Conception de diaphragme à l’aide de parement d’acier léger

Introduction
Le vent soufflant le long d’un toit à pignon typique exerce des pressions directement sur le parement et la structure, de même que des forces de renversement pour le bâtiment en entier. Des contreventements discrets ou une ossature de tiges rigides permettent de résister à ces pressions. Une approche différente consiste à utiliser la force de flexion du parement de toit en acier afin qu’il agisse comme un diaphragme de cisaillement pour transférer ces forces sur les fondations. Les forces de base agissant sur le bâtiment sont illustrées dans la Figure 1 ci-dessous.

Méthode de conception de diaphragme 

Une aide pour la conception est offerte pour aider à faire le choix et le détaillage du parement en tôle d’acier et des attaches requis pour créer un diaphragme de parement d’acier léger. Ainsi, la publication de la MCA (Metal Construction Association) intitulée “A Primer on Diaphragm Design”, First Edition 2004 est offerte sur le site Web www.metalconstruction.org. Ce manuel fournit une collection de tableaux de calcul comme celui reproduit ci-dessous (voir la Figure 2) de même que des exemples et des commentaires. Malheureusement, les profils utilisés pour les valeurs de calcul disposées en tableau ne sont pas typiques des produits canadiens. Cependant, une expression générale de conception est incluse dans le manuel et peut être utilisée pour calculer les valeurs de la force de cisaillement et de rigidité pour d’autres profils de parement. 

Étendue 

Le manuel fournit des tableaux pour une variété de systèmes de mur et de toit qui comprennent les composants suivants: 

Profils de parement: 

• Panneau d’une largeur de 36 po, d’une profondeur de 1,5 po avec un espace de 7,2 po entre les nervures avec des forces d’élasticité de 33 et 50 ksi 
• Panneau d’une largeur de 36 po, d’une profondeur de 1,25 po avec un espace de 12 po entre les nervures avec des forces d’élasticité de 80 ksi 

Isolant: 

• Fibre de verre R19 
• Polyisocyanurate ou blocs thermiques de 3-1/4 po Modèle de fixation: 
• Configurations de 3, 4 et 5 vis sur la largeur de la feuille de tôle Attaches: 
• Vis no 12 ou 14

Utiliser avec calcul aux états limites au Canada 

Les tableaux fournis par le manuel de la MCA donnent des valeurs pour les forces de cisaillement qui sont des charges permises aux États-Unis avec la méthode Allowable Strength Design. Au Canada, nous utilisons le Calcul aux états limites, donc les valeurs de cisaillement doivent être converties. Les charges permises disposées en tableau sont multipliées par le facteur de sécurité approprié (FAC = 2,35 dans le modèle de tableau ci-dessous) pour déterminer la résistance nominale. La résistance nominale est ensuite multipliée par le facteur de résistance approprié pour obtenir la résistance pondérée pour une utilisation dans une conception calcul aux états limites. Les facteurs de résistance pour les diaphragmes en acier plié à froid sont indiqués dans le Tableau D5 CSA-S136-07 de Spécification nord-américaine pour le calcul des éléments de charpente en acier formés à froid. Ces valeurs sont reproduites ci-dessous.

Cliquez pour télécharger Quelques mots sur la tôle d'acier #25: Conception de diaphragme à l’aide de parement d’acier léger

Diaphragm Design using Lightweight Steel Cladding

Introduction 

Wind blowing across a typical gable-roof building produces forces that act directly on the cladding and structural members, as well as overturning forces for the building as a whole. Discrete wind bracing or rigid pole-framing are methods for resisting these forces. Another approach is to utilize the in-plane strength of the steel roof cladding to act as a shear diaphragm to transfer these loads to the foundation. The basic forces acting on the building are illustrated in Figure 1 below.

Diaphragm Design Method 

A design aid is available to assist in the selection and detailing of the sheet steel cladding and fasteners, needed to create a lightweight steel cladding diaphragm. This aid is a publication of the Metal Construction Association titled “A Primer on Diaphragm Design”, First Edition 2004, and is available through their website at www.metalconstruction.org. This manual provides a collection of design charts like the one re-produced below (see Figure 2) as well as worked out examples and a commentary. Unfortunately the profiles used for the tabulated design values are not typical of Canadian products. However, a general design expression is included in the manual that can be used to develop design shear strength and stiffness values for other cladding profiles. 

Scope 

The manual provides tables for a variety of different roof and wall systems that include the following components: 

Cladding profiles: 

• 1.5”deep x 7.2”rib spacing x 36”panel width in 33 and 50 ksi yield strengths
• 1.25”deep x 12”rib spacing x 36”panel width in 80 ksi yield strength 

Insulation: 

• R19 fiberglass 
• 3-1/4”polyisocyanurate or thermal spacer blocks Fastener pattern: 
• 3, 4 and 5 screws configurations across the sheet width Fasteners: 
• #12 or #14 screws 

Use with Limit States Design in Canada 

The tables provided in the MCA manual give design shear values that are allowable loads for use in the US with the Allowable Strength Design methodology. In Canada we use Limit States Design, and so the design shear values need to be converted. The tabulated allowable loads are multiplied by the appropriate safety factor (FAC=2.35 in the sample table below) to determine the nominal resistance. The nominal resistance is then multiplied by the appropriate resistance factor to get the factored resistance for use in an LSD design. The resistance factors for cold-formed steel diaphragms are given in Table D5 of CSA-S136-07 North American Specification for the Design of Cold-Formed Steel Structural Members. These values are reproduced below. 

Click to download Fact Sheet #25: Diaphragm Design using Lightweight Steel Cladding

Standard Thicknesses for Canadian Lightweight Steel Framing Applications

The Canadian manufacturers have adopted a common set of standard base steel thicknesses for lightweight steel framing components (e.g. studs and joists). The North American cold formed steel industry has been pursuing the benefits of standardization by harmonizing both framing products as well as engineering design practices. The North American Specification for the Design of Cold-Formed Steel Structural members (the CSA-S136-07 standard in Canada) is the current design document for cold formed steel structural members, and is the first North American structural design standard. This one document applies in Canada, United States and Mexico. With the further adoption of common thicknesses, the manufacturers of lightweight steel framing products in North America can work towards standardizing product geometries that will also benefit the construction industry. The current standard thicknesses are provided in Table 1 shown below.

The gauge numbers listed in Table 1 are only provided as a convenience and are not to be used when ordering or specifying steel. They are shown for reference purposes only. It is also important to note that these thickness gauge equivalents are different than the more commonly used Manufacturers Standard Gauge (MSG) system used for other structural sheet steel products (e.g. cladding and deck). With these unique LSF thicknesses it is even more important than before not to use gauge numbers: doing so will cause mistakes. Using the decimal thickness is the only way of ensuring that the product specified is the product delivered to the site. 

The colours listed in Table 1 are used by the manufacturer to identify the product thicknesses. Typically this colour is spray-painted on the end of a bundle of members (e.g. stud, joist or track) for easy identification. These colour-thickness combinations are consistent with the requirements in the ASTM C955 Standard Specification for Load-Bearing (Transverse and Axial) Steel Studs, Runners (Track), and Bracing or Bridging for Screw Application of Gypsum Panel Products and Metal Plaster Bases and the AISI S201 North American Standard for Cold-Formed Steel Framing - Product Data.

Sheet steel products for LSF members must be coated with a metallic coating of either zinc or 55% aluminum-zinc alloy. Zinc coated sheet steel shall conform to ASTM Standard A653/A653M Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process. The 55% aluminum-zinc alloy coated sheet steel shall conform to ASTM Standard A792/ A792M Standard Specification fro Steel Sheet, 55% Aluminum-Zinc Alloy-Coated by the Hot Dip Process. For structural framing members the minimum metallic coating is Z180 (G60) or AZM150 (AZ50). For non-structural steel studs the minimum metallic coating is Z120 (G40) or AZM100 (AZ35). 

The minimum thicknesses of typical hot-dipped metallic coatings are given in Table 2. These metallic coating thicknesses must be added to the base sheet thickness when determining the delivered sheet thickness. Metallic coatings are also subject to manufacturing tolerances the same as the base steel. Therefore, the actual thickness of the metallic coating will be greater than the minimum thicknesses listed in Table 2. This factor needs to be considered when attempting to verify the base steel thickness of a coated product. There are other coating types and weights in addition to those listed in Table 2 that may be used with other cold formed steel products.

Click to download Lightweight Steel Framing Technical Bulletin Volume 7, Number 2: Standard Thicknesses for Canadian Lightweight Steel Framing Applications

Hardcopy Publications on Sale Now!

Most of our hardcopy publications are now on sale. The purchase price for the documents is now FREE*, just pay shipping and handling so we can cover our costs to ship the documents to you. Order now while supplies last!

Documents on sale:

• CSSBI 51-06: Lightweight Steel Framing Design Manual - 2nd Edition (Regular Price: $35)
• CSSBI 56-00: Residential Steel Framing - Training Curriculum (Regular Price: $75)
• CSSBI B13-06: Design of Steel Deck Diaphragms - 3rd Edition (Regular Price: $50)
• CSSBI B15-07: NBC 2005: Snow, Wind and Earthquake Load Design Criteria for Steel Building Systems (Regular Price: $10)

Go to the Resources section of the CSSBI website to place your order.

* Exclusion: Design of Design in Cold Formed Steel Seminar Handout

New Design in Cold-Formed Steel Seminars Scheduled - Register Now!

Our sister organization, the Cold-Formed Steel Engineers Institute Canada is hosting a Design in Cold Formed Steel Seminar. Below are the details.

Design in Cold Formed Steel: Using the North American Specification for the Design of Cold-Formed Steel Structural Members CSA Standard S136-12

When & Where

Tuesday, April 7, 2015 - Ottawa, ON

Tuesday, April 14, 2015 - Vancouver, BC

Wednesday, April 15, 2015 - Winnipeg, MB

Registration:

CFSEI Members - $225

Non-Members - $275

Each registrant will also receive a comprehensive set of lecture notes full of explanatory material and worked examples.

Registration is limited to 50 people on a first-come first-serve basis.

About the Seminar

The primary objective of this seminar is to make the designer conversant with the latest edition of CSA Standard S136-12 (North American Specification for the Design of Cold-Formed Steel Structural Members). This is a harmonized document between Canada, the US and Mexico, and supersedes the 2007 edition (including Supplement 2010). The Specification was developed through a joint effort of the American Iron and Steel Institute’s (AISI) Committee on Specifications and the Canadian Standards Association’s S136 Technical Committee. In comparison to the 2007 edition of S136 (including Supplement 2010), a number of significant changes have been incorporated into the North American Specification, in part due to the harmonization process and in part due to latest research developments. 

Topics
The intent is to bring the participant up-to-date with the current design provisions contained in the new North American Specification for the Design of Cold-Formed Steel Structural Members (S136-12), highlighting significant changes from the 2007 edition of S136. As well, numerous illustrative examples will be presented.

• Introduction 
• Materials
• General Design Considerations
• Elements in Compression
• Members in Tension
• Members in Bending
• Members in Compression
• Combined Bending and Compression
• Connections
• Member Bracing
• Testing and Fatigue
• Direct Strength Method

Also, the latest Editions of the AISI North American Design Standards for Cold-Formed Steel Framing will be reviewed since these design standards are referenced by CSA S136 for use in Canada.

Registrants are encouraged to bring a copy of the S136-12 Standard to the seminar. If necessary, this can be purchased from CSA by telephone [416-747-4044, or 800-463-6727], E-mail [sales@csa.ca] or by visiting their web site at www.csa.ca.

Schedule
8:00 am     Registration and coffee
8:30 am     Seminar begins
10:15 am   Break
12:00 pm   Lunch provided
12:45 pm   Seminar resumes
2:30 pm     Break
4:30 pm     Seminar concludes

Who Should Attend

• Anyone involved in the design of cold formed steel structural members. This seminar will provide a quick and effective means of learning about the 2012 edition of CSA S136 (North American Specification for the Design of Cold-Formed Steel Structural Members).
• Anyone who would like the opportunity to have questions answered concerning all aspects of cold formed steel design.

Speakers

Dr. R.M. Schuster, P.Eng.
University of Waterloo
Waterloo, Ontario

Dr. S. R. Fox, P.Eng.
General Manager
Canadian Sheet Steel Building Institute
Cambridge, Ontario

Application of the National Energy Code for Buildings 2011 to a Steel Building System

Dockside Green Power Generation Building - Victoria, BC

The National Energy Code for Buildings (NECB) was published in the fall of 2011. It is a National Model Code which will be adopted by the Provinces and Territories to the extent it meets their plans. It applies to the construction of new buildings that are required to meet the provisions of Part 3 of the National Building Code of Canada, or the applicable Provincial Building Code.

There are four paths through which building designs may comply with NECB 2011:

1. The Prescriptive Path (Section 3.2) in which assemblies and components must meet minimum prescribed performance requirements.
2. The Simple Trade-off Path (Section 3.3.3) in which certain assemblies or components may not meet the prescribed performance requirements, while other assemblies or components exceed the prescribed performance requirements, such that the overall performance of the building will not use more energy.
3. The Detailed Trade-off Path (Section 3.3.4) in which a computer model is used to establish a reference building envelope energy target. Some components are permitted to be less energy efficient provided it can be demonstrated the building envelope will not transfer more energy than the building envelope energy target.
4. The Performance Path (Section 3.4) in which the Trade-Off methodology is extended to include equipment inside the building, (i.e. fans, appliances, elevators, etc.) and a computer model is used to determine that the building assemblies, components and equipment in aggregate, will not use more energy than the reference building envelope energy target. 

The Prescriptive Path provides maximum overall thermal transmittance for the building walls, roof, fenestration and doors (refer to Section 3.2.2). The Prescriptive Path also allows certain percentages of total wall area for vertical fenestration and doors (Section 3.2.1.1_1). For heating degree days (HDD) less than 4000, the ratio of area of doors and fenestration to total wall area can be a maximum of 40%. For HDD between 4000, and 7000, this ratio is calculated as (2000-0.2xHDD)/3000. For HDD over 7000, this ratio is 20%. As it gets colder, the allowable area for doors and windows decreases. The Prescriptive Path further allows up to 5% of the gross roof area to be skylights (Section 3.2.1.4_2)

The Simple Trade-off Path demonstrates that the sum of the areas of vertical (or horizontal) assemblies of the building envelope multiplied by their respective overall thermal transmittance is not more than the corresponding assemblies in the reference building. The reference building for the Simple Trade-off Path is the same building used with the Prescriptive Path.

If certain components are more energy efficient than those prescribed in the Prescriptive Path, the trade-off calculation is permitted to take this increased performance into account.To perform the calculation, vertical (wall) portions of the building envelope can only be traded off against other vertical (wall) portions, and horizontal (roof) portions of the building envelope can only be traded off against other horizontal (roof) portions.

The Simple Trade-off Path cannot be used for semi-heated buildings, which require less energy than a reference building with a temperature set point of 18°C, and have heating capacity no greater than the heating load plus 5%. For semi-heated buildings, the Detailed Trade-off Path or the Performance Path, must be used.

The Detailed Trade-off Path and the Performance Path utilizes computer energy modelling and the services of a professional engineer proficient in this work.

To learn more about Steel Building Systems and the National Energy Code for Buildings, download our Fact Sheet #37: Application of the National Energy Code for Buildings 2011 to a Steel Building System

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