Structural Steel Design and Analysis

Structural Steel Design and Analysis

Structuralv Steel analysis is the process of calculating the forces, moments and deflections to which the members in a structure are to be subjected. There is a vast range of analysis tools offering speed, precision and economy of design; 3-D, FE modeling, bespoke portal frame, cellular beam or plate girder design software are now widely available. Modeling centenary actions, cold formed member performance or grillage analysis - all these are now commonplace for structures, where analysis hand is impossible. Increasingly sophisticated analysis methods continue to improve the accuracy with which the behavior of structures can be predicted.

Modeling the real world behavior of a structure is made easier by the use of full model generating software, with load generating tools enabling frame stability verification along with member checks. The design can be performed to British or European Standards.

This article explains the basics of common structural modeling approaches and describes the differences between various types of analysis. Emphasis is placed on the verification of models and analysis results to ensure a safe and economic structure is obtained at the end of the design process.

Example of splitting a frame into a series of 2D models

It is recognised that with the universal presence of computer analysis, an intuitive understanding becomes increasingly important, both in the creation of analysis models and critically, in the appraisal of the analysis results, such as the deflected shape, distribution of moments or distribution of reactions.

Numerical analysis of structures relies on the designer's understanding of structural behaviour, choice of appropriate software, method of analysis and above all the use of engineering judgement to know when the answers are reasonable.

An intuitive approach uses broader, more dynamic reasoning skills to evaluate the behaviour of any particular structure. The key principles involved in developing this kind of understanding of structural behaviour are:

To consider the deformed shape of a structure

To use statically determinate simple systems, so that good appreciation of the behaviour of the real structure with all its complexities can be gained.

It is helpful to use the graphic options of the software to review input data, such as loads, and output data, such as deflections and bending moments.

Most orthodox buildings are a series of repeating 2-D frames and often it is convenient to model in this way. Most steel sections are highly efficient in one primary direction and moment resisting connections to the minor axis can be difficult and expensive.

However, many multi-storey buildings are modelled in 3-D, as it is very effective to copy and repeat similar floors together with defined load patterns. 3-D modelling is also useful for analysing complex frames and for cataloguing member size, type, location, etc. within the whole building model.

Braced structures with nominally pinned connections, are most cost effective. Analysis can accommodate continuous design, but the connections are more expensive.

Analysing the Structure of Steel Design Analysis

1. Analyse and design the structure to resist gravity actions (self-weight, imposed actions, snow loads, etc). This structure comprises floors, often composite floor decks acting compositely or non-compositely with steel beams and columns. It is recommended to model:

(i) One typical floor first - ensuring common member sizes are used where possible to maximize standardization

(ii) Using this floor, replicate it within the building as many times as possible; design all floors and the columns for gravity combination of actions.

2. Analyse and design for lateral actions (arising from wind and initial imperfections etc.) and design the lateral load resisting system. This system can consist of one or more of the following:

Braced frames -

with bays containing diagonal braces or cross-bracing which resist the lateral loading in tension and/or compression Continuous frames with bays resisting lateral load due to frame action and moment-resisting connections between beams and columns Concrete shear walls which are typically planar elements or groups of planar elements (cores) which resist the lateral load in shear or shear and bending respectively.

Trusses and lattice girders

Fastrak gallery Hanger Iktisas 800x600 new.png

Model of a building with trusses

(Fastrak model courtesy of Trimble)

There are a variety of models which may be used for the analysis of a truss. These include:

Pin jointed frames

Continuous chords and pin jointed internal, i.e. web, members

Rigid frames.

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