Steel Sheds The steel sheds is to give an intro into the the materials used in all steel buildings and the common common names of some of the members used. It will also lead on to other pages which go into more detail on specific items.
Sketch of a medium sized steel framed building showing the main components.
Portal Frame, Portal Arch Or Just Portal. Portal frames are the basic item in all steel sheds of a certain size. You will see them, maybe just one in a small garden shed, or huge things weighing in at many tonnes in industrial buildings.
Steel sheds - A detail of a warehouse type building to illustrate some of the terms used.
1. Portal column, Universal column "I" section 2. Portal rafter, Universal beam "I" section 3. Strut tie, pipe section 4. Vertical wall bracing. equal angle section 5. roof plane bracing. equal angle section 6. "Z" section wall girt 7. Cleat for girt. MS flat 8. Overlap joint of two "Z" girts 9. Outrigger supporting last fascia fascia purlin, MS angle. 10. Insulation blanket, double sided foil, supported on wire safety safe ty mesh. 11. High tensile structural bolts connecting all members. Apologies once again for the crap photo, but I have to keep the file size down for people with slower connections. if you could see this shot in high resolution, you would be able to see that all the bolts have a marking pen cross on them. What happens is the structural engineer will specify the amount that the structural bolts shall be tightened to. When the steelwork is erected, a guy goes around with a torque wrench and tightens them up to the correct setting, and then marks them off to show that it has been done.
Footings Footing are an integral part of the whole structure. What I have drawn here are cylindrical pier footings. In this case the footings are drilled by machine and filled with concrete, with hold down bolts set into the wet concrete. The main structural steelwork would then be erected, and the floor slab poured. With the slab poured than the rest of the building is finished with a good surface to work on. There are many different ways of providing support and hold down for steel work, so I will go into it in more depth. Check out the menu on the right.
Braces
Steel Sheds -Wall brace consisting on a mild steel flat with a welded on turn-buckle for tensioning (and plumbing the portals). • • •
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The portals keep the building rigid in a direction parallel to the portals. To keep the building rigid in the direction at right angles to the portals we add braces. The braces I that I have drawn in the sketch above are stiff members that can work in compression and tension. This is normal in larger type steel sheds. They would also have a rigid connection to each other where they cross. Typically called cross braces. Other designs may have single ones at one end of the building and single ones at the other going in the opposite direction. On lighter construction the bracing could take the form of flat straps screwed to the tops of the purlins (speed brace etc.). Rods or straps tensioned with turn-buckles. Straps and rods only work in tension. Bracing is a basic element of modern construction and is essential in all buildings, not just the steel framed buildings we are t we are talking about here.
Again there is a fair bit to be said about braces, you wouldn't believe how many different types there are, so I will go into them in more detail. Check out the menu on the right for updates to this section.
Steel sheds - Lightweight kit t ype prefabricated shed using double "C" section steel purlins as the portal members. In this case the roof surface is braced with flat straps fixed to the underside of the steel purlins. The designer in this case has added a collar tie across the top of the portal rafters.
Looking at the sketch above you will see that the end walls have extra columns, to carry the wall girts and to frame around the door. This means because of the extra support then the end frames f rames are not true portals, spanning the whole width, and as such they are carrying less load and many times can be made lighter. The photo above shows this, the end frames consist of single "C" purlins. while the main portal frames are made from double purlins. For this reason the designer has chosen to place the roof bracing between the true portals (the second bay from the end), that are carrying more load.
Strut Ties Strut ties join/separate the portal frames at the top. As the name suggests they act as a s struts when under compression loads and as ties when under tension. On small residential jobs they are quite often site welded into position, and as the jobs get larger they use bolted connections.
Steel Purlins And Girts
Steel sheds - Sketch showing the layout of the steel purlins on the roof, and steel girts to the side wall. The girts to the end walls and braces etc not shown for clarity.
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Purlins are horizontal members fixed to the frame that support the roof sheeting. Girts are horizontal members that are fixed to the vertical faces of the frames to support the The bridging pieces shown here are at mid span. They stiffen the purlins and stop them twisting out of shape under load. The purlin and girts are made out of the same standard cold rolled sections, usually 'C" or "Z" sections. The shapes are the same, the names change depending on whether they are on the roof or the walls. There are quite a few points to think about regarding purlins and girts, so once again I have done a separate page on these brilliant items. You would think that the humble bridging pieces would only take a paragraph to describe. Not so, you should know me by now. There are a few options, not the least is making your own, or doing without them entirely. Check out the menu on the right for my steel bridging page here.
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Portal Frame, Portal Arch Or Just J ust Portal.
Sketch of a laminated timber portal arch. •
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Porta is word from Latin that basically means door, which has spread to many modern languages. It was used to refer to a gate in a Roman City. From this is derived the word Portal which in Architecture can be defined as "an impressive or monumental entrance". When I was a young journeyman I used to work for a company that made large laminated timber portal frames for churches, which were called portal arches. Another definition of portal is the structural engineer's version, which is what we are interested in here. "A structural framework consisting of a beam be am supported by two columns to which it is connected with sufficient rigidity to hold unchanged the original angles between the intersecting members". The words to take note of here are, sufficient rigidity. rigidity. To stop the building leaning over or racking in the width, the columns and the beams have to be sufficiently strong enough in themselves to resist sideways forces, and the connecting joints holding them together should also be able to resist those forces. One other thing is needed for a portal frame to be stable along it's length. It has to be fixed down rigidly. That is attached solidly to it's foundations. In the photo below, the end frames have additional columns added to them to support the outside wall members. These extra columns make the end e nd frames not strictly portal frames as the columns help to share the load, so depending on circumstances and cost the engineer might decide to make these lighter than the other true portals that are spanning the full width.
Industrial Type Portal Frames
An industrial building under construction. The frames shown here are made out of heav y sections, workshop fabricated and grit blasted and coated with zinc silicate. A detail of this building is on the steel sheds intro page
Light Weight, Kit Type, Portal Frames
A light weight frame detail using a stiffener at the column/rafter connection know as a knee brace. Another detail of this building i s on the steel sheds intro page, showing the top/collar tie connection.
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Portal column consisting of of two "C" section purlins bolted together The joints have a 10mm MS steel plate sandwiched between the "C" "C" sections Portal rafter consisting of two "C" section purlins bolted together Fly brace, flat galv. strap bolted to to rafter and roof purlins. Stiffens the rafter Knee brace consisting of two "C" section purlins bolted together "Z" section purlin bolted direct (no cleat)through the the bottom flange to the the portal rafter rafter "C" section girt bolted direct (no cleat)through the flange to the the portal column column
Manufacturers seem to be tying to outdo each other by bringing out cheaper and cheaper version of this type of this construction. Used mainly for fo r rural storage sheds and cheap garages, these kits have almost no welded parts in there fabrication. The only parts that are welded in the above shed are the column / baseplate joint. The purlin to rafter bolted connection, without the use of a cleat welded to the rafter is not a recommended way of doing it.
A Simple RHS Type Portal Framed Carport
A simple portal frame carport.
Just about as basic as you can get, a simple carport consisting of two 125 x 75 x 5.0 RHS portal frames. The site welded joints at the column/rafter connections have the same RHS size gussets to stiffen the connection. Strut ties out of the same RHS material give stiffness the other way.
