The Concept of Complex Structures

In this guest post, Phalguni Kar talks about identifying complex structures, the responsibility of structural engineers, and some examples of simple structures modified to complex structures.

Identifying Complex Structures

In structural analysis and design, it is always debatable whether a structure is complex or not. Sometimes it is crooked configuration, sometimes it is loading pattern, and sometimes it is selection of the structural configuration due to functional requirement which make the analysis and design comparatively complex. Structural engineers always try to conceive a simple and symmetrical configuration for the project but due to functional and/or aesthetic requirements, simplicity and elegance cannot always be achieved.

Certain structures are basically complex in nature and, in my opinion, the first attempt of analysis and design of these structures by any structural engineer needs specialist guidance. The following structures fall under the category of specialised structures involving complex analysis and design and need specialist guidance:

1)    Analysis and design of special (long span, skewed, curved etc.) bridge sub-structures and prestressed and non-prestressed super-structures.

2)    Analysis and design of foundation and super-structure of tall buildings (50 stories and above) and tall and slender structures.

3)    Analysis and design of foundations and structures supporting heavy rotating or reciprocating equipment.

4)    Analysis and design of anchored and non-anchored sheet piling structures and reinforced earth retaining structures.

5)    Analysis and design of modular structures during handling, load out and shipment.

6)    Analysis and design of dock structures, water-front structures, cargo berth or jetty structures.

7)    Analysis and design of offshore oil and gas production structures.

 

Offshore Complex Structures

Offshore Complex Structures

Apart from above, the analysis and design of routine foundation and structures for dwelling houses, pipe racks, conveyer galleries, electrical transmission line towers etc. normally don’t need specialist guidance except for specific needs due to functional requirements and difficult founding conditions. Analysis and design of high roofed buildings (eaves height > 30m without any intermediate floors), with or without crane gantries, and housing electrical or electro-mechanical equipment need special attention or specialist guidance.

Responsibility of Structural Engineers

Like every professional, structural engineers have important responsibilities. We, structural engineers, should always remember that we are supporting all other engineering professionals involved in a project and our priority should be selecting structures to fit the functions the structure is being designed for. Functional design.

In short, structural engineers should remember that the system is being built for safely enclosing or supporting people or equipment. This can include conveying system like pipes for liquids and gases or conveyor belt system for bulk materials or structures that are required for public health / environmental projects such as stacks, chimneys, water & effluent and sewage treatment plants.

With the above logic in mind, structural engineers should also ensure that the functional requirements of the structures are adequately maintained during structural analysis and design as per data provided by functional specialists and technical authorities. Any assumptions or decision on functions must be approved by the responsible technical authorities. Any changes required in functional design for improving structural configuration must be discussed with technical / functional authorities and approved before being implemented into the design. In many sectors, a full validation and verification process is required.

For smooth structural engineering, some preliminary knowledge of the functional concept is helpful for the structural engineers but they should remember that structures are meant for a particular function and that the structural design should not jeopardise the functional requirements.

Structural engineers should remember that engineering is not only about running analytical models in a structural program or preparing calculation using excel / Mathcad routine templates. It involves solid technical decisions acceptable to project and functional authorities. These often require difficult trade-offs to be made that are best informed by experience.

Bridge - Complex Structure

Bridge – Complex Structure

Examples of Simple Structural Configuration Modified to Complex

I would like to provide two examples where a normal structural configuration changed due to the functional requirements and the structural analysis and design became complex.

1.)   In a two tier steel pipe rack, the top and bottom longitudinal beams were tied by diagonal members to take advantage of truss effect as the span of the long beams was around 16 metres.

Just before sending the structure for fabrication,  piping engineer requested us to remove one diagonal member. This was because in one bay a large diameter pipe needed to have a nozzle protrude above the lower tier longitudinal beam.

Now the simple truss problem did not remain simple. We had to strengthen the top and bottom tier members and adjacent verticals at the specific bay and this fulfilled the functional requirement.

2.)   One pipe rack foundation was constructed before an adjacent small equipment foundation details were available at site.

Once the details of the small equipment foundation were available, it was found that the foundation of the equipment was interfering with the foundation slab of the pipe rack. Now analysis and design of both pipe rack foundation and equipment foundation became complex.

We revised the depth of the foundation of the equipment and redesigned it to rest on the pipe rack foundation. A slight adjustment of equipment location was required; this was done with the agreement of functional specialist.

Sometimes, due to the existence of a large diameter underground pipe line at the structural column foundation location, analysis and design of the structure become complex. Such problems are very common while undertaking extension or revamp projects within existing or running plants.

Structural engineering in industrial projects is more dependent on other engineering functions. Sometimes, other engineering disciplines gets affected due to variations and this effects the structural engineering in a big way. To undertake such structural modifications, experienced and specialist structural engineering guidance is absolutely necessary.

Two major issues normally are involved in such modifications:

  • modifications in the layout of structures and buildings,
  • modifications to equipment and the surrounding functional layout.

In both of these types of modifications involvement of multidisciplinary engineering activities are required and the structural engineer is normally required to release documents before finalisation of modified scheme /layout and unit plot plans. It is advisable that the structural engineer should wait for the approved modified layout and design data before releasing documents for construction.

Working out structural arrangement with minimum wastage normally depends upon experience and competence of specialist engineers of the different disciplines involved in the project. Timely flows of data to the structural discipline reduce engineering time and help to avoid complexity in the structural engineering analysis and design. It is desirable to involve engineers of different disciplines with adequate experience and competence to work in process modification scenarios.

 

Phalguni KarThis article is contributed by Phalguni Kar, a freelance professional engineer who provides consultation services on conceptual civil and structural engineering design. He obtained his bachelor’s degree in Civil Engineering at Indian Institute of Technology in year 1972.
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