Technical
NSC 27
May 19
angularity, but for a quick check, assume that C1 = 1.13, mainly
for easy use of the look-up tables in the Blue Book.
For the trial section of a 533 × 210 × 101 UB in S275 (note
that all beams are S355 nowadays!), a buckling length of 9 m
and C1 = 1.13, the buckling resistance Mb = 288 kNm. As a coarse
check, this is quite reassuring when compared to the computed
value of 277 kNm*.
A further approach is to use the look-up tables in the back
of P3625, where χLT depends only on h/tf and L/iz, which more
mature designers will recognise as D/t and L/ryy in previous
nomenclature. The tables in P362 assume C1 = 1.0, so are likely
to deliver a smaller resistance than computed with precision.
h/tf = 536.7/17.4 = 31
L/iz = 9000/45.7 = 196
Using Table E2 from P362, χLT = 0.38 with some approximate
interpolation.
Therefore Mb = 0.38 × 2610 × 103 × 265 × 10-6 = 262 kNm
This seems to offer reassurance that we are in the correct
parish, at least, when compared to the computed value of
277 kNm*.
What has not been addressed!
In the opinion of the author, the challenge with gantry girders
is not in fact the member verification, but the determination of
the applied actions in accordance with EN 1991-3, a problem
which was not mentioned in Verulam. A treatise on the subject is
available for download6, but the topic is complex.
Other issues not addressed here are the deflection limits for
crane supporting structures, which may be more important than
the member resistance. Designing the supporting structure
to control the spread of the gantry beams will be important.
Finally, fatigue design may govern the size of the member – an
introduction to the subject7 and example calculations8 have
been published in NSC.
*Footnote
Readers trying to replicate the calculation of Mcr as quoted
in P385 may have some difficulty. The correct value of Mcr
Figure 3 Bending moment diagram
appears to be between 336 and 340 kNm and consequently
Mb = 288 kNm. Although it would be tempting to blame
the software, it appears the user calculated the level of load
application as 533/2 + 65 = 331 mm, when 286 mm should have
been used (the load is applied at the top flange, not on top of
the 65 mm rail).
References
1 Verulam, The Structural Engineer, March 2019
2 Handbook of Structural Steelwork, BCSA and SCI (second
edition of 1991)
3 Design of steel beams in torsion, (P385) SCI, 2011. Available
on steelconstruction.info
4 A brief history of LTB, New Steel Construction, February &
March 2016
5 Steel Building Design: Concise Eurocodes (P362) SCI, 2017
6 Sedlacek et al Actions induced by cranes and machinery
https://estudijas.llu.lv/pluginfile.php/127337/mod_resource/
content/1/20100609%20Exemple-Aachen%20Piraprez%20
Eug%C3%A8ne.pdf
7 Henderson, R. Introduction to fatigue design to BS EN 1993-
1-9. New Steel Construction, September 2018
8 Henderson, R. Illustration of fatigue design of a crane runway
beam. New Steel Construction, January 2019
/The_Blue_Book
/Fatigue_design_of_bridges
/Eurocode_Design_Guides
/NSC_Feb16_tech.pdf
/NSCmarch16Tech.pdf
/20100609 Exemple-Aachen Piraprez Eugène.pdf
/20100609 Exemple-Aachen Piraprez Eugène.pdf
/20100609 Exemple-Aachen Piraprez Eugène.pdf
/NSC_Sept18-Tech.pdf
/NSC_Jan19Tech.pdf
/
/www.rainhamsteel.co.uk
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