Girder Design Load History
SDC has spent 48+ years designing of crane girders for general industrial buildings and the steel industry. In 1974 SDC designed a crane girder for an industrial building with a 35T overhead crane. The girder was a standard cross section that included a rolled beam with a cap channel. The girder was designed for a lateral load equal to 10% of the lifted load per the 7th Edition of AISC Manual of Steel Construction. Welds connecting the beam and channel were calculated by determining the horizontal shear between the members.
The August 1979 version of AIST Technical Report 13 specified the lateral crane girder load for a mill crane to be 20% of the lifted load while the AISC requirement remained at 10%. The August 1991 version of AIST Technical Report 13 changed the lateral load requirement to a percentage of either the maximum lifted load or crane wheel load. The load is determined 3 different ways but is generally about 40% of the lifted load distributed to the crane girders across the bay from each other by lateral stiffness. The 1991 AIST lateral load requirement has not changed in 30 years as found in the current August 2021 AIST Technical Report 13.
AISC requirements are now found in ASCE/SEI 7-10 which requires a lateral load of 20% distributed to the crane girders across the bay from each other by lateral stiffness. In summary, the steel mill industry requires crane girders to be designed for twice the lateral load as for general industry.
Girder Structural Design History
Our 1974 crane girder design was based on what is known as flexural analogy. The lateral flexural load was resisted by the cap channel, top flange and part of the girder web. The torsion load was calculated by taking the lateral load times the depth of the crane rail. The torsional moment was distributed the top and bottom flanges of the girder by dividing by the girder depth. AISC does not address crane girder design.
All revisions of AIST Technical Report 13 use flexural analogy. The big difference from crane girders used in general industry is both the magnitude of the lifted loads along with the severe crane duty cycles. The doubling of the lateral loads for steel mills leads to the design of horizontal thrust plates or horizontal trusses to transfer the loads back to the building columns. These crane girders are considered to be unsymmetrical. AIST addresses the design of these unsymmetrical crane girders by determining the effective elements of the girder cross section for X and Y bending stresses.
The August 2003 version of AIST Technical Report 13 has an important change to Section 5.8 for Crane Runway Girders. AIST states that “the effect of torsional moments and out-of-plane forces at the rail-to-top-flange interface shall be considered”. AIST go on to state that “an exact analysis and design solution is complex and beyond the scope of this document”. AIST states that crane girders designed using the techniques contained in the Technical Report have satisfactory performance without additional strengthening.
The 2005 AISC Specification integrated both the ASD (Allowable Stress Design) and LRFD (Load Resistance Factor Design) Specifications into a single set of rules for the design of steel structures. The new specification unified the design standards with the most current knowledge and design practices.
For crane runway girder design, the section regarding plate girders found in the 9th edition of the code has been eliminated. Section F12 has been added to include the design of unsymmetrical structural shapes such as built-up plate girders. For standard structural sections, meeting the new requirements is not that difficult. However, for crane girders constructed from built-up sections, meeting the requirements of Section F12 is extremely challenging. The design engineer needs to determine the shear flow in the girder that conforms to zero warping shear stress at the terminal ends of the girder cross section and calculate the torsional warping constant (Cw) in order to apply the code equations.
SDC Girder Structural Design
SDC’s experience has been that all steel mills where we have provided engineering services have crane runways that do not have satisfactory performance. The warped 116 ft. long crane shown in Photographs #1 and #2 in Crane Girder Failure Examples proved to SDC that the exact analysis that AIST does not address due to complexity is the root cause of the girder failure.
SDC began writing the first version of CRANE GIRDER PRO in 1990 to address the major structural issues with crane girder design. The first and most difficult problem to solve is locating the shear center of an unsymmetrical crane girder. In order to obtain the correct value of torsion, all crane loads including gravity wheel loads must be resolved about the shear center as described in Torsion Warping Constant (Cw). SDC uses our Warping and Rotational Properties Program (WARPP) to calculate all of the torsional properties of the girder cross section.
The next step was to develop a module to determine the torsional stresses. Torsional stresses are computed based on the equations and their derivatives as noted in Definition of Terms for each end condition of interest (pinned or fixed). It should be noted that flexure stresses are solved with to elastic principal axes while torsional stresses are resolved about the shear center of the girder.
Another key CRANE GIRDER PRO module compiles influence line analysis results for all structural response categories owing to a single unit load running from end to end. The module then combines the effect of each influence category for the series of wheels arranged according to the actual wheel spaces grouped under the end truck as it bridges down the runway. A long girder will accommodate wheels for two fully loaded cranes. This design condition exists in many mill buildings. Using Microsoft Access, numerous linked database tables are updated dynamically to track I/O information including all stresses and deflections as crane bridges down the crane girder. As the crane girder is divided into zones of interest, the critical stresses for each zone are mined from the database.
CRANE GIRDER PRO generates varieties of custom report that are used to assess the adequacy of girder design suiting various qualification criteria. One of the most important reports is the Fatigue Strength Assessment Report. The exact state of tensile stress fluctuation and shear stress reversal at each node of girder cross section is calculated by a series of load combinations that meet AIST Technical Report 13 criteria. Girder Failure Summary shares our experience with unpredictable crane girder behavior. However, in many cases, the combination of weak axis bending stress and warping normal stress could induce sufficient amount of tension in the top flange to overcome the high compressive stress, which in turn causes many failures owing to fluctuation of tensile stress.
For a complete print out of the computer model and stress report for a W40x324 girder with a 3/8″ thrust plate and W14x30 back-up beam, go to the Crane Girder Pro Calculation Output (pdf). A summary calculation is shown in the Crane Girder Analysis Summary Calculation (pdf) for a similar girder. These types of girders are not specifically addressed by AIST Technical Report No.13.