Gerber Roof Girders

The third module in CISC’s Design Modules lineup, “Gerber Roof Girders ” has been published and is now available. The first two modules on “Eccentrically Loaded Bolt Groups” and “Eccentrically Loaded Weld Groups” were described in earlier blog posts.

Design Module 3 (DM3) covers the design of roof girders in cantilever-suspended-span construction, the dominant form of Gerber girder application. Design tables are provided to assist designers in selecting the cantilever girder and drop-in segment sizes for typical building applications with evenly spaced columns and joists.

The tables are applicable to flat roofs subject to dead, live, snow and vertical wind loads, and are based on climatic parameters representing many Canadian locations. Various girder and joist span combinations, lateral bracing configurations and load conditions are included. Solved examples are presented to illustrate practical applications.

DM3 is intended to be used in conjunction with the National Building Code of Canada (2015) and CSA Standard S16-14, Design of Steel Structures. It applies to wide-flange (W-shape) beams produced to ASTM A992/A992M and A572/A572M Grade 50[345].

The most popular roof framing system for single-storey buildings in North America features sheet steel roof deck, open-web steel joists (OWSJs) and wide-flange girders in Gerber girder configuration. It generally offers steel savings and efficiency in steel erection, resulting in greater economy versus simple-span girder construction. The most effective applications involve structures with a relatively uniform loading and regular column grid.

 

Figure 1
Structural Configuration

 

The Gerber girder configuration featured in DM3 consists of continuous cantilever girder segments and simple-span (drop-in) segments in alternating spans as shown in Figure 1. Thus, the girder system is also known as the Cantilever-Suspended-Span (CSS) construction. The analysis and design of Gerber roof girders are more challenging, as compared to simple-span girders, due to two main factors:

a) Patterned roof loadings and counteracting load effects in multiple load combinations, and
b) Lateral-torsional buckling under various loading and lateral bracing conditions.

This new CISC publication aims to cover these considerations for common structures and loading conditions, and to provide a comprehensive set of design tables and examples.

 

Figure 1
Structural Configuration

Module 3 is available in print at this LINK