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Using diaphragms in dynamic analysis

Introduction

One important use of diaphragms is the analysis of the structural behaviour of buildings under lateral loading, in particular seismic loading.

In SCIA Engineer, dynamic and seismic analysis (including ELF method) requires the definition of masses. Masses may be defined manually or by converting gravity loads into masses. SCIA Engineer always takes into account the self-mass of the structure. However, it is necessary to refine that last statement for composite & metal decks, especially when using diaphragms.

The self-weight is the action of gravity on the self-mass. By default, the self-weight is a load acting in the vertical direction downwards, equal to the self-mass multiplied by the acceleration of gravity. However, although linked, both quantities are stored separately in SCIA Engineer and may be altered separately as well.

Manual input of masses on diaphragms

As a general rule, masses defined directly attached to diaphragms cannot be taken into account in the analysis model and are subsequently not exported to the solver when running the dynamic calculation. The reason for that is, that masses are usually acting equally in all directions. That means, for instance, that a surface mass distributed on a diaphragm, which has no bending stiffness, would not be held in the direction perpendicularly to the plane of the diaphragm.

Masses must be redistributed to supported entities (e.g. beams), in the same way as loads are. However, load panel functionality, whihc is used in diaphgrams for load transfer, is designed to redistribute only loads, not masses.

The procedure for defining masses on diaphragms is, nevertheless, straight forward, as, in most cases, masses for the dynamic analysis are deducted from the static loading of the structure:

• use only mass groups linked to a load case
• make sure that the option Keep masses up-to-date is ticked ON in the properties of the mass group
• if masses must be defined manually, make sure that they are attached to supporting members instead of diaphragms

Enabling the Keep masses up-to-datesetting causes the loads to be converted to masses after being distributed to the supporting members of the diaphragm (or load panel). Typically, line loads on beams are converted to line masses on beams, instead of the initial surface load applied on the diaphragm or load panel.

Self-weight and mass of structural members

Non-composite members

Non-composite members also include the steel profile of composite beams.

The self-weight of non-composite members is taken into account in one operation, through a self-weight load case. That load case contains the entire self-weight of such members.

The self-mass of non-composite members is taken into account automatically during dynamic analysis. SCIA Engineer includes the self-mass in each mass combination without intervention of the user.

A self-weight load case contains no explicit load for non-composite members. As a consequence, linking a mass group to a self-weight load case will produce only an empty mass group.

Both self-weight and self-mass of non-composite members may be altered, separately, using property modifiers.

Composite & metal decks

The mass and self-weight of composite decks is made of 3 components:

• the profiled steel sheeting
• the fresh concrete
• the dry concrete

For metal deck, the concrete parts are simply left out, but the other principles remain.

By default, partial self-weight is enabled for composite decks, which means, that different density values are used for concrete, depending on the considered check. For construction stage checks, the steel sheeting and fresh concrete are used. For final stage checks, the steel sheeting and dry concrete are used. Separate, partial self-weight load cases are used for the 3 parts listed above.

The partial self-weight feature may be disabled in the composite setup. In that case, only the dry density of concrete is considered in all cases for the analysis. Only one, standard self-weight load case is then used, which contains the self-weight of the entire structure, including composite & metal decks.

Taking self-weight & mass into account in the analysis

The tables below summarize how the self-weight and self-mass are sent to the solver, for various combinations of settings. For masses, when specified, a mass group must be created manually and inserted into the relevant mass combination(s).

Notations:

• SW = standard self-weight load case
• SW_fresh = partial self-seight load case with fresh concrete density
• SW_dry = partial self-weight load case with dry concrete density

Composite deck with Standard FEM element behaviour, partial self-weight enabled

Do not create mass groups linked to SW_fresh or SW_dry. As the mass of concrete is automatically included in mass combinations, this would double the mass of concrete.

Composite deck with Diaphragm element behaviour, partial self-weight enabled

Make sure to use either a mass group linked to SW_fresh or a mass group linked to SW_dry in a mass combination. Using them together in the same mass combination would double the mass of concrete of the composite deck.

Composite deck with Standard FEM element behaviour, partial self-weight disabled

Composite deck with Diaphragm element behaviour, partial self-weight disabled

In this case, the mass group linked to SW will include the mass of both the steel sheeting and the concrete topping.

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