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  4. Tutorial de un diseño de viga compuesta

Tutorial de un diseño de viga compuesta

Hope you have gone through our earlier blog Ejemplo de diseño compuesto AS2327 which gives an overall idea about the Composite Design Model. If you have done so already, I would request you to please go through it and come back here. For those who have visited the same, please read on.

For today, let’s have a walkthrough to understand the process of designing a composite beam and the step by step calculations obtained using SkyCiv’s Composite Beam design program.

Determination of effective section of concrete portion (ancho efectivo)

The first step in defining a composite beam cross section is to access the width of concrete flange available to act compositely with the steel section. Los anchos efectivos se expresan en relación con la luz de la viga. The constant value of effective width is taken as L/4.

Determination of effective portion steel section:

Cuando la viga de acero bajo compresión, its section will be buckled if it is non-compact or slender. En este caso, only effective area is considered in the design. Para esto, la sección debe clasificarse en la categoría a saber. Compacto/No compacto/Esbelto.

Las secciones compactas son preferibles a las secciones no compactas. En caso de que alguno de los elementos caiga en la categoría de sección No Compacta, su porción efectiva debe ser considerada en cálculos posteriores mediante el cálculo de anchos reducidos. Para secciones compactas, se supone que toda la sección transversal es efectiva sin ninguna reducción. Sección transversal que tiene esbelto elemento deberá no be used.

The effective width of steel plate will be determined in accordance with AS4100 which leads to the calculation of element slender ness on account of local buckling. The element slenderness for local buckling shall be checked as follows:

Design of beam for Strength

Designing the composite beam for strength criteria involves calculation of moment capacity. The module is capable of evaluating the sagging moment capacity of a beam as a case of Simply Supported beam.

    • Moment capacity is calculated considering full shear connection (FSC) es decir. β=1.0
    • As a part of FSC, 3 Different cases for the Plastic Neutral Axis (PNA) positioning are considered for evaluation of moment capacity.
    • When the concrete slab is stronger than the steel beams, la PNA will lie within the concrete slab como se muestra en la figura (1). Para este caso, the ultimate flexural strength is determined from a simple couple force.

Higo(1) : PNA lies in concrete slab

    • When the steel beam is stronger than the concrete slab, the plastic neutral axis will lie within the steel beam as shown in Fig (2). Para este caso, the moment strength can be obtained by summing moments about the centroid of the tension force. There can be two sub-cases in this category viz. PNA lies within top flange of steel beam Higo (2-a)o PNA lies in the web Higo (2-b).

Higo(2-a) : PNA lies in Top Flange of Steel Beam Higo(2-b) : PNA lies in Top Flange of Steel Beam

    • When there is a corrugated metal deck below the concrete slab, the moment of resistance for FSC is worked out in a similar way considering 3 possibilities of PNA position. The deck orientations are also taken into consideration while evaluating the moment of resistance viz. deck is parallel to span of beam (θ=0), deck is perpendicular to span of beam (θ=90) or any angle made by the decking with beam span in the range of 0 a 90(0<θ<90)
    • Además de esto, the Moment capacity is calculated considering partial shear connection (PSC) es decir. for value of β=0.1 to 0.9
    • The program also estimates the moment capacity of only steel beam section i.e. the case where there is NO COMPOSITE ACTION. This is the case where β=0 and thus, the concrete slab in the cross section does not play any role in the flexure design. This can be the case during construction for a very short duration.
    • As prescribed in AS:2327, the relationship between the degree of shear connection si and the moment of resistance ratio (es decir. ratio of moment corresponding to specific value of β to moment corresponding to β=1) for various values of β ranging from 0 a 1.0 are plotted.
    • User can get an idea about the moment capacity for certain degree of shear connection for the given cross sectional dimensions and shear connectors. The number of trials for shear connectors in terms of size and spacing can be performed using the program and the graph.

Design of Beam for Shear

    • Shear check is calculated for vertical shear as well as longitudinal shear.
    • Longitudinal shear is evaluated at the interface between concrete slab and steel beam.
    • For the given size and spacing (or numbers) of shear connectors, the longitudinal shear carrying capacity is evaluated. Así, user gets provided value of si for the shear connection. The minimum required value of β is calculated by the program as per Cl. 3.5.8.3. The required value of β for the desired moment of resistance can be obtained from the graph above.
    • The above evaluation can guide the user about optimization in case of shear connectors based on the criteria of longitudinal shear resistance.
    • The vertical shear resisted by the given cross section is evaluated based on contribution from slab (as per AS2327), acero estructural (as per AS4100) and the shear connectors (as per AS2327).
    • User can specify whether to consider or ignore the shear contribution from the concrete slab in the shear capacity calculations.
    • Two types of shear connectors are supported by the program viz. shear studs and structural bolts.
    • The program output provides the intimation to the user about the detailing provisions of shear connectors viz. minimum dia. of connector, minimum and maximum allowed spacing, edge distances, number of rows etc.

Design of Beam for Serviceability

    • The serviceability calculations involves estimation of deflection under the following cases:
      • construction stage (Steel beam only)
      • service stage-short term effects (composite section)
      • service stage-long term effects due to shrinkage (composite section)
      • service stage-long term effects due to creep (composite section)
    • The program is capable of calculating the deflection for the above cases based on the theory of either UN-CRACKED section or CRACKED section. The choice is given to the user to specify the section type viz. cracked or un-cracked.
    • In case of analysis based on un-cracked section, the transformed area of concrete in terms of structural steel is evaluated by the program and subsequently the other section properties which are needed for deflection calculation.
    • Cracked section analysis is based on the assumption that concrete is ignored in the composite section.
    • Total deflection is calculated for the above cases which is considered as the permissible deflection for the given cross section and compared with the actual deflection.

Esté atento a este espacio para el próximo blog sobre un tutorial similar para las columnas compuestas.

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