CROSS Safety Report
Design of tall asymmetric structures
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Overview
A reporter touches on the topic of analysis and design of the structural systems for tall buildings which are asymmetric and may be irregular in plan.
Key Learning Outcomes
For civil and structural design engineers:
- It is good practice to check and validate all design outputs from proprietary design software
- If you are concerned with any of the output’s, consider raising this with the software technical support team and seeking clarification
- The importance of validating software is noted in the Institution of Civil Engineers Civil Engineering Journal August 2013 - The importance of understanding computer analysis in civil engineering
Full Report
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A reporter has touched on the topic of analysis and design of the structural systems for tall buildings which are asymmetric and may be irregular in plan. These might have a concrete core and an external steel frame with floors spanning between core and frame. Tall buildings which are complex in plan raise interesting issues concerning the methods of analysis that should be used, and there is conjecture that a first order linear analysis may not represent the structure adequately. It has been argued that non-linear geometry effects should be included so as to provide a better representation of the behaviour of the structures.
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The problems of validation of analysis models; that is the consideration of whether the model is capable of properly representing the real structure was the subject of the study ‘The use of computers for engineering calculations’ published by the Institution of Structural Engineers in a 2002 report prompted by a recommendation from the tenth biennial SCOSS report. The guidelines advocate a logical and disciplined approach to computer assisted engineering based around a process where each stage has built-in checks. The questions to be expressed and answered are:
Is the model satisfactory in its representation of structural behaviour?
Is the software and the way it is used appropriate and suitable?
Are the results correct?
Analysis software must be used within the limitations of its applicability. It is all too easy to believe that because a structure has been computer modelled the output is therefore accurate. As buildings become taller and more complicated, issues customarily ignored in smaller structures start to become critical. In tall buildings, for example, differential elastic shortening of vertical supports can cause significant redistribution of horizontal member moments. Moreover non linear effects such as those due to elastic sidesway from non uniform vertical loading start to become important as do P-D effects. It is vital to be able to check whether the software has a built in ability to consider both global and member buckling capacities.
It is vital to be able to check whether the software has a built in ability to consider both global and member buckling capacities.
Non-standard structures
Asymmetry in a structure may exacerbate such effects under both vertical and horizontal loading but whether these could lead to potential buckling problems would depend upon the example being studied. In designing a non-standard structure, the process should start by ensuring that all possible aspects of behaviour can be represented by the model until there is confidence that they can be ignored. Some of the validation issues can be resolved by carrying out sensitivity analysis e.g. making runs with and without non-linear geometry and comparing the results.
Aspects not to be ignored are the horizontal and vertical elastic changes that occur during construction (perhaps exacerbated by asymmetry) which will have an effect on the stress distribution and stability in the completed structure i.e. dependent on when separate parts are rigidly linked up. Thus, the modelled stress distribution in the final structure is influenced by the sequence and manner of phased construction.
Robustness of structures
There is now an international consensus that ‘robustness’ ought to be a consideration in all structural designs yet there is no consensus about what ought to be done for tall, novel structures (Class 3 structures in UK terminology). The Eurocode rules for lateral load envisage some proportion of this load is notional (as in UK practice) but some lateral load will arise due to out of plumb in the erection of the columns.
In tall asymmetric structures this ought to be a consideration, but the effects may not be apparent from the model representation of the structure. Codes that advise on robustness all draw attention to the special considerations that must be given to transfer structures. The Institution of Structural Engineers report on robustness: “Practical guide to structural robustness and disproportionate collapse in buildings” published in October 2010, whilst not dealing with Class 3 structures, makes the following very important point:
‘A key presumption is that for any one building, there should be one engineer in overall charge of both stability and robustness and not least when multiple structural disciplines are involved as in hybrid structures.’
Mandatory checks
In view of the complexities involved the question can also be raised as to whether buildings like these should have a mandatory check and, separately, a review, as part of Regulatory Control procedures. As mentioned in the comments on report 235 should there be a process for checking on the competency of organisations and individuals to be responsible for such designs in addition to general safety legislation in the UK?
The validation of software, and its proper use, is a matter that needs to be addressed both in practice and in education.
CROSS would be interested in receiving other views on this important topic.