing method for several reasons. First, during construction, wall studs go unsheathed for many months, but are subjected to significant construction loads.This is especially true for load-bearing, mid-rise structures. Second, some sheathing products, including gypsum wallboard, can be easily damaged and rendered ineffective if subjected to water or moisture. Third, much higher bracing loads can be achieved using mechanical bracing. IBC Section 2211.4 permits Designers to design steel bracing for axially loaded studs using AISI S100 or S211. However, S100-07 requires the brace to be designed to resist not only 1% of the stud nominal axial compressive strength (S100-12 changes this to 1% of the required compressive axial strength), but also requires a certain brace stiffness. S211 requires the Designer to design the bracing for 2% of the stud design compression force, and it does not have a stiffness requirement. . AISI S100 is silent regarding combined loading, but S211 provides guidance. S211 requires that, for combined loading, the Designer designs for the combined brace force determined using S100 Section D3.2.1 for the flexural load in the stud and either S100 or S211 for the axial load. In addition, the bracing force for stud bracing is accumulative as stated by S211 Commentary section B3. As a result, the periodic anchorage of the bracing to the structure such as strongbacks or diagonal strap bracing is required.
Some benefits and challenges of steel clip and bridging bracing include:
Proprietary solutions, such as the Simpson Strong-Tie SUBH bridging connector, can significantly reduce installed cost since many situations require only one screw at each connection.
Unlike strap bracing, u-channel bracing can be installed from one side of the wall.
U-channel bracing does not create build-up that can make drywall finishing more difficult.
Extra coordination may be required to ensure that u-channel bridging does not interfere with plumbing and electrical services that run vertically in the stud bay.
Bracing for axial loaded studs requires periodic anchorage to the structure, such as using strongbacks or diagonal strap bracing.
Bracing of laterally loaded studs does not require periodic anchorage since the system is in equilibrium as torsion in the stud is resisted by bridging (e.g., U-channel) bending.
Some benefits and challenges of steel flat strap and blocking bracing include:
May be installed at other locations than stud punchout.
Required to be installed on both sides of wall.
Bumps out sheathing.
Bracing for axial loaded studs requires periodic anchorage to structure, such as using strongbacks or diagonal strap bracing (same load direction in stud flanges).
Bracing for laterally loaded studs requires design of periodic blocking or periodic anchorage to the structure (opposite load direction in stud flanges).
There are several good examples Designers may reference when designing CFS wall stud bracing. They include AISI D110 Cold-Formed Steel Framing Design Guide that may be purchased from www.cfsei.org, SEAOC Structural/Seismic Design Manual Volume 2 Example 3 that may be purchased from www.seaoc.org, and the Simpson Strong-Tie wall stud steel bracing design example on page 60 of the C-CFS-15 CFS catalog.
Cold-formed steel framing is a versatile construction material, but Designers need to carefully consider the bracing requirements of the AISI specification and wall stud design standard. What cold-formed steel wall bracing challenges have you encountered and what were your solutions?
The Wei-Wen Yu Center for Cold-Formed Steel Structures has issued a Call for Papers for the 22nd International Conference on Cold-Formed Steel (CFS) Structures, to be held Nov. 5-6 in St. Louis, MO. The goal of the conference is to enable sharing of state-of-the-art information pertaining to CFS design.
Both engineering researchers and practitioners have provided valuable contributions to the conference. Past proceedings are available online.
Researchers and practitioners are encouraged to submit abstracts for consideration by the conference steering committee. Application-oriented topics highlighting innovations in CFS applications are strongly encouraged. The deadline is Dec. 31. Abstracts may be submitted by e-mail to email@example.com.
Simpson Strong-Tie is sponsoring the 23rd Short Course on Cold-Formed Steel Structures hosted by the Wei-Wen Yu Center for Cold-Formed Steel Structures (CCFSS). The course will be held on October 15-17, 2013 at the Drury Plaza Hotel at the Arch in St. Louis, MO.
This course is for engineers who have limited or no experience designing with cold-formed steel (CFS), as well as those with experience who would like to expand their knowledge. Lectures will be given by industry-recognized experts Roger LaBoube, Ph.D, P.E., and Sutton Stephens, Ph.D, P.E., S.E. The course is based on the 2007 and 2012 AISI North American Specification for the Design of Cold-Formed Steel Structural Members with the 2010 Supplement (AISI S100-07/S2-10 and S100-12) and will reference the Cold-Formed Steel Design (4th Edition) text.
Topics include design of wall studs, floor joists, purlins, girts, decks and panels. For more information and to register, visit the CCFSS website.
For the first half-dozen years of my professional career, my experience with cold-formed steel (CFS) consisted of sizing studs for non-structural walls and red-marking the bracing details on architectural plans. When the dotcom bubble burst, my firm needed to shift its focus from high-tech commercial and industrial to more multifamily design work. Several developers we worked with built with CFS, so in addition to designing condominiums instead of cleanrooms, I was designing CFS.
Less than 10% of engineers have any exposure to CFS design as part of their undergraduate education. OK – so this is based on an informal survey of about 50 colleagues, but I suspect if I were to hire a market research firm for lots of money, I’d pretty much get the same response.
For the first half-dozen years of my professional career, my experience with cold-formed steel (CFS) consisted of sizing studs for non-structural walls and red-marking the bracing details on architectural plans. When the dotcom bubble burst, my firm needed to shift its focus from high-tech commercial and industrial to more multifamily design work. Several developers we worked with built with CFS, so in addition to designing condominiums instead of cleanrooms, I was designing CFS. Less than 10% of engineers have any exposure to CFS design as part of their undergraduate education. OK – so this is based on an informal survey of about 50 colleagues, but I suspect if I were to hire a market research firm for lots of money, I’d pretty much get the same response. Continue Reading