EngineeringHome » Steel eNotes - Fall 2010 » Engineering
The American Iron and Steel Institute has published Supplement 2 to AISI S214-07 (North American Standard for Cold-Formed Steel Framing – Truss Design 2007 edition). It completely replaces Supplement 1 and revises and clarifies provisions related to design responsibilities, loading, quality criteria and bracing for greater consistency with building codes and industry practice. Specifically, the purpose of this supplement is to better harmonize with the provisions desired by involved industry groups, including NCSEA and SBCA, which were initially documented in IBC Proposal S209-07/08.
The contents of this document are referenced in IBC 2009.
With regard to Cold-Formed Steel Truss design, reference should be given to Sections B2 and B4 of the Supplement. The following is a summary of these sections that are relevant to the design and submittal of the truss package.
B2 – Responsibility of the Truss Design Engineer/Truss Designer
- The Truss Design Engineer shall supervise the preparation of truss drawings
- The Truss Designer shall be responsible for the individual truss component designs
- The Truss designer shall make available as part of the truss submittal package, upon request, design calculations, including the following:
- loads and load combinations considered;
- axial forces, moments, and shears resulting from the applied loads and load
- combinations; and/or
- design assumptions.
- The truss design drawings shall consist of the individual truss design drawings and referenced details, if any. The truss design drawings shall be part of the truss submittal package and include, at a minimum, the information specified below:
- applicable building code used for design, unless specified on a cover/truss index sheet;
- slope or depth, span, and spacing;
- number of plies if greater than one;
- bearing locations and minimum bearing lengths;
- design loading(s) as applicable, including:
- top chord roof or floor live load;
- top chord roof snow load;
- top chord dead load;
- bottom chord live load;
- bottom chord dead load;
- additional loads and locations;
- environmental design loads (e.g., wind and snow) and all applicable factors
- as required to calculate the truss loads; and other lateral loads, including drag strut loads;
- reaction forces and direction, including maximum downward, lateral and uplift reaction forces, where applicable, based on nominal [specified] loads;
- location of all truss member connections;
- gusset plate locations, sizes, and material specifications;
- fastening type, size, quantities, and locations;
- shape and material specification for each truss member;
- maximum axial compressive force in all truss members based on nominal [specified] loads;
- truss-to-truss connection and truss field assembly requirements;
- calculated span-to-deflection ratio and/or maximum vertical and horizontal
- deflection for nominal [specified] live and total load, as applicable;
- locations of required permanent individual truss member restraint in accordance with Section B6(a) or B6(c), if required; and
- design and details for individual truss member reinforcement in accordance with Section B6(b), if required.
B4 Responsibilities of Building Designer/Registered Design Professional for the Building
The building designer or registered design professional for the building, through the construction documents, shall provide information sufficiently accurate and reliable to be used for facilitating the supply of the structural elements and other information for developing the design of the trusses for the building, and shall provide the following:
- All truss and structural element orientations and locations;
- Information to fully determine all truss profiles;
- All structural element and truss support locations and bearing conditions;
- The location, direction, and magnitude of all dead, live, and lateral loads applicableto each truss including, but not limited to, loads attributable to: roof, floor, partition, mechanical, fire sprinkler, attic storage, rain and ponding, wind, snow (including snow drift and unbalanced snow), seismic; and any other loads on the truss;
- All truss anchorage required to resist uplift, gravity, and lateral loads consisting of
- approved hardware or methods designed by a registered design professional;
- Truss to structural element connections (truss to truss connections are the responsibility of the truss designer, as stated earlier)consisting of approved hardware or methods designed by a registered design.
- Comment—Aegis Metal Framing provides a complete line of truss to structure connection products and details. We are more than happy to work in conjunction with our truss fabricator partners to provide these products and services to any project professional;
- Permanent building stability bracing; including truss anchorage connections to the permanent building stability bracing;
- Criteria related to serviceability issues including:
- Allowable vertical, horizontal or other required deflection criteria;
- Any dead load and live load deflection criteria for flat roofs subject to ponding loads;
- Any differential deflection criteria from truss-to-truss or truss-to-adjacent structural member;
- Any deflection and vibration criteria for floor trusses including any strongback bridging requirements or any dead load and live load deflection criteria for floor trusses supporting stone or ceramic tile finishes; and
- Anticipated moisture, temperature, corrosive chemicals and gases expected to affect the trusses and requirements for any additional corrosion protection.
B4.5 Permanent Individual Truss Member Restraint/Bracing
The building designer and/or the registered design professional for the building shall be permitted to specify the method of the permanent individual truss member restraint/bracing in accordance with Section B6.
B6 Design of Permanent Individual Truss Member Restraint/Bracing
Where permanent individual truss member restraint/bracing is required, it shall be accomplished by one of the following methods:
(a) Standard Industry Details. Standard industry permanent individual truss member restraint/bracing details supplied in accordance with B3.4.
(b) Substitution with Reinforcement. Truss member reinforcement designed by the truss design engineer or truss designer to eliminate the need for permanent individual truss member restraint/bracing. The permanent individual truss member reinforcement design and details shall be noted/shown on the truss design drawings or on supplemental truss member buckling reinforcement details provided by the truss design engineer or truss designer.
(c) Project Specific Design. A project specific permanent individual truss member restraint/bracing design specified by any registered design professional, as specified in the contract documents and/or construction documents, and supplied in accordance with B3.4.
Source: North American Standard for Cold-Formed Steel Framing – Truss Design 2007 edition.
Aegis Metal Framing can take the responsibility of the Truss Design Engineer in that we supervise the preparation of the truss design drawings. We also work to meet the requirements of the truss design drawing.
Section B4 is a critical section in that it clearly describes the responsibilities of the Building Designer or Registered Design Professional of the building.--specifically, the requirement to determine and specify all loads to be supported or resisted by the individual trusses. In some cases the Building Designer will attempt to put the requirements of calculating loads on the Truss Designer or Truss Design Engineer. If that becomes the case, refer him/her to this section of the Truss Standard Supplement 2.
B4.5/B6 simply states that the building designer or registered design professional, when it comes to truss bracing, can choose how to address it. Specify Standard Industry Details, some of which can be found in the Structural Building Components Association document CFSBCSI, or have a specific permanent bracing design done by a registered design professional. The registered design professional can certainly be Aegis Metal Framing. We design hundreds of bracing design layouts involving Ultra-Span® trusses each year. We understand the Ultra-Span product, how it is to be braced, available products and the best methods for attachment for Ultra-Span.
This supplement will become part of the next AISI Truss Standard, but for now the supplement is available for free download from the AISI website, www.steel.org. Click on Construction on the left hand list, and then select Codes and Standards.
>Mike Pellock, P.E.
Vice President - Technical Services