In last week’s issue, we tackled some of the basic aspects of steel structures which include properties of steel; supports; challenges with steel; among others.
This week we continue to look into the steel structures as we tackle other aspects of it. As we highlighted last week, a steel structure is usually used for constructions that require complicated framework systems and the ability to withstand tremendous forces. These constructions need extensive spans and flexible structures that can easily be assembled and disassembled.
Types of steel structures
We have different types of steel structures. Understanding the advantages and disadvantages of different structural systems is essential to design your project correctly.
There are four main steel frame structures namely: basic building frame structure; portal frame structure; truss structures and grid structures.
However, there are many other types and variations of frames that are used in more specialised or unique applications.
Basic steel building frame structures (beams and columns): used to construct high rise and super high-rise buildings, multi-storey buildings and commercial office buildings. Can be adjusted to suit your needs. It comprises beams and columns that can withstand vertical and horizontal loads. The columns, beams, bracing and other members are connected to form a flexible layout and create a larger space
Portal frame structure: is a structure with connected beams and columns. It’s a simple structure with simple construction and reasonable stress. Its widely used in industrial, commercial, agricultural and institutional buildings. Due to their simplistic design, they can be constructed quickly
Steel truss structure: comprises several rods hinged or fixed at both ends of each rod. Truss generally comprises top chord, bottom chord, vertical members, diagonal web and inter-truss support. It has lighter weight and more rigid. It is often used in roofs with large spans, bridges, TV towers, mast towers, industrial and civil buildings
Steel grid structures: comprises mainly rods connected to form a grid pattern that distributed stress across the structure, have small space stress, lightweight and high rigidity. It can be used for rain roofs, canopies e.g. service station canopies; exhibition halls; gymnasium and sports stadiums
Steel building frame, Portal frame structure, Steel truss structure
Other types of structures include
Arch structures eg arch bridges
Cable stayed bridges
Components of a steel structure.
A steel structure mainly includes the following components
- a) Concrete base and stub columns complete with holding down bolts
- b) Base plates and vertical columns
- c) Trusses
- d) Steel gusset plates on joints
- e) Horizontal beams — purlins
- f) Wind braces
These members are riveted, welded or bolted together
- g) Roof sheets and cladding
- h) Ridges and edge strips
The sheets and ridges are screwed onto framework using self-drilling screws and hook bolts
Design of steel structures
In Chapter 3 Section 1 and 4 of “The Model building by-laws”, every building or portion of a building shall be designed and constructed to sustain the most adverse combination of dead loads, superimposed loads and forces to which it is expected to be subjected to.
The design of the structure should make provision for the combination of loads and forces producing the most adverse conditions of induced stress, deflection and structural instability. The design and construction of structural steel work should satisfy the requirements of the by-laws in accordance with C.A.S. 157.
The foundations of the structures should be designed factoring in the top load. The foundations, anchorages or other fixings should provide a moment that exceed the overturning moment of the building or part of the building due to adverse combinations of dead and superimposed loads and forces by at least 50 percent.
When making structural designs its important to consider the following aspects
Span to be covered
Height / size of the building
Possibilities of natural disasters e.g. fire, earthquakes, cyclones, etc.
Weather and climate
Direction of prevailing wind
Cost of the materials
Availability of materials
Steel structure erection process
A steel structure needs a high level of expertise and equipment in order to erect it well. The erection process consists of:
Architectural drawings — shows the outlook of the finished building. This is done by architect; engineer or a competent person
Structural drawings — shows the layout of the structural components such as concrete works and steel structural framework. This is done by and engineer
Manufacturing / working drawings — usually done during the production process and it also includes the joining details. Can be done by engineer or draughtsman
Setting out — transference of the structure from paper to the ground. This can be done by competent artisans
Excavation — the pits for each column are dug to the required depths as advised by the engineer or as per soil tests results
Steel fixing of bases, starter bars and columns
Pouring of concrete for bases and stub columns with holding down bolts
Fabrication of columns and trusses
Erection of columns and trusses
Laying of purlins and bracing
Sheeting and ridging
Innocent Chatikobo is an engineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in structural engineering and construction. For your comments, views, questions and feedback he can be contacted on the following platforms. Cell: +263 777 950 224; +263 712 376 037; +263 782 502 732/Email: [email protected] /Facebook: AtroServe Engineering Zimbabwe