design of a multi-storey steel structure

7/27/2019 Design of a Multi-storey Steel Structure

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DESIGN OF A MULTI-STOREY STEEL STRUCTURE

Computation run on an un-braced frame with 3 spans and 6 storey, the loads beingpresented in the table below:


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Calculus of the structural imperfections

This evaluation is made by considering an equivalent geometric imperfection, expressed bythe initial bow:

Where :

but:

Initial bow may be replaced by a system of horizontal equivalent forces at every level.Total load at every level is: 60kN/m6m3=1080 kN.Horizontal equivalent force is: 0.00261080=2.81 kN.


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DESIGN OF THE STRUCTURAL CONNECTIONS: BEAMS TO COLUMNS JOINTS

End plates 25 mm and high strength M24, gr.10.9 bolts, not preloaded. Weak component of the connection: web panel of the column. The joint is considered a rigid one having a full strength by most of the engineers but its

characteristics places it in the semi-rigid connections area, with partial strength. Increasing the strength of the web may be done:

A- use a HEM section with a stronger web or a build-up section ( plate girder) increase thesteel consumption;

B- strengthen the web with supplementary steel plates welding increase the costs of theexecution and it induces a fragile behaviour under repeated loading (dynamic actions).

Supplementary web plates, ENV 1993-1-8


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DESIGN OF THE BEAM TO COLUMN CONNECTION

Ed

cr cr

F

F


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OSERVATIONS:Strength of the joint almost equal with the strength of the beam de (89% from the plastic

resistance of the beam and the stiffness of the joint with respect to the stiffness of the beam

is 13.2, that is smaller than 25, whic makes a joint to be considered fully rigid in an un-braced frame.Due to the fact that the external bending moment, M j,Ed is smaller than 2/3 from the

resistance of the joint, M j,Rd, the stiffness of the joint S j is equal withthe initial sitffness S j,ini. In this case both the situations will be considered in the design:1) the joint is fully rigid;2) The joint is semi-rigid, this behaviour being modelled.

The structural analysis must consider the sensitivity to lateral sway (I or II order) and alsothe plastic resistance of the structure (elastic or plastic).In this regard, a frame may be considered with fixed joints or the sway may be considered asdepending on the ratio between the vertical total force F Ed applied on the structure and thecritical elastic force F cr causing lateral instability (sway).

The structure with rigid connections, the buckling analysis shows a factor cr=11.1 > 10,then the structure is classified as with rigid connections. And normally according to ENV1993-1-1, an elastic I order analysis with global imperfections may be run .

Still, being close to the limit, an elastic II order analysis will be run also. For the semi-rigidconnections, the buckling analysis show a factor of cr= 8, 96 and because is a value under10, the structure will be considered with swaying connections and a II order elastic analysis isimposed.


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Buckling analysis with SAP 2000


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First buckling shape for the structure: a) with fully rigid connections; ) with semi-rigid connections


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I and II order analysis of the structure with SAP 2000


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The reserve in the resistance of the structure up to a plastic hinge development, aplastic analysis must be run both for the rigid frame and for the sway frame. Plasticanalysis gives cr


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Definition of plastic hingesfor columns (P-M)


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Distribution of the bending moment on the beam in the stage of I plastic hinge


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design of a multi-storey steel structure

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