tubular, core, and outrigger structures

7/30/2019 Tubular, Core, and Outrigger Structures

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CE 636 - Design of Multi-Story StructuresT. B. Quimby

UAA School of Engineering


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Many different types of Tube structures:framed-tube, tube-in-tube, bundled tube,braced-tube, and composite tube

All forms evolved from the traditional rigidjointed structural frame. The basic design philosophy is to consider the

entire floor plan as one section and place asmuch as possible the load carrying materialaround the external periphery of the building tomaximize the flexural rigidity of the crosssection.


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Consists of four orthogonally rigidly jointed framepanels place around the perimeter of the structureforming a tube in plan.

The columns are closely spaced and connected bydeep, stiff beams. The columns are aligned such thatthe strong bending plane is in the plane of the panel.

Placing the frames on the exterior of the building

maximizes the inertia of the tube section. The stiff beams mobilize the flanges of the tube.The stiffer the beams, the less shear lag, and moreparticipation by the columns in the frames normal tothe direction of force.


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The uniformity of the members makes it possible touse mass production techniques in the construction.

Interior layout is very flexible, making this system

good for office buildings. There are no interior shearwalls or braced frames in the way.

The closely spaced columns create problems wherelarge openings are required. Can resort to transfer

trusses/girders or inclined columns to solve theproblems. The closely spaced columns and uniformity are not

always considered to be aesthetically pleasing. Repetitive floor systems are possible.


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Complex behavior: Acts like a perforated tube,which is somewhat more flexible than a solidtube.

Frames parallel to the neutral axis take much ofthe moment in tension/compression. (Columnstress ~ Mc/I) Shear lag resulting fromdeformation in the spandrel beams tends tokeep all columns from participating equally.

Frames perpendicular to the neutral axis takethe shear.


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The function of the floor system is to transmitthe horizontal forces to the different vertical

structural elements. Floor diaphragms are rigid in plane. This prevents

distortion of the tube section.

Floor diaphragms are flexible out of plane. Out of

plane actions are generally negligible.

Most tubes are doubly symmetric, a featurethat can make analysis much simpler.


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Additional web frames are added to reduceshear lag in the flange frames.

Resulting effects: stresses in flange columns is more uniform

reduced shear forces in web frames (interior

frames constrained to act with exterior frames by

rigid floor diaphragms.)


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Diagonal bracing is added to the frames. The addition of bracing stiffens the web frames

and reduces shear lag in the flange frames.

Behavior is much closer to that of a truecantilevered tube. May reduce the number of columns in the

frames. Large scale bracing tends to be more practical

than single bay/story bracing due to reducednumber of connections. All columns connectedto the bracing.


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Bracing constrains all the columns to deflecttogether axially, creating a redistribution (orequalization) of gravity loads among the

columns. Many of the columns may actually be in tension

when subjected to gravity loads. Spandrels must be able to take the tension

created by the compressive forces in thediagonals under gravity loading and thecompression created the tensile forces in thediagonals under lateral loading.


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Generally core structures make use of anelevator or utility core in the building byenclosing the core with shearwalls or braced

frames. The core behaves as a single large structural

component (a vertical cantilever thin walledbeam) with large moments of inertia.

Due to the thin wall nature, the section is proneto warping resulting from torsion and/or thinplate buckling/distortions resulting frombending compressive stress.


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Most cores are open thin walled elements. Under the influence of torsional forces the

internal forces are a result of both pure shearand warping shear. Warping may result in large normal stresses. This is the same as seen in thin walled

structural steel sections subjected to torsion.


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Warping of the core has a significant effecton the response of the core.

See Mechanics of Material texts and Steeltexts for detailed discussion of torsion in thinwalled elements. These principles can beapplied to core structures.


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Hand methods require the solution of the differentialequations associated with the warping behavior.

Computer finite element membrane analysis

eliminates the need for in-depth understanding ofwarping behavior and the computation of warpingsection properties. Auxiliary beams may be requiredto make rigid connections to the membraneelements.

An analogous frame can be created if membraneelements are not available.

The problem can be simplified according to the twocolumn analogy or a single warping column model.


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Consist of a core which is stiffened byattachment to axial force columns by flexurallystiff outriggers. Stiff spandrel girders located

at the end of the outriggers are used to mobilizeall the columns on a building face.

The core takes all the shear. Most of the moments is taken by the columns at

the ends of the outriggers. Outriggers must be very stiff. The structure may be unsymmetrical.


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Approximate Analysis

Four simplifying assumptions (see pg. 356 of text)

May be used in optimizing the location of theoutriggers.

The approach uses compatibility of rotations.

Final Analysis

Stiffness computer methods.

tubular, core, and outrigger structures

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