Centre of lateral stiffness at centre of mass. Eccentric floor plates twist under seismic conditions, generating large additional deformations and forces.
Consider load path for seismic base shear and moment. Foundations will be subject to very high forces in seismic events.
13.1.3 Structural engineering
targets
Minimise weight of floor slab and framing. This will reduce column loads, increase the natural frequency, and reduce seismic demands. It should also reduce the embodied carbon and resource demand. Be aware of the economics of relevant industry; the viability of your project may depend on it. Maximise proportion of the building dead load carried by the stability system.
The stability system must resist overturning, generating moments or uplift forces.
Columns, walls and foundations are all simpler, stiffer and cheaper if they remain in compression at all times. Will also reduce the need for foundations resisting tension.
Size the stability system to achieve no tension in service. In vertical elements of the stability system, cracking has a very significant effect on the stiffness of a reinforced concrete element. If walls and columns do not see tensile stress in service, assume un-cracked section properties in the analysis. This reduces the need for tension capacity in foundations.
Overall deflection under wind load less than, for example, H/500. The aim here is to arrive at a design achieving the occupant comfort criterion, to limit p-delta effects and allow practical detailing of the cladding. H/500 will be superseded in later design stages by more specific checks for comfort and cladding movement.
Prefer no horizontal movement under vertical loading. Effects can be offset by presetting but this complicates the construction process.
Prefer differential shortening of vertical structure less than span/500. Where span
= distance between vertical elements. This is a serviceability criterion and is not widely codified. Could go to span /200, as long as the effects of this are followed through in building design and the specification of the following trades.
Prefer that wall compression reinforcement does not need containment links.
Applies only in areas of very low seismicity, and is code-dependant. At 2%
compression steel, BS 8110 requires the addition of links, and costs increase disproportionately.
Integrate wall layout with lifts, stairs and risers. Core efficiency can have a significant effect on the economic viability of the project.
Integrate outriggers with building function, such as plant floors.
Coordinate service routes with lintel beam depths and/or openings.
Design core walls as self-stable during construction. This will make the construction process simpler and safer and will reduce the build cost.
13 Structural design - concept
13.1.4 Analysis methods
Build an analysis model. Use your favourite analysis software. Some people work by hand at this stage. The model should be simple to build and amend, so that alternatives can be looked at quickly. It does, however, need to be detailed enough to pick up the significant structural effects.
Model core using linked stick elements or using 2D elements. The 1D element modelling method recommended by Cross in the book edited by Melchers and Hough represents each section of shear wall as a single stick element and links them with horizontal stiff arms and lintel beams. This produces accurate results with a small number of elements. It is straightforward to use the force and moment output in ultimate capacity checks and rebar design. Alternatively, programs such as ETabs and Strand allow representation of the walls using 2D elements.
Use sufficient elements to model behaviour when using 2D. Meshes need to be carefully refined at lintel beams, lintel beam connections and around builders’ work holes. 2D meshes that are too coarse can cause significant errors.
In 2D analysis be careful that linked freedoms do not limit the structural action of the lintel beam elements. Linking freedoms at floor levels will cause errors if lintel beams are modeled using 2D elements.
Model rotational flexibility of lintel beam connection to shear wall. Lintel span can be increased by d/2 each end or effective EI can be reduced.
Floor diaphragm action can often be modelled by linking freedoms in the model.
A rigid link in the xy plane at each floor level is a convenient way to simplify the model. Care is required, however, to ensure this does not mask important structural actions. Only use if in-plane effects are not significant. Be careful of using it with outriggers or mega bracing.
In stick element modelling, consider neglecting minor axis and element torsion effects. If minor axis and torsion effects are not required for the functioning of the stability system, consider setting I and J values for the core wall to zero so that they are not considered in the analysis.
Use appropriate E values for duration of loading. Short-term E can be used for wind loading; long-term E is required for dead and live load analysis.
Use cracked-section-properties analysis. Cracking has a marked effect on the distribution of forces in the structure and overall deflection of the building. It is probably sufficient at this stage, however, to use I(cracked) = I (un-cracked)/2.
Sections without tensile stress in service will be un-cracked. Ideally this will include all columns and walls. Sections subject to tensile stresses will be cracked, probably including all lintel beams and other sections.
Use wind load from code, with appropriate dynamic augmentation. If doubt exists, consult a competent specialist.
Include notional horizontal load in accordance with the code you are using.
Use live-load reduction in accordance with the code you are using.
P delta analysis may be required. The response of the stability system to horizontal load may well be amplified by the combined influence of the gravity loads and the deflection of the system. Evaluate this effect and use P-delta analysis if it is significant.
Structural design - concept 13
Additionally, in zones of low, medium or high seismicity:
Extend your static analysis model. Extend your static analysis model for use in linear response-spectrum analysis. The model will need to represent all the building mass, its distribution in plan over each floor plate and the columns supporting it.
Carry out a linear response-spectrum analysis to EC8 or IBC. This will offer a first approximation to the seismic deflections and forces.
Analyse sufficient modes to mobilise at least 90% of the building mass in x, y and z directions. Analysis package should enable scaling of the modal dynamic results to the appropriate response spectrum for the site.
Use CQC method. Combine the effects of individual modes into the total response for each direction.
Combine results from x, y and z responses. In accordance with code; the SRSS method is recommended.
Choose q or R appropriately. Take expert advice if doubt exists about the value of q or R to use. This will have a very significant effect on the design because of the direct effect on section strength demands.
Add ‘accidental torsion’. In accordance with code.
Confirm all sections can be designed with sufficient ductility. Use rules from the selected design code.
Use over-strength factor. In calculation of strength demand for non-ductile elements such as collectors, diaphragms and foundations.
13 Structural design - scheme
13.2 Scheme design
13.2.1 Objectives
Finalise dimensions and setting-out of structural system. Position and size of all vertical structure, elements of the stability system, and slab edges should be agreed with the design team and fixed. Detailed coordination with other disciplines. Finalise the interface between following trades and the stability system.
Produce scheme-stage drawings. A report describing the system and how interfaces with the building design is often useful at this stage as well.
Produce estimates of concrete and rebar quantities. Usually required for cost-checking.
13.2.2 Essentials
Determine whether wind-tunnel testing is required. Check the limits of application of your wind code. The limits in BS EN 1992-1.4 are a useful guide:
z H < 200m
z H/d < 5; where d is the minimum horizontal dimension perpendicular to the wind.
z Plan form is rectangular z Building is prismatic.
If your building lies outside these limits, wind tunnel testing will probably be required.
Seek the advice of a competent specialist.
Determine whether Non-Linear Time History Analysis is required.
CTBUH 2008 Recommendations for the Seismic Design of High-rise Buildings, sets out the consensus view of the world’s most experienced designers of high-rise structures in seismic zones. It recommends non-linear time history analysis (NLTHA) is required when:
z H > 50m
z Seismic hazard is moderate or high.
In zones of low, medium or high seismicity:
All structure and cladding to remain elastic during a seismic event with an average return period of 50 years. Elastic response spectrum analysis can be used.
See appendix B of CTBUH recommendations (2008).
No structural collapse during a seismic event with an average return period of 2,500 years. Non-linear time history response analysis will probably be required.
Refer to CTBUH recommendations (2008). Consult a competent specialist if lacking the capability for this type of analysis.
In zones of medium to high seismicity:
In seismic analysis, deformation is the critical parameter. Collapse is prevented by the structure’s ability to accept a sufficient extent of inelastic deformation.