Lateral Torsional Buckling of a Principal Beam with an I-Section According to EN 1993-1-1

Technical Article on the Topic Structural Analysis Using Dlubal Software

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Technical Article

This example is described in technical literature [1] as Example 9.5 and in [2] as Example 8.5. A lateral-torsional buckling analysis must be performed for a principal beam. This beam is a uniform structural member. Therefore, the stability analysis can be carried out according to Clause 6.3.3 of DIN EN 1993-1-1. Due to the uniaxial bending, it would also be possible to perform the design by the General Method according to Clause 6.3.4. Additionally, the determination of the moment Mcr is validated with an idealized member model in line with the method mentioned above, using an FEM model.

Structural System

Cross-Sections:
Principal beams = IPE 550
Secondary beams = HE-B 240
Material:
Structural steel S235 according to DIN EN 1993-1-1, Table 3.1

Image 01 - Structural System

Design Loads

LC 1 Selfweight:
gd = 1.42 kN/m
LC 2 Imposed Load:

Formula 1

f1,d = 145.4 kN · 24 m = 72.70 kN/m


Formula 2

f2,d = 198.5 kN · 24 m = 99.25 kN/m

Image 02 - Loads

Design Internal Forces

Image 03 - Distribution of Bending Moment My for Load Combination CO1 = LC1 + LC2

Stability Analysis Without Considering Secondary Beams According to [3] Clause 6.3.2

Under the assumption of a lateral and torsional restraint available at the member's start and end, an ideal critical moment for lateral torsional buckling Mcr of 368 kNm is determined in RF-STEEL EC3 in line with the verification according to [3], Clause 6.3.2. Thus, the design according to Equation 6.54 results in 1.64. Hence, the ultimate limit state design cannot be fulfilled without the stabilizing effect of the secondary beams.

Stability Analysis Considering Secondary Beams According to [3], Annex BB.2.2

The rules of DIN EN 1993-1-1, Annex BB.2.2 assume a continuous rotational restraint over the beam's length. Therefore, the discrete rotational restraint available in the model is "smeared" to a continuous rotational restraint.

Determination of available continuous rotational restraint:
The values are taken from [2] and adjusted only to the notation of annex BB.2.2.
Cθ,R,k = 11,823 kNm (portion from flexural deformation of secondary beams)
Cθ,D,k = 359 kNm (portion from cross-section deformation of principal beam, connection to web is considered)

Conversion to continuous rotational restraint Cθ with average distance of secondary beams:

Formula 3

xm = 2.5 m  2.7 m2 = 2.6 m


Formula 4

Cθ = 1111 823  1359 · 2,6 = 134 kNm/m

Determination of required rotational restraint:

Formula 5

Cθ,min = Mpl,k2EIz · Kθ · Kυ = 65,330221,000 · 2,670 · 10 · 0.35 = 266.4 kNm/m


where
Kυ = 0.35 for the elastic cross-section ratio
Kθ = 10 according to DIN EN 1993-1-1/NA, Table BB.1

A reduction of Cθ,min by (MEd / Mel,Rd)² is possible:

Formula 6

Cθ,min = 266.4 * 452.7521.32 = 200.9 kNm/m

Verification:
Cθ,prov = 134 kNm/m < Cθ,min = 200.9 kNm/m

The design in the form of the verification of a sufficient restraint for the principal beam's lateral deformation according to Annex BB.2.2 cannot be performed.

Stability Analysis Considering Secondary Beams According to [3], Clause 6.3.4

Determination of the available discrete rotational restraint:
The values are taken from [2] and adjusted only to the notation of annex BB.2.2.
Cθ,R,k = 11,823 kNm (portion from flexural deformation of secondary beams)
Cθ,D,k = 359 kNm (portion from cross-section deformation of principal beam, connection to web is considered)

Formula 7

Cθ = 1111,823  1359 = 348 kNm/rad

With this rotational spring, it is possible to describe the structural model of the notionally singled out set of members for the design according to Clause 6.3.4 in Module Window 1.7.

Image 04 - Rotational Spring in Window 1.7

During the verification according to 6.3.4, a resolver for eigenvalues implemented in RF-STEEL EC3 determines the factor αcr,op, by which the smallest ideal critical buckling load can be reached with deformations from the structural system plane.

Image 05 - Factor αcr,op Determined by RF-STEEL EC3

The critical buckling load factor is shown among the intermediate values (see the result windows) and the corresponding mode shape can be displayed in a separate window. Thus, the result is a moment Mcr of 452.65 kNm ∙ 2.203 = 997.2 kNm.

Hence, the design according to Equation 6.63 results for the model in 1.01. For the calculation of αcr,op the load application point was applied in accordance with the detail settings in a destabilizing way on the upper flange. Keeping in mind that the real point of load application is between the upper flange and the shear center, it is possible to ignore the slight exceeding and consider the design to be fulfilled.

Image 06 - Design in RF-STEEL EC3

Determination of Mcr on an FEM Model

With the "Generate Surfaces from Member" function and other available modeling tools, you can easily create a surface model of the structure in a minimum amount of time. It is possible to determine and graphically display the moment My in the beam using the "Result Beam" member type. The critical buckling load factor that is needed can be calculated on the entire model with the RF-STABILITY add-on module.

Image 07 - My in Beam (above) and Critical Buckling Load Factor in RF-STABILITY (below)

With this FEM model, we get a moment Mcr of 447.20 kNm ∙ 2.85 = 1,274.5 kNm. This is a bit higher than the result on the member model with corresponding discrete rotational springs. Consideration could be given to an even more accurate modeling of the connections of the secondary beams.

Reference

[1] Kuhlmann, U.: Stahlbau-Kalender 2013 - Eurocode 3 - Anwendungsnormen, Stahl im Industrie- und Anlagenbau. Berlin: Ernst & Sohn, 2013
[2] Lindner, J.; Scheer, J.; Schmidt, H.: Stahlbauten - Erläuterungen zu DIN 18800 Teil 1 bis Teil 4. Berlin: Beuth, 1993
[3] Eurocode 3: Design of Steel Structures - Part 1-1: General Rules and Rules for Buildings; EN 1993-1-1:2010-12
[4] National Annex - Nationally Determined Parameters - Eurocode 3: Design of Steel Structures - Part 1-1: General Rules and Rules for Buildings; DIN EN 1993-1-1/NA:2015-08
[5] Training Manual EC3. Leipzig: Dlubal Software, September 2017

Author

Dipl.-Ing. (FH) Frank Sonntag, M.Sc.

Dipl.-Ing. (FH) Frank Sonntag, M.Sc.

Sales & Customer Support

Mr. Sonntag coordinates the Dlubal Software office in Leipzig and is responsible for sales and customer support.

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  • Updated 23 August 2021

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