Lattice v Portal Frame Roof Structures Q5 - Estate Agent's Examination Part 1&2

Q.
(a) With the aid of labeled diagrams explain the following roof structure for large single-storey factory building:

i) Lattice roof structure.
ii) Portal frame roof structure. (15 marks)

(b) Discuss the functional requirements of the roof for a single-storey factory building. (10 marks)

(25 marks, 2015 Q5)

A.
Similar question was asked in 2012 Q6 here.

(a) (i) Lattice Roof Structure

Labeled lattice roof structure

Duo-pitch Truss

Long-span industrial buildings can be designed with lattice trusses, using channel, beam or tubular sections. Lattice trusses tend to be beam and column structures and are rarely used in portal frames. Various configurations of lattice trusses are illustrated. The two generic forms are W or N bracing arrangements. In this case, stability is generally provided by bracing rather than rigid frame action.

Using lattice structures, a comparatively high stiffness and load bearing resistance can be achieved while minimising material use. Besides the ability to create long spans, lattice structures are attractive and enable simple service integration.

Please refer to the website of Single Storey Industrial Building by SteelConstruction.Info - The free encyclopedia for UK steel construction information - for a complete information on Industrial Buildings.

(ii) Portal Frame Roof Structure

The vast majority of single storey, steel framed buildings are portal frames. These were first widely used in the 1960s. During the 1970s and early 1980s they developed rapidly to become the predominant form of single storey construction. Using plastic design techniques first developed at Cambridge University, for spans up to about 50m portal frames are the most economical solution available. Large column-free areas can be achieved at relatively low cost. Often on multi-span frames the intermediate valley columns are omitted (‘hit-and-miss’) so that on, say, a 45m span frame, with bay centres of 8m, each column-free ‘box’ covers an area of over 700m2, which is nearly a fifth of an acre!

Portal frames typically use hot-rolled beams and columns for the roof rafters and supporting columns, although cold formed sections may be adequate for some small span structures. Portal frames come in a variety of different shapes and sizes, with flat and pitched roofs. The schematic arrangement of a typical single storey portal frame building is shown.

Labeled portal frame roof structure

Portal frames are the most commonly used type of structural frame for industrial buildings because they are a highly cost-effective solution. They are widely used because they combine structural efficiency with functional application. Various configurations can be designed using the same structural concept as shown. Multi-bay frames can also be designed, as in (e) and (f), using either single or pairs of internal columns.

These simple types of structural systems can also be designed to be architecturally more appealing by using curved members, cellular or perforated beams etc. Innovative structural systems have also been developed in which portal frames are created by moment resisting connections using articulations and ties.
Ref:
http://www.steelconstruction.info/Single_storey_industrial_buildings#Types_of_portal_frame
http://www.steelconstruction.info/Single_storey_industrial_buildings#Lattice_structures

(b) functional requirements of the roof for a single-storey factory building