Ten tips to secure good design

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    Structural engineers often create “best practice” lists for specific design and detailing tasks. The benefits of such lists are many, but at face value, they serve to remind veteran and new practitioners alike of what to consider for specialized areas of design. One common project task is designing with mechanical fasteners for attachment of steel deck roof and floor systems. This article is not intended to be comprehensive or to “re-invent the wheel,” but rather provides a simple checklist to help structural engineers avoid some common problems on future projects.

    1. Use available design references

    The best place to begin any project is to use available design references, including ICC Evaluation Service Reports (ICC-ES ESR), design software, and manufacturer catalogs.

    Technical resources

    Many fastener and steel deck manufacturers still offer printed technical design manuals and catalogs with diaphragm shear load tables, material specifications, and installation guidelines. Structural engineers can contact these manufacturers for copies of their latest technical literature or download from the company websites. These can be compiled into a binder or reference file for future project use. It’s prudent to check periodically for updates.

    ICC-ES ESRs are essential for demonstrating compliance with the International Building Code. ICC-ES has a long history of providing these independent product evaluations and they are considered by most building officials and structural engineers as the gold standard for construction product evaluations, including fasteners and steel deck. It is important to ensure that the ESR is applicable for the code to which the project is being designed. Structural engineers can visit the ICC-ES website for free downloads of ESRs, acceptance criteria, and other technical documents relevant to their project work.

    Free software exists for the design of steel deck roof and floor systems. Structural engineers can visit the Hilti webpage for a free download of the Profis Direct Fastening (DF) Diaphragm Design software that allows for steel deck diaphragm and wind uplift design with all types of connectors and deck profiles.

    2. Specify all attachment methods, and then let the contractor decide

    It’s common to let the contractor decide on the means and methods for the steel deck installation as long as the project submittal requirements are satisfied and all building code evaluations and approvals are in place, including ICC-ES ESRs, Underwriter Laboratories (UL), and Factory Mutual. Powder-actuated fastening systems can be specified alongside or in place of traditional arc spot puddle welds and screw fasteners. This practice allows contractors to bid the project based on the method most efficient for them, which can result in lower overall costs for the owner. Use of available design and optimization software makes this relatively simple for the designer.

    3. Ensure choice of proper fasteners for environmental and other conditions

    Use materials suitable for the conditions. In general, do not specify zinc electroplated fasteners for exterior, exposed conditions, but recognize that some fasteners may not be possible to hot-dip galvanize and added corrosion protection may increase material cost. Consider stainless steel fasteners or methods to seal the connections of steel deck in exterior, exposed conditions — zinc-rich compounds or paint will reduce galvanic corrosion but will not protect against stress-corrosion cracking. When used as part of a built-up roof system and concrete-filled floor decks, zinc electro-plated fasteners and unprotected welds are typical. Due to the heat developed during welding operations, zinc coatings and paint that might be applied to steel deck surfaces can be consumed or damaged. This may require touch-up protection if the steel deck is used in exterior, exposed conditions, as an added work step. However, if it’s not specified or inspected, touch-up may not occur, and this can lead to unsightly red rust, corrosion staining, and possible connection failure later.

    Mechanical fasteners have published application limits and these should be understood by the structural engineer. Application limits include base steel thickness, tensile strength, and steel deck thickness. Proper mechanical fastener types must be selected based on applications verified by laboratory tests. Not all fasteners are intended or have been qualified for all applications. Verify with the manufacturer or check its technical literature for details. The American Iron and Steel Institute (AISI) provides guidance and limitations on welds and screw-fastened steel connections in the North American Cold-Formed Steel Specification.

    Welding of steel deck when moisture is present on the surface may present a serious electrical shock safety hazard. In fact, Annex E of AWS 1.3 includes the following: “…to prevent shock, the work area, equipment, and clothing should be kept dry at all times. The welder should stand on a dry board or insulated platform.”

    For this reason, many contractors simply choose not to weld steel deck when there is moisture present. So, if your project will be constructed in an area where there is frequent rain or snow, it may make more sense to specify mechanical fasteners to minimize work stoppages. At the very least, this is an instance where mechanical fasteners should be specified as an option, as discussed above in tip 2.

    If an interlocking deck profile is preferred, it should also be noted that some manufacturers offer an interlocking screwable sidelap profile.
    Hilti

    4. Follow manufacturer installation guidelines

    This is obvious, but all too often, unfamiliar installers can make unintended mistakes. Project specifications can require that manufacturer representatives shall be on the project site before work begins to ensure that installers are properly trained. Similarly, AWS welding procedures should be followed; otherwise, unintended consequences may result. Due to the nature of welding, periodic or even continuous inspection may be required when welding is used. The SDI Manual of Construction with Steel Deck, MOC2, should also be consulted and referenced. A free copy of this reference can be downloaded from the SDI website.

    5. Understand what it takes to make specified connections

    If you are not well-aware of what your details look like in the field, you might not be getting what you are specifying. We’ve all heard the horror stories of trying to implement connection details that may look fine on paper or in CAD details, but are near impossible to complete in the field. The same holds true with steel deck fastening. Putting oneself “in the boots of the installer” can help guide decisions on connections. Following are questions for structural engineers to consider:

    • What is the most reliable, consistent way of making steel deck attachments?
    • How can I reduce the variability in workmanship that may exist between installers?
    • How reliable will connections be when installed “in the air” on my roof structure?

    6. Avoid specifying endlaps

    Multiple layers of thick steel deck can pose a serious challenge for arc spot welding and mechanical fastening alike. If you must specify thick steel deck panels, it might be better to butt the deck panel ends and add an additional row of connections just to be safe. Otherwise, you’re relying on the installer to tightly nest (or beat with a sledge hammer) the panel endlaps to reduce the gaps.

    Beating steel deck panels into place can also damage them, especially if panels are heavy zinc-coated or painted.

    In addition, 22- and 20-gage steel deck installed with mechanical fasteners has proven ductility that has been documented in various technical research investigations. For this reason, it might be prudent to use mechanical fasteners (pins or screws) with lighter gage deck whenever possible, if connection ductility is needed or desired.

    7. Consider thermal expansion forces

    Thermal expansion can be an issue on steel deck projects installed in hot weather or in environments with larger temperature differentials. This affects both welds and mechanical fasteners. If these environmental conditions exist, covering the steel deck soon after installation may help reduce these effects. The SDI Diaphragm Design Manual provides an example. How and where these design details are implemented is the structural engineer’s responsibility, but it can have a big impact on installation and constructability.

    8. Operators must be trained on the fastening system

    This is another obvious requirement that is overlooked. ANSI A10.3 Safety Requirements for Powder-Actuated Fastening Systems should be included in the Project Specifications and Structural General Notes where these mechanical fastening systems are used. This is similar to requiring deck welders to be AWS certified. These installer safety training and qualification requirements should be viewed as good installer attributes, rather than obstacles or barriers.

    9. Hold pre-construction conferences

    Proactive team meetings to review steel deck installation and verify connection types, patterns, zones, et cetera, are well worth the effort. Taking a little time up front to review your specification for roof and floor deck attachment with the installer can really pay off with productivity gains later on.

    Manufacturer representatives should be included in pre-construction meetings when mechanical fasteners will be used. As discussed in tip 4, it also makes sense for a manufacturer representative to be at the jobsite at the start of work, and periodically throughout the job. In today’s tough business environment, this is something that the best manufacturers will be able to offer.

    10. Specify nestable deck profiles

    Although there are two types of steel deck profiles available — nestable and interlocking — the nestable deck profile may offer some advantages with respect to constructability.

    Nestable deck positioning can be adjusted on the project site to compensate for slight misalignments that may occur during layout. Steel deck manufacturing tolerances may also result in slightly different dimensions between manufacturers. This may affect nestable and interlocking deck profiles alike.

    As mentioned in tip 6, industry research into the inelastic cyclic/seismic performance of steel deck diaphragms has also been done with nestable profiles and mechanical fasteners, but to date, this type of cutting-edge research has not involved interlocking deck. If an interlocking deck profile is preferred, it should also be noted that some manufacturers offer an interlocking screwable sidelap profile. This will allow for use of sidelap screws that are proven as ductile with interlocking deck.

    Finally, if traditional button punch sidelap connections are specified, be aware that these may not be possible in thicker 18- and 16-gage interlocking deck panels. Manual compression is extremely difficult with this much steel and reliable connections are difficult to achieve. This is also why the SDI assigns a zero Qs sidelap connection load value to button punches in diaphragm shear tables. Nestable deck profiles allow for screw-fastened sidelap connections which can be very reliable, consistent, and quick to install with today’s stand-up screw fastening tools.

    As with any list, the tips presented are not meant to be all inclusive, but represent a good place to start. If you have any other ideas or suggestions for adding to this list, please let us know.

    William Gould, P.E., is director of codes and approvals with Hilti, Inc., and specializes in fastener qualification testing evaluation. Andrew Liechti, P.E., is technical services manager with Hilti, Inc., and specializes in powder-actuated fastener qualification testing and evaluation. They can be reached at william.gould@hilti.com and drew.liechti@hilti.com, respectively.