SCI P354

The SCI P354 design guide ^{(Ref. 1)} suggests that the following aspects of the floor analysis model should be considered.

The dynamic Young’s modulus of concrete may be taken as 38 GPa and 22 GPa for normal and lightweight concrete, respectively.

All connections should be assumed to be rigid.

Column sections should be provided, and they may either be assumed to be pinned at their theoretical inflexion points, usually mid-height between floors, or fixed at their remote ends.

In some cases non-loadbearing cladding (or partition walls) might be judged to provide vertical restraint (translationally and rotationally free).

The interfaces at core walls should be modeled as fully fixed.

CCIP-016

According to CCIP-016, the presence of full height partitions of (i) masonry, (ii) studwork, and (iii) glass can significantly modify the dynamic properties of floors. As a consequence, if the presence of those elements is required for the floor to serve its purpose, they should be included in the analysis. This is not the case for part-height and movable partitions.

Due to small strains associated with footfall analysis, pinned beam connections should be converted to fixed.

To account for the additional stiffness and restraint provided by facades and concrete core walls, vertical edge restraints should be provided in their locations.

Since columns increase the frequencies of floors, due to their rotational stiffness, the columns both above and below the considered level should be included.

AISC DG11

DG11 recommends that the FAM be limited in its size and indicates that considering a large floor area might be unconservative. For a slab item of interest the FAM should include only those slabs immediately adjacent. So for a regular building a ‘middle’ slab would create a model of 9 similar slabs including the one of interest. This is entirely at the user’s control in Tekla Structural Designer.

Balconies can be included as a whole, but it’s recommended to include only the seating sections between several rackers if the relevant vibration modes span over very large areas.

In case of staircases and footbridges, flexible supporting elements should be included, as their flexibility is most likely to provide additional damping and decrease the natural frequencies.

When modeling a floor with composite action, the greater stiffness of concrete on metal deck under dynamic loading needs to be accounted for by increasing the concrete’s Young’s modulus by a factor of 1.35.

To predict natural frequencies of slabs more accurately, the orthotropic material properties should be defined, with the flexural stiffness parallel to deck ribs being between two to five times higher than the stiffness perpendicular to the ribs.

Rolled beam and girder end connections should be considered as continuous, even if they are designed and detailed as shear connections.

Slabs

For 2-way spanning slabs the slab mesh is generated using 2D elements in accordance with the meshing controls specified in the FAM Properties.

To have their stiffness included in the analysis, each of the different 1-way spanning slab types can optionally be meshed using orthotropic 2D elements.

If 1-way composite slabs are meshed, the orthotropic properties can either be auto calculated based on their thickness, (using an X factor of 1.000 and a Y factor of 0.010), or user defined properties can be specified. To use the latter option you would also need to select 'Analyze slab' in the composite slab item properties in order to define the analysis parameters required.

For those slabs that are unmeshed you can specify the proportion of their self weight mass to be included.

Each of the above choices are set via Analysis Model Options

Column stacks and wall panels

The column stacks are modeled as either full height with fixed supports, or half height with pinned supports.

Walls are modeled either with fixed supports, or modeled supports.

The releases of members attached to the columns and walls can either be fixed or the actual releases defined in the model can be used.

Each of the above choices are set via Analysis Model Options

Beams

The supporting beams are subdivided into 1D elements connected to the slab 2D element mesh to produce a well discretized analysis model of both beams and slabs.

A ‘Max node spacing’ parameter in Analysis Model Options ensures that beam lengths with no meshed slab attached are still subdivided to some degree.

While any non-composite steel beams included in the FAM are by default treated as non-composite, the general expectation is that any steel beam with a minimum nominal composite behavior should be considered as composite for footfall analysis, even if for design purposes it's still a non-composite beam. To cater for this a 'Treat as composite' option can be set in the 'Floor vibration' section of the beam properties. Enabling this option allows the concrete flange width above the beam to be calculated taking into account the adjacent composite slab(s) in the same way as it would be for a beam that is defined as composite.

Facade restraints

The restraining effect provided by building cladding can be taken into account by selecting 'Generate Facade restraints' in the FAM Properties. When selected, the solver nodes along the perimeter of the FAM are fixed in their Fz degree of freedom.

Once facade restraints have been generated you can then add or remove them from individual beams and slab edges as required.

Spandrel beams (AISC DG11 only)

Spandrel beams (beams that support the facade) are usually significantly restrained by cladding.

In accordance with DG11 any beams identified as spandrel beams have a multiplier applied to their major axis stiffness for the footfall modal analysis.

They are taken into account by selecting 'Generate Spandrel Beams' in the FAM Properties. When selected, the multiplier is applied to any beams along the perimeter of the FAM.

Once spandrel beams have been generated you can then add or remove them from individual beams as required.

The default multiplier is 2.5, as this is the value recommended by DG11. You can edit the multiplier if required via Analysis Model Options

SCI P354 and CCIP-016

These loadcases normally include unfactored self-weight of the structure with superimposed dead loads such as the weight of the ceilings and services, unless the analysis is for a bare-state structure, then those can be ignored.

If certain load cases are guaranteed to exist in the finished structure, semi-permanent loads can be included as well. This does not apply for dance or aerobic floors.

Although UK National Annex to EN 1990 ^{(Ref. 6)} recommends the inclusion of 30% of the imposed loads in SLS verification, SCI P354 and CCIP-016 recommend to either ignore them or apply no more than 10% of the nominal imposed loads, following the research conducted by Hicks et al.^{(Ref. 7)}

Overestimating the mass used in the modal analysis can result in non-conservative structural response.

AISC DG11

Recommendations are given in DG11 Section 3.3.

Expected day-to-day dead and live loads should be estimated and used. Normal mechanical and ceiling installations can be modeled by adding 4 psf to the structural system weight, although those values can be adjusted if needed.

DG11 Table 3.1 specifies live loads that can be used for vibration analysis based on the occupancy type.

Supported partition wall masses can be applied to slab shell nodes along the wall, as line masses assigned to zero-stiffness frame elements in the plane of the slab, or as uniform mass applied to the slab shells.

Damping ratio - SCI P354

Damping ratio - CCIP-016

Damping ratio - AISC DG11

When working to DG 11, to establish the damping ratio the designer sums contributions from each of the floor components separately.

Acceptable accelerations (Figure 2-1 of DG11)

This figure depicts three curves for different situations e.g. office floors and pedestrian bridges. In Tekla Structural Designer these have been named Curves A to C where,

• Curve A - offices and residences, quiet areas

• Curve B - indoor pedestrian bridges, shopping malls

• Curve C - outdoor pedestrian bridges

Frequency Response Function (FRF)

DG11 uses the Frequency Response Function (FRF) method to identify the worst frequency of the floor. This frequency is called the ‘dominant frequency’ and has an associated ‘magnitude’ as a percent of acceleration due to gravity, g.

FRF Frequency

• Resonant Response: FRF for frequencies below 9 Hz

• Transient Response: FRF for frequencies below 20 Hz

The peak accelerations at the dominant frequencies are then compared directly with limiting accelerations obtained from the selected curve in DG11 Figure 2-1 to determine the pass/fail status.