PORTAL FRAME BUILDINGS AND SHEDS
1. THE PORTAL FRAME
The portal frames are the main structural elements of the builiding, being so to speak its “skeleton”. They consist of (i) Grade 350 steel I-section columns and rafters, and (ii) the column foundations. The frame joints at B,C and D are shop welded and site bolted, and are designed and fabricated to achieve a rigid structural unit.
The frames are designed for the following loads:-
Roof loads such as workmen, snow or hail
Roof loads can be positive as on AB, or negative (i.e. suction) as on BC, CD and DE.
2. THE IMPORTANCE OF PROPERLY DESIGNED JOINTS
If the joints at B, C and D are not rigid, they will “open up”, and the frame will be unstable when subjected to loads (“pack of cards effect”)
3. THE IMPORTANCE OF PROPER FOUNDATIONS
3.1 Vertical loading on the frame results in A and E tending to be pushed outwards. If the foundation cannot resist this horizontal push, outward movement will occur, and the frame will lose structural strength. (The portal frame is similar to an arch, and movement of the foundations of the frame weakens it, just as an arch is weakened if movements occur.)
3.2 Wind subjects the portal frame to uplift forces (the roof tends to “fly-off) like an aeroplane wing), to overturning forces on the sides and ends of the building, and also to “drag” forces on the roof and sides of the building.
These destabilizing forces are resisted essentially by the weight of the building, and in this regard, the foundations contribute significantly to this weight. Generally speaking it is a fact that portal frame buildings of this kind are light weight structures, and as such they tend to collapse “sideward” and “upwards” rather than downwards”. The effect of wind on a light building cannot be overemphasized. The destabilization it causes is a major design consideration, and in this context, foundations can be regarded as the building’s “anchors”.
4. THE IMPORTANCE OF BRACING
4.1 ROOF BRACING
4.1.1 Buckling of Rafters
The rafter of the portal frame is a slender structural element, and unless it is restrained it will buckle when loaded.
In a braced roof this restraint is provided by the purlins acting together with a braced bay. The purlins provide the restraining force for the rafters, and the braced bay acts as a “buttress” which absorbs these purlin restraining forces.
While this system is effective in restraining the top flange of the rafter I-beam, the bottom flange remains relatively unrestrained, and to achieve the requisite restraint, short lengths of angle iron are connected at intervals between the bottom flange of the I-beam and the purlins as shown. This simple and necessary anti-buckling feature is sometimes neglected in the design of portal frames.
4.2 SIDE BRACING
A building subjected to wind forces along its length will tend to collapse as shown a above, while a building with a braced side bay as shown below will be stable, since the braced bay will function as a “buttres” to resist the wind forces, and transfer them to the foundations.