Input requirements for DG11 floor vibration

Tekla Structural Designer
2021
Tekla Structural Designer

Input requirements for DG11 floor vibration

General

The simplified method for the analysis of the vibration of floors given in the AISC Publication DG11, on which on which the Tekla Structural Designer check is based, is only applicable to regular structures which, by and large, are created from rectilinear grids.

Of course the floor layouts of 'real' multi-storey buildings are rarely uniform and Tekla Structural Designer therefore provides you with the opportunity to select the more irregular floor areas to be assessed with grids that are other than rectilinear.

In so far as the selection of the beams and girders to be used in the analysis is concerned, only beams or girders with Non-Composite, Steel Joist or Composite attributes are valid for selection and, within these confines, the user should be able to:

  • select a single beam or girder
  • select a girder span as critical plus an adjoining span (in a two or three span configuration)

In all cases, and subject to the above restrictions, which beams and girders from the selected area of floor are chosen is entirely at your discretion and under your judgment, but it is expected that the beams and girders chosen will be those that are typical, common or the worst case. Irrespective,Tekla Structural Designer will take these beams as those that form the idealized floor layout. There is no validation on what the you select (although there is some validation on which beams and girders are selectable i.e. those which have no slab for part of their length, those from angle sections, those with no adjoining span when a 2-span configuration is chosen, and those with no adjoining span at both ends when a 3-span configuration is chosen will not be selectable).

Data Derived from Tekla Structural Designer

Note that, where appropriate, the derived data is for each design combination under SLS loads only.

Unit supported weight

The unit supported weight is used to establish the 'effective panel weight' - that is the weight of the floor and its permanent loading that has to be set in motion during vibration of the floor. It is taken as the slab self-weight (and to be accurate, the beam self-weight), other permanent 'Dead' loads and the proportion of the 'Live' loads (expressed as a percentage) that can be considered as permanent.

The unit supported weight is obtained by summing all the loads (or the appropriate percentage in the case of live loads) that act over or in the selected area. This includes any blanket, area, line and spot loads that are present within the selected area. The component of any of these load types that lie outside of the selected area are ignored. Nodal loads directly on columns are also ignored. The total load is then divided by the area selected.

The slab self-weight will usually be in the Slab Dry loadcase - note that in the case of composite slabs this includes the weight of decking. The beam self-weight is in a separate protected loadcase. For simplicity this component of the unit supported weight is ignored. This leads to a slight inaccuracy in the participating mass that is conservative (more mass is advantageous).

Note that the use of live load reductions has no effect on the floor vibration check.

Slab data

If there are more than one set of slab attributes in the selected area then you have to choose which of these it is appropriate to use. From the designated slab attributes the following information/data is obtained,

  • the transformed inertia in mm4 per mm width [in4/ft].
  • the short-term modular ratio for normal or lightweight concrete as appropriate.

If the designated slab attributes are for a 'generic' slab, then you are asked for the transformed inertia and the dynamic modular ratio.

Beam data

When these are non-composite beams, the inertia is obtained from the sections database. When these beams are of composite construction the inertia is the gross, uncracked composite inertia based on the dynamic modular ratio that is required. Steel joist inertias from the database are assumed to be 'gross' inertias of the chords and are editable. Following guidance contained in AISC Steel Design Guide 11 Ref. 1, section 3.5, the gross steel joist inertia is factored by quantity Cr and displayed as the 'effective' inertia in the results viewer. Cr is derived from the following formulas,

For joists with angle web members:

Cr
 = 
0.90*(1 - e-0.28(L/D))2.8
 ≤ 0.9 
valid for 6 ≤ L/D

For joists with rod web members:

Cr
 = 
0.721 + 0.00725*(L/D)
 ≤ 0.9 
valid for 10 ≤ L/D

Where

L
 = 
Joist span
D
 = 
Joist nominal depth
Note: There is no guidance in DG11 what to do if L/D < 6 for angle web members or L/D < 10 for rod web members. Therefore, in these cases Tekla Structural Designer calculates Cr with L/D = 6 (angle) or L/D = 10 (rod).

The span of the critical/base beam and the adjoining beams is required.

The deflection of the critical beam under the permanent loads is required. To calculate this value, the deflection under the Dead loads and the appropriate percentage of the Live load deflection is summed.

Girder data

The same data is required as that for the beams.

Floor plate data

The dimensions of the floor plate in the idealized cases are defined in one direction by the number of beam bays and in the orthogonal direction by the number of girder bays. In practice, given that the idealized case may not attain, the floorplate dimensions are derived from the slab items you select as participating in the mass.

User Input Data

Secondary Beam Spacing

You must confirm the spacing of the secondary beams - an average value when the spacing is non-uniform.

Permanent Live Loads

You are required to specify the proportion of the permanent live loads that are to be used in the vibration analysis as a percentage of the live load.

Beam Continuity Factor

This has a value of either 1.0 or 1.5 and you are offered the choice when a two or three span beam configuration is selected. The default value is 1.0.

Girder Continuity Factor

This has a value of either 1.0 or 1.5 and you are offered the choice when a two or three span girder window is selected. The default value is 1.0.

This factor is not generally applicable for girders as they usually frame directly into columns and this potential increase in the effective panel weight does not apply under those circumstances.

Beam and Girder location

You are required to specify whether the beam and the girder under consideration is an "internal" location or an "internal edge" location. This information is required to set the constants that are used in calculating the effective widths of the beam and girder panel modes.

Number of bays used to establish effective panel weight

You are required to specify the number of bays in the direction of the beam span, nj, and the number of bays in the direction of the girder span, ng, that are to be used to establish the effective panel weight. The number of bays ranges from 1 to 4 for both directions.

Damping ratio

Floors do not vibrate as a free mass but have some damping i.e. dissipation of the energy in the system. Values of the damping ratio for individual components, βi are recommended in DG11 as,

Recommended Component Damping Values
Component Ratio of Actual Damping-to-Critical Damping, βi

Structural System

0.01

Ceiling and ductwork

0.01

Electronic office fit-out

0.005

Paper office fit out

0.01

Churches, schools and malls

0.0
Lightly furnished quiet spaces 0.005

Full-height dry wall partitions in bay

0.02 to 0.05*
*Depending on the number of partitions in the bay and their location; near the center of the bay provides more damping.

For example, a floor with ceiling and ductwork supporting an electronic office area has β = Σβi = 0.01 (floor) + 0.01 (ceiling and ductwork) + 0.005 (electronic office area) = 0.025, or 2.5% critical damping.

Since an even higher damping ratio might be justified for storage floors for example, a range of up to 10% is offered.

Acceleration limit

You must enter acceleration limits appropriate to the floor under consideration, one limit for floors with frequencies in the range 3-9 Hz and a separate limit for High Frequency floors (in the range 9-15 Hz). These limits will be based on your engineering judgement and the advice given in DG11 - which gives a range of values between 0.5% and 5.0% depending on structural form (for building floors see the table below).

Table 4-1. Recommended Tolerance Limits for Building Floors

Occupancy

Acceleration Limit

ao/g x 100%

Offices, residences, churches, schools and quiet areas

0.5%

Shopping Malls

1.5%

Footstep frequency

You should enter the footstep frequency to be used if a high frequency floor is detected. The range being 1.2 Hz to 2.2 Hz.

Sensitive use

You have to specify the Use case and walking speed if this analysis is performed. See: Sensitive use analysis DG11

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