MDX Software - FAQ #21
How can I get the program to successfully generate a girder design?
I keep getting abort messages that begin such as: "Computation difficulty
during search..."
-----------------------------
Until one acquires a feel for the way the design mode behaves and a skill
for setting range limits that will induce convergence, we suggest:
1. Fix web depth as appropriate for the span (eg., Span/30 for a composite
beam).
2. Except for box girders, fix bottom flange width at about web depth/3
3. Except for box girders, fix top flange width at 2 to 4 inches less than bottom flange width. It may have to be larger for LRFD if this constrains the plastic neutral axis below total depth/5 from the top of the slab. For box girders fix top flange width at an appropriate value.
4. Set lower limits on web thickness at something near the minimum appropriate
for the depth. Do the same for flanges.
The purpose is to somehow obtain a preliminary design, which then can be
fine tuned in rating mode until a practical configuration is obtained. Some
modification normally will be required to avoid overstressing, etc., because
approximate stiffnesses and user specified noncomposite dead load are required to generate
a design, after which updated stiffnesses and the self weight approach can be used to
generate stresses and deflections upon reanalyzing. This will cause some force shifting with
probable minor overstressing. (Note: Define:Loading:Noncomposite Dead Load will allow the
self weight approach after girder dimensions have been established.)
We recommend only generating an initial set of girder designs (or a design if a line girder project), and after that point the user should concentrate on tuning the design in rating mode.