Engineering Observations

• Roof Gerber System

  The Gerber system is indeed remarkable and efficient, widely used as a roof framing system across North America. One intriguing aspect of Gerber systems is the concept of the unbraced length of the cantilever. Unlike slender members in tension, slender members in compression buckle. As the distance between support points increases, members in compression rapidly lose capacity. Moreover, the unsupported length of a member could be longer than the actual length of the member. What controls this, is the support conditions, giving rise to the term, effective length. The effective length of a cantilever can surpass its actual length, depending on the stability conditions provided at both the root and tip of the cantilever.

  In the following case, lateral-torsional support is provided at the root of the cantilever. This support is achieved by the joist bottom chord extension and the two full-depth web stiffeners. However, at the tip of the cantilever, the joist is positioned on the link beam rather than the cantilever itself, resulting in a lack of support. This absence of lateral support for the top and, more crucially, the bottom in compression flange at the tip necessitates the use of an effective length factor (K) ranging from 1.5 to 2.5 as per the CISC 1989 Design Module Since, stability is not a game of numbers, an effective length factor (K) of 2.5 should be opted for in this situation.

  

  
• Columns and Unbraced Lengths

  The importance of unbraced length in column design is a focal point of my attention. I am constantly vigilant in identifying and analyzing the unbraced lengths of columns in various structures. This aspect is crucial as it directly influences the buckling behavior and overall stability of the structural system, prompting me to assess and optimize column designs for enhanced structural performance and safety. In modification projects where parts of existing floor slabs are demolished to create internal atrium's, assessing unbraced lengths of columns that have been exposed becomes critical.

  Interestingly, the column in the photo has an unbraced length larger than the entire adjacent four-story building. At its top, this column doesn't support a mere sign or a simple beam but rather the edge of the tower above it, highlighting the complexity and importance of analyzing unbraced lengths in structural design for both safety and functionality.

  

  
• Structural Steel CBF

  I recently came across a new structural steel concentrically braced frame. Notably, bracing is placed on one bent across this side of the structure, which featured a double cross bracing strategically positioned at the center, utilizing tubes rather than plates or rods due to the higher lateral loads. Additionally, a beam was placed at about mid-height to reduce the unbraced length of the tubes when in compression, optimizing structural stability. The column, what seems to be a larger than usual W12 was oriented with the strong axis facing the wind direction, especially crucial given the approximately more than 30 feet (~8.5m) unbraced length on one axis, dependent on girt placement.

  For the cantilever, the designer went to town with two stiffeners at the column and a steel joist directly over the column with a bottom chord extension so that both the Gerber's top and bottom flanges are laterally supported at the column, while at the tip the joist is placed on the cantilever side to the left of the pin and not on the link beam or to the right of the pin, I rarely see this, but it is good practice because now the top flange of the tip of the cantilever is laterally supported. Most design offices assume a (K) value of 1.5, regardless of the conditions. As a matter of fact, this is the appropriate situation at both the column (root) and tip of the cantilever, where a (K) value of 1.5 should only be allowed.

  I am yet to see the combination where the joist is placed on the cantilever, to the left of the pin, as shown but also with a bottom chord joist extension which would brace the bottom flange of the cantilever, which is in compression. This is best practice and the cantilever tip is lateral torsionally supported as both the top and bottom flanges would be braced in this case. I wonder if a bottom chord extension on the joist placed on the tip of the cantilever and using a (K) of 1.0 would actually offset the extra weight of the cantilever beam, when opting for heavier beams influenced by a (K) > 1.5. The search continues for such scenario.

  

  
• Reinforced Concrete Transfer Slab

  In a recent project featuring a concrete braced frame, a distinctive arrangement was observed on the second-floor. It became apparent that several columns did not continue all the way down but rather terminated at the second-floor level, necessitating the transfer of their vertical loads. Additionally, discontinuous walls also required transferring both their vertical and lateral loads at the second-floor level. Each transfer beam would require two columns to support it at the ground-floor level.

  To address this challenge without compromising the architectural vision of an unobstructed ground-floor space, conventional transfer beams were avoided. Instead, a strategic solution was implemented: the second-floor slab was designed to be thicker than the typical floor slab. This thicker slab not only met the shear strength requirements for transferring the gravity loads from the columns and walls but also served as a diaphragm. This diaphragm effectively transmitted the lateral loads from the walls that terminated at the second-floor level to the stiffer cores located at the ground-floor level.

  

  
• Bridge Columns Loads

  The comparison between the two bridge columns in the photos reveal distinct design strategies based on their load conditions. In the eccentric column subjected to tension, heavy stiffeners reinforce the beam-column junction, accompanied by numerous large-sized anchor bolts and substantial steel plates. These elements collectively bear the tensile force, ensuring structural integrity under significant loads.

  On the other hand, further down the bridge where the geometry allowed for a central column. The compression-loaded column features lightly loaded anchor bolts also the anchor bolts are fewer, primarily serving to counter any incidental tensile forces due to eccentricities or unbalanced loads. This differential approach reflects a nuanced understanding of load distribution, with each design tailored to effectively manage the predominant forces acting on the respective column.