Engineers must be ever mindful of the need for the structures they design to have continuous load paths to transfer gravity, wind and seismic loads through the structure and into to the foundation.
The best load paths are usually direct ones. This is true for both global load paths (i.e., loads traveling through the structure) as well as for connection load paths (loads traveling through connections). For steel-framed structures, connections with smooth load paths are the easiest to design, fabricate and erect – and they are usually the most economical. “Kinked connections” are those through which loads traveling through the connection weave and “zig-zag”.
Kinked connections can cause problems, because they often create secondary stresses in members and connection elements
These secondary stresses must be considered by both the Engineer of Record (EOR) as well as by the connection design engineer. Failure to consider secondary stresses can result in connection failures such as the failure that occurred in 1981 when a hanger rod pulled through a box beam in the Hyatt Regency skywalk in Kansas City. In this tragedy, a series of events unfolded which resulted in the collapse of the skywalk structure – killing 114 people. The steel fabricator revised a connection that was only partially detailed on the structural drawings – and neither the Engineer-of-Record or the steel fabricator’s connection design engineer investigated the subtleties and secondary stresses occurring in the revised connection detail. The original connection had a kink in it, and the revised connection added two additional kinks in the connection load path.
Who is responsible for connection design?
The American Institute of Steel Construction’s Code of Standard Practice (AISC 360-16) states that the EOR is ultimately responsible for the safe design of the entire structure, including the connections, even when that engineer delegates connection design to the steel fabricator.
Section 3 of the AISC 360-16 lists three options for EOR’s to choose from when dealing with connections:
- Option #1: Show the complete connection design on the structural design drawings.
- Option #2: Require on the contract documents that connections be selected or completed by an experienced detailer.
- Option #3: Require on the contract documents that connections be designed by a licensed professional engineer working for the fabricator.
Option #3 is commonly referred to as “delegation of connection design”, and is common practice on the east coast. Regardless of which option is used, the EOR is ultimately responsible for insuring that all connections are properly designed.
Conveying connection design requirements on the structural drawings
With Option #1, the EOR designs the connections and details all connections on the structural drawings.
For Option #2 the EOR requires the steel detailer to select or complete the connections using tables provided in the AISC Steel Construction Manual, or tables provided on the structural drawings. Connection engineering is performed by the EOR. The detailer completes the connections using tables.
For Option #3 the EOR delegates connection design to the steel fabricator. The connections are designed by an engineer working for the fabricator, and signed and sealed connection design calculations are submitted to the EOR.
For Options #2 and 3, the EOR is required to provide details on the structural drawings showing all member reinforcement (i.e., stiffener plates, web doubler plates, etc.), and showing concept connection details (referred to as conceptual configuration details in the COSP) showing the EOR’s general requirements as to how the connection is to be configured. Dimensions, and sizes of member reinforcement must be shown on the structural drawings for option #2. Details and minimum dimensions and sizes of member reinforcement must be shown for option #3.
Concept connection details and member reinforcement details are required on the structural drawings to inform bidders as to the complexity of the connections so that they can properly price the connections in their bids.
Dealing with kinked connections
Engineers must identify and deal with kinked connections during design. Quite often the best (and most economical) way to deal with a kinked connection is to alter the framing and eliminate the kink. That option (altering the framing) is not available if the kinked connection is not discovered until shop drawing review. By then the steel is ordered and it is usually too late to change the framing.
EOR’s must review all connections on their projects during design, even when delegating connection design, and ask themselves the following questions:
- Are the connections designable? Can they be easily and economically designed?
- Are the connections constructable?
- Is member reinforcement required at any of the connections?
- Is there anything unusual about how loads travel through the connections that will result in secondary stresses in the members on either side of the connection? In other words, are there any kinked connections?
- Can any of the connections be simplified by altering the framing – and if doing so, will that result in a more economical framing scheme?
All unusual connections, kinked or otherwise, must be documented with concept connection details on the structural drawings. The need for member reinforcement must be determined by the EOR, and that member reinforcement must be shown in connection details on the structural drawings.
Adhering to the requirements outlined in Section 3 of AISC 360-16 for dealing with connections minimizes the chances of mistakes slipping through and minimizes the likelihood of RFI’s, change orders and arguments between stakeholders. The safety of steel-framed building structures is enhanced when engineers understand and follow the AISC Code of Standard Practice.
The process for insuring a successful project
Designing safe, constructable and economical building structures requires years of experience and good engineering judgement. The process for insuring that such experience and judgement will be integrated into every project requires that structural engineering firms recognize the need for monitoring quality throughout the project – from design development through construction administration. Key to Quality Assurance is the designation of a Quality Assurance Manager – a engineer preferably with 20+ years of experience, to monitor all projects and serve as a second set of eyes on every project.
Today’s technology has given engineers the ability to design building structures many times faster than was possible before the use of computers – however that technology does not imbue engineers with the skill and engineering judgement needed to design safe and economical structures. It is only through the adoption of a formal Quality Assurance process, one that is ongoing and continual, that engineers – particularly young engineers, learn the art of structural engineering.
When a Quality Assurance program is integrated into the process, everyone benefits. The owner gets a safe, cost effective and efficient design. The contractor gets a constructable design and high-quality contact documents – insuring smooth construction with minimal RFI’s and change orders, and the architect’s job is made easier by working with a structural engineer who integrates technology with experience into the design process.
As with everything else, quality does not come free. Owners and architects selecting a structural engineer must be mindful that high-quality design may not come with the lowest fee.
At The Harman Group we integrate experience and technology with a high level of customer service. This results in structural design that is safe, economical and constructible.