Relative Economy of Pre-Engineered Buildings

Over past two decades, popularity of so-called Pre-Engineered Buildings have increased over conventional truss structures in industrial and warehouse construction. Technically, Pre-Engineered Buildings are portal frames with moment transferring connections and are not new. In fact, portal frames had been less popular alterrnative to truss buildings from economy considerations.

The current popularity of portal frames is due to the following reasons:

1. 
   
   Introduction of High Grade High Carbon Steel at prices comparable to that of Mild Steel. Use of Grade 350MPa Steel in place of conventional Grade 250MPa results in slender structures achieving greates economy.
2. 
   
   Advent of Submersion Arc Welding that imparts built-up sections homogenity and strengths comparable to that of Hot Rolled Standard Sections. However, SAW is not still feasible on all joints of built-up sections.
3. 
   
   Popularity of Cold Rolled Light Guage Sections such as Zed and Lipped Channnels for Purlins. Cost of these sections have now come down and their lower thickness is more friendly to self-tapping screws of Galvalume sheet roofing.
4. 
   
   Galvalume sheet roofing has itself served as a viable allternative replacing disdvantages of conventional forms of roofing.
5. 
   
   Flatter slopes are possible with Portal frames, (as less as 1 in 10) since slopes are not required from structural considerations. Flatter roofs reduces sheeting areas and make appearance more aesthetic.
6. 
   
   Aggressive marketing efforts of PEB manufacturing companies.

It is generally seen that use of high grade steel is more economical in PEB construction than using conventional MS sections. The cost saving in using 350MPa Grade Steel reduces weight of structture by around 20% whereas cost differences are about 10%. It is advisable to check supply of MS grade structures whereas designs are made for higher grades.

However, slender structures are vulnerable to higher defflections than conventional structures. In most cases, PEB structures cannot meet the requirements of deflection control as per IS:800. Bracing using rods also fails to satisfy the slenderness ratio criterion of IS Codes. In practice, performance of rod braces are subject to the extent of  prestressing achieved by tightening the same. These are specifications that have  to be decided between the client and the vendor.

It is generally seen that strength per unit weigt (in terms of moment carrying capacity) of built up sections are sensitive to section parameters in decreasing order as  follows: Width of flange, Thickness of Flange, Depth of Section, Thickness of Web. Stiffness per unit weight (in terms of MI on major axis) are sensitive respectively as Depth of section, Flange Dimensions, Thickness of Web.