Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Lateral-torsional buckling (LTB) is an important failure mode that needs to be taken into account during
the design of steel beams. The fundamental equation for determining the elastic critical moment of
a beam was derived with the assumption that the beam is subjected to a uniform bending moment
distribution. Loads on steel structures generate a great variety of bending moment distributions. The
effect of the bending moment distribution is taken into account by a parameter known as the equivalent
moment factor. The procedure outlined in the South African National Standard for limit-states design of
hot-rolled steel work, SANS 10162-1:2011, for determining the equivalent moment factor was originally
developed for a bending moment that is uniformly or linearly distributed, however it is currently used
for all bending moment distributions.
A Finite Element (FE) model was developed in this investigation for determining the equivalent moment
factor. The numerical model included residual stresses and initial geometric imperfections commonly
found in hot-rolled steel beams. To validate the assumptions made during the development of the FE
model an in-depth experimental investigation was conducted on simply supported beams. Three different
load configurations were considered in the experimental study in order to simulate various bending
moment distributions. A comparison of the equivalent moment factor between the numerical results and
the results obtained from various steel specifications, including SANS 10162-1:2011, was carried out in
an attempt to quantify the positive and negative attributes of the various methods employed by steel
design specifications.
The experimental investigation concluded that the FE model is able to successfully represent a simply
supported beam with realistic characteristics that include residual stresses and imperfections. The comparative
study illustrated that for a bending moment distribution with a constant moment gradient,
SANS 10162-1:2011 provides excellent results. However, for the other distributions considered in this investigation
highly conservative results were obtained for the equivalent moment factor. The relevance of
these findings were made clear by considering three design cases found in steel structures. The resistance
moment of the beams in each of these cases was calculated according to each of the steel specifications. It
was found that the use of a highly conservative procedure for determining the equivalent moment factor
can lead to the uneconomical design of a structure. / AFRIKAANSE OPSOMMING: Laterale-torsie knik is ’n belangrike falings modus wat in ag geneem moet word tydens die ontwerp van
staal balke. Die fundamentele vergelyking vir die bepaling van die elastiese kritieke moment van ’n
balk is afgelei met die aanname dat die balk onderworpe is aan ’n eenvormige buigmoment verdeling.
Belastings op staalstrukture genereer ’n groot verskeidenheid van buigmoment verdelings. Die effek van
hierdie buigmoment verdelings word in ag geneem deur ’n parameter wat bekend staan as die ekwivalente
moment faktor. Die prosedure uiteengesit in die Suid-Afrikaanse Nasionale Standaard vir die ontwerp
van warm-gewalste staalwerk, SANS 10162-1:2011, vir die bepaling van hierdie faktor is oorspronklik
ontwikkel vir ’n buigmoment wat uniform of linieêr verdeel is oor die lengte van die balk, maar dit word
tans gebruik vir alle buigmoment verdelings.
’n Eindige Element (FE) model is ontwikkel in hierdie ondersoek vir die bepaling van die ekwivalente
moment faktor. Die numeriese model sluit die residuele spannings en aanvanklike geometriese imperfeksies
wat in die algemeen teenwoordig is in warm-gewalste profiele in. Die aannames wat gemaak is tydens
die ontwikkeling van die FE model is bevestig met ’n in diepte eksperimentele ondersoek oor die gedrag
van eenvoudig opgelegde balke. Drie verskillende las konfigurasies is oorweeg in die eksperimentele studie
om verskeie buigmoment verspreidings na te boots. ’n Vergelyking van die ekwivalente moment faktor
tussen die numeriese resultate en die resultate verkry van verskeie staal spesifikasies, insluitend SANS
10162-1:2011, is uitgevoer in ’n poging om die positiewe en negatiewe eienskappe van die verskillende
metodes wat gebruik word in verskillende staal ontwerp spesifikasies, te kwantifiseer.
Die eksperimentele ondersoek het tot die gevolgtrekking gelei dat die FE model in staat is om ’n eenvoudige
opgelegte balk te verteenwoordig, met realistiese eienskappe wat residuele spannings en imperfekies
insluit. Die vergelykende studie toon dat SANS 10162-1:2011 uitstekende resultate bied vir ’n
buigmoment verdeling met ’n konstante moment gradiënt. Dit was egter gevind dat vir ander verdelings
wat in hierdie ondersoek oorweeg is, SANS 10162-1:2011 hoogs konserwatiewe resultate bied. Die
toepaslikheid van hierdie bevindinge is duidelik gemaak deur drie ontwerp gevalle wat algemeen in staalstrukture
gevind word te bestudeer. Die weerstandsmoment is in elk van die gevalle bereken volgens elke
staal spesifikasies. Daar is gevind dat die gebruik van ’n hoogs konserwatiewe prosedure vir die bepaling
van die ekwivalente moment faktor kan lei tot die ontwerp van ’n onekonomiese struktuur.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/95863 |
| Date | 12 1900 |
| Creators | Smalberger, Hermanus Johannes Wessels |
| Contributors | Van der Klashorst, Etienne, Stellenbosch University. Faculty of Engineering. Department of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xv, 151 : ill. |
| Rights | Stellenbosch University |
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