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301 Butler-Carlton Hall
1401 N. Pine St.
Rolla, MO 65409-0030
USA
Phone: 573-341-4471
Fax: 573-341-4476
Email: ccfss@mst.edu
1.A classic truss analysis assumes a single node point for the intersection of the chord and web members. However, when designing a truss using cold-formed members, i.e. C-, hat, tubes, etc., the members do not typically permit a single node connection. Thus, is the assumption of a single node analysis valid?
3. So, sheathing offers nothing to enhance the structural performance of the truss?
4. Is the buckling coefficient, k, the same regardless of the cross-section geometry?
7. What are the design requirements for permanent bracing?
10. Where can one find additional information regarding truss design?
A classic truss analysis assumes a single node point for the intersection of the chord and web members. However, when designing a truss using cold-formed members, i.e. C-, hat, tubes, etc., the members do not typically permit a single node connection. Thus, is the assumption of a single node analysis valid?
Answer No. 1:
Section D4.6 of the Standard for Cold-Formed Steel Framing - Truss Design permits analysis using multiple or single node analysis. However, when using a single node analysis additional supplementary structural review is required to ensure that the eccentricities resulting from a simplified single node analysis do not compromise the structural integrity. The Commentary on the Standard for Cold-Formed Steel Framing - Truss Design contains additional discussion regarding this topic.
Question No. 2:
When evaluating the strength of a compression chord that has sheathing attached to its top flange, can a composite section be considered?
Answer No. 2:
The Standard for Cold-Formed Steel Framing - Truss Design does not specifically address the use of a composite section. However, because the connection of the sheathing is typically accomplished by using a self-drilling screw spaced at least 12 in. on center, it is unlikely that adequate horizontal shear transfer can be accomplished to achieve composite behavior. Based on full-scale truss tests conducted at the Missouri University of Science and Technology , the cross-section analysis should be based on the steel cross-section alone.
Question No. 3:
So, sheathing offers nothing to enhance the structural performance of the truss?
Answer No. 3:
No, tests at the Missouri University of Science and Technology have demonstrated that the sheathing does contribute to the structural performance of a truss. This enhanced performance is reflected in Section D4.2.1 of the Standard for Cold-Formed Steel Framing - Truss Design by permitting the use of buckling coefficient, k, equal to 0.75 when sheathing is directly attached to the chord.
Is the buckling coefficient, k, the same regardless of the cross-section geometry?
Answer No. 4:
No. The cross-section stiffness influences the choice of the buckling coefficient, and therefore Section D4.2.1 of the Standard for Cold-Formed Steel Framing - Truss Design specifies different values of the buckling coefficient based on the cross-section profile.
Question No. 5:
A common truss profile is to use C-sections connected web-to-web at the nodal points. This profile does not concentrically load the web member. Must one consider the eccentricity when designing the web member?
Answer No. 5:
Yes. Tests at the Missouri University of Science and Technology have shown that the web member is influenced significantly by the end eccentric loading. Section D4.4 of the Standard for Cold-Formed Steel Framing - Truss Design provides design guidance.
Must my truss design conform to the design rules contained in the Standard for Cold-Formed Steel Framing - Truss Design ?
Answer No. 6:
Yes. The Standard for Cold-Formed Steel Framing - Truss Design has been adopted by reference in the ICC and NFPA building codes and thus is legally required when the local building code adopts the these building codes.
Question No. 7:
What are the design requirements for permanent bracing?
Answer No. 7:
The Standard for Cold-Formed Steel Framing - Truss Design does not provide specific design rules for sizing permanent bracing. Section F2 merely states that the permanent bracing shall be designed and specified by the building designer. The Commentary on the Standard for Cold-Formed Steel Framing - Truss Design provides discussion on this topic and cites the Light Gauge Steel Engineers Association's Tech Note 551e, Design Guide for Permanent Bracing of Cold-Formed Steel Trusses as a resource for design.
Question No. 8:
Must proprietary truss cross-section profiles be designed in accordance with the Standard for Cold-Formed Steel Framing - Truss Design ?
Answer No. 8:
Yes. The Standard for Cold-Formed Steel Framing - Truss Design does not reference a specific manufacturer, but the Standard does present design rules for generic cross-section profiles, i.e. hat and Z-shape. For example, several manufacturers market truss chord cross-section profiles that are variations of a hat shaped profile (Figures b, c, d, e). Also there are manufacturers that market a profile that is a modified Z-shape (Figure a).
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(a) (b) (c) (d) (e)
Why is the building designer and not the truss designer responsible for the design of the permanent bracing? As the building designer I do not know what bracing the truss designer has specified.
Answer No. 9:
For bracing to function properly it must be anchored. The truss designer typically does not have information regarding the complete building system and thus is unable to design a complete bracing system. By specifying that the building designer is responsible for the bracing design the Standard for Cold-Formed Steel Framing - Truss Design is hopefully creating an environment in which the truss designer and the building designer communicate to ensure that the roof system conforms to the design load requirements.
Question No. 10:
Where can one find additional information regarding truss design?
Answer No. 10:
For addition information regarding truss design, additional design information can be found by contacting truss manufacturers, the Center for Cold-Formed Steel Structures, ccfss@mst.edu, or the Light Gauge Steel Engineers Association, www.lgsea.com or steeldon@earthlink.net.