Chapter 3 – Advanced Introduction (Part 2 of 2)
Table of Contents
3.1 Could Unsymmetrical Sectioned CRG Experience Lateral Torsional Buckling?
3.2 Revisiting Principal Axes
3.3 Get Going By Engineering Judgement?
3.4 The Die Hard Shear Center
3.5 Interaction among Longitudinal Stresses
3.6 The Inseparable Links between Shear Center and Longitudinal Stresses
3.7 What Does LTB of Unsymmetrical Sections Look Like?
3.8 If Not LTB Then Should It Be Imperfection?
3.9 What Could Practitioners Do About Unsymmetrical Sections?
3.10 Play Safe – A Better Defense
3.11 Avoiding Unsymmetrical Sections
3.12 Local Buckling – From Gross Section to Effective Section
3.13 Loads and Load Combinations – Data Processing Woes
3.14 Effective Section – Post Buckling Geometry
3.15 The Tricky Effective Sections
3.16 Maximum Stresses – In Most Cases, Some Cases or All Cases
3.17 Stress Distribution – Then Hidden Intricacy
3.18 Stress Distribution – The Pitfall
3.19 Combined Stresses
3.20 Fatigue Stress Ranges
3.21 Live Load On/Off Cycles – The Exact Count
John Fong (馮永康) . Bill Vanni
Structural Design Corporation
1133 Claridge Dr., Libertyville, IL 60048
©March 2010 − October 2021, 2023
3.1 Could Unsymmetrical-Sectioned CRG Experience Lateral Torsional Buckling?
While bringing up this somewhat puzzling question per se, it turns up another one:
By the asking, does that (or not) make any R&D sense beyond our own common sense?
It’ll be too soon for a straight answer without serious R&D-tailored contemplation. Then only on self-awareness purpose, a very subtle cross examination that may hit us not that hard but wondering, who really symbolizes R&D? Nevertheless, questing for a modest advice over such an exceptional issue on unsymmetrical sectioned members’ behalf does seem fit as breaking through one of those technical bottlenecks mentioned at the beginning of Chapter 2, or is it not?
Anyhow, whether out of curiosity or pure interest, at some point or another, many Structural Engineering Practitioners would need an R&D signoff for it, just to be sure.
Knowing the fact that well-founded (or well-funded?) discussion on this matter was rare or rather vague for time being (as of this writing,) so let’s see if there is any easy way out on our own to piece together the puzzle on the design exclusively of unsymmetrical sectioned girders by further posting the following;
- How shall we deal with Lateral Torsional Buckling (LTB)?
- Do we really have to bother with LTB issue(s) at all?
Or so for a much wider coverage, those two question(s) might be consolidated into something like this:
Should the occurrence of LTB be fully dependent on or selectively be independent from the profile geometry at all?
Perhaps it can go either way, but here’s the main reason why bringing up the subject of interest:
As explained in many textbooks as we’d learned, LTB obviously pertains to symmetrical sectioned (mostly I-shaped) members, but the practical notion on LTB’s happening (or not happening) to non I-shaped unsymmetrical sectioned members seemed somewhat tentative or otherwise hesitant if not consistently missing
Beyond I-shaped members on Practical Structural Engineering Rationales, the need of clarification and advice on LTB were very specific: As we normally think, it should apply to all sizes and configurations of Crane Runway Girders (CRG); or does it?
At this point even though there isn’t much inspiration on pressing a sophisticated engineering logic (as we would start forming the not-so-sophisticated judgement a few pages down) to the issue, but the absence of such an interesting (important or not) subject has to be cracked open somehow through some means – without reinventing the wheels but drawing on reliable resources already existed out there − since there weren’t “Official R&D-blessed words” on this subject in the short run so what is about to transpire next should be assimilated with a grain of salt.
3.2 Revisiting Principal Axes
First thing first, let Y-load be pointing along the gravitational axis as always. It is important to point out the Y-axis as we speak of is the one that matches the loading axis whence the orientation it points to might not match the innate principal Y’-axis for unsymmetrical section.
To simplify the loading scenario and treatment on matter of concern, one could put aside the effect due to longitudinal force by letting Z-load ≈ 0 for now thus leading to the most basic XY loading condition; further on if also letting lateral thrust X-load ≈ 0 then the CRG is mainly influenced by (1) vertical Y-load, along with which (2) a companion effect from Py–δx torsion − Mz (or Mt) − due to misaligned Y-load eccentricity ex with respect to not the girder web but to the Shear Center (SC).