Adequate strength prevents excessive deformation. However, an excess of strength will lead to excessive forces in the structure.
Consider including gravity framing. This can stiffen the building significantly and can be advantageous to include in the NLTHR analysis.
Brittle sections must remain elastic. Elements without deformation capacity beyond yield are not permitted to experience inelastic deformation
Deformation demand calculated in the analysis must be less than the permissible value for that detail in ASCE 7-10 (2010). For every structural element. If this condition is met, the collapse prevention requirement has been met.
13.2.3 Structural
engineering targets
Ensure all sections can be reinforced economically. Check all elements, particularly outriggers framing into core walls, shear in lintel beams, anchorage where beams span perpendicular to a thin wall, areas of tensile stress in core walls, and clashes where several beams frame into the same piece of concrete. Storey drift under wind load coordinated with the cladding design. The relative movement of one storey relative to the next will be very important in the design of the cladding system. Limit used in design should be agreed with the cladding supplier.
Values in the range of h/500 to h/200 have all been used successfully in the past and h/300 probably strikes a good balance, while h/400 is easier for cladding suppliers to deal with.
Lateral acceleration under wind load is acceptable to occupants. A dynamic consideration calculated in the wind-tunnel laboratory: it is influenced by the wind climate, building geometry, building mass distribution and stiffness, the geometry of the surroundings and the available damping.
13.2.4 Scheme stage
analysis methods
The scheme-stage analysis model should include sufficient detail to allow the design of every significant structural element. There are particular requirements if wind-tunnel testing and/or non-linear time
13.2.4.1 Analysis if wind-tunnel testing is not required
Analysis methods are as described in the section on concept design but with a greater level of precision and more thorough coordination with the design team.
It is likely that lateral accelerations will not cause significant discomfort or negative comment from building users.
Either limit overall deflection to H/500 or check the lateral accelerations, as predicted by the National Building Code of Canada, are within acceptable limits.
13 Structural design - scheme
13.2.4.2 Analysis if wind-tunnel testing is required
Carry out a dynamic analysis for wind response.
First, define:
The analysis origin. In 3D, at foundation level, as near as possible to the shear centre of the building.
The analysis axes. To coincide with directions of movement in first two modes, if possible.
Things to remember:
Model rotational moment of inertia of floor plates. In order to model torsional effects.
Ensure first two modes are translational. Rather than torsional.
Probably won’t need pdelta analysis. Lambda crit for the service state should be well above 10.
Then, use a modal dynamic analysis to prepare this data to send to the wind-tunnel laboratory:
Modal frequencies. For first three modes.
Mode shapes. For first three modes.
Floor-by-floor masses and positions. Of centre of each floor mass, in 3D.
13.2.4.3 Wind tunnel testing
Take advice from a competent specialist, who is likely to recommend either a High Frequency Force Balance (HFFB) test or a Simultaneous Pressure Integration test. The process will be in two parts: model testing and post-processing. The post-processing stage can be re-run without re-testing.
For the testing stage, the wind-tunnel laboratory needs 3D external geometry of the building and the surroundings and the analysis origin and axes defined relative to the 3D geometry data.
For the post-processing stage, the laboratory needs:
Results from your dynamic analysis.
Structural damping coefficient. The amount of reliable data on actual damping in buildings is limited. Traditionally, engineers have used damping values of up to 3 %.
Recent research into the behaviour of completed buildings suggests, however, that this may be an over-estimate. Further information can be found in CTBUH 2008, the paper by Smith, Merello and Willford, EN1991-1-4, and other research data.
Range of frequencies and mass to use for sensitivity analyses. Use, for example, +/- 20 % to account for difference between calculations and real building behaviour.
Level of highest occupied floor for analysis.
Building use. Office or residential?
Acceptance criteria. Use ISO 10137 (2007), as well as the appropriate national code.
Structural design - scheme 13
The wind-tunnel laboratory should give you
Prediction of horizontal accelerations. At upper occupied floor.
Acceptability of accelerations. Relative to criteria.
Floor by-floor-loads. To use as wind loading cases in detailed analysis.
13.2.4.4 Analysis, if non-linear time history is not required
If H < 50m: Design to EC8 or IBC using the elastic response spectrum analysis.
If H >50m but seismic hazard is low: Elastic response-spectrum analysis can be used. Appendix B of CTBUH 2008 sets out recommendations for the analysis:
Seismic hazard based on 2,500-year return period.
Response spectrum based on 2% damping.
Maximum demand to capacity ratio = 2. Effectively equivalent to an assumption of q, or R, =2.
Ductile detailing required when demand to capacity >1. Structural components with strength demand to capacity ratios >1.0 should be detailed as components of an intermediate framing system to ASCE 7-10. When strength demand is less than capacity, specific seismic detailing is not required.
Foundations designed for elastic demand. Or the maximum base moment and shear that the structure can deliver to the foundation, accounting for all possible sources of reserve strength.
13.2.4.5 Analysis if non-linear time history is required
If H>50 m and seismic hazard is moderate or high: Take advice from a competent specialist: There are two stages to the procedure: No damage in 50-year event, no collapse in 2,500-year event. A code-based analysis may be required in order to comply with local regulations, as well as to carry out the ‘performance based design’ procedure described below.
Elastic response spectrum analysis for 50-year return-period event:
Model similar to that required for wind analysis.
Seismic hazard based on 50-year. This will, in fact, vary with the jurisdiction and building importance.
Response-spectrum based on 2% damping. See CTBUH 2008 appendix A.
Accidental torsion need not be included.
All structure to remain elastic.
13 Structural design - scheme
Non-linear time history response analysis for 2,500-year return-period event:
Seismic hazard based on 2,500-year return period. This will, in fact, vary with the jurisdiction and building importance.
Select earthquake histories. In accordance with ASCE 7-10 (2010) and the seismic hazard assessment for the site.
Initial stiffness should take account for cracking up to the point of yield. ASCE 41-06 (2007) Supplement 1 provides suitable guidance.
Damping prior to onset of yielding should not be greater than 2 %. See CTBUH 2008, appendix A. Energy absorption after the onset of yielding will be explicitly modelled in the NLTHR analysis.
Post-yield force-displacement relationships based on ASCE 41-06 (2007). Or other industry standard relationships, where they exist, and where they are appropriate.
Floor diaphragms. Modelling must allow for calculation of diaphragm and collector forces and in-plane forces from transfer structures and outriggers.
Second order and p-delta effects will be required. This analysis needs to account for the effects of significant deflections.
Accidental torsion need not be included.
Number of analyses. Required. Will depend on the choice of ground motion histories.
Output peak racking deformations and floor accelerations. These are used for assessment of non-structural components. Racking deformations are often more meaningful than storey drifts.