Metal buildings face constant stress from wind forces including torsion, shear, compression, and lift. To counteract this pressure, steel buildings utilize different forms of bracing in the roof and walls to transfer the loads.
For most steel buildings under 60' wide, the diaphragm action provides all the bracing that is needed, but when high winds, heavy snow loads or even a large number of openings creates extra stress, a steel building may require one of the following types of additional reinforcement: X bracing employs steel rods or cables in the shape of an "X" to connect various parts of the frame tightly, weak axis bending increases the size of the main-frame base plates to prevent columns from twisting under heavy stress, and a wind column is an additional vertical member which reinforces a column.
Diaphragm BracingDiaphragm bracing utilizes the diaphragm resistance of the wall panels to transmit lateral wind or seismic forces to the foundation. Diaphragm action utilizes undisturbed sheeting, floor to roofline, and assumes all wall panels are installed correctly.
Cable or Rod Bracing
When diaphragm bracing is inadequate or not allowed (ie: in high windload situations), the first alternative is to use cable or rod bracing between columns in the form of a letter x. Sometimes referred to as "x-bracing" or "cable-bracing" or "rod-bracing", this method consists of sets of cables or rods in the roof (between the rafters) and in the walls (between the columns). The quantity of locations to be braced is determined by the design criteria.
Fixed Base or Wind Column BracingA "fixed base" or "wind column" is an additional vertical member, which reinforces a column, that attaches to the foundation. This form of bracing is commonly used when x-bracing causes an interference with framed openings in a particular bay. Wind columns are an economical substitute for standard x-bracing but do require modifications to the foundation design.
Portal Frame or Wind Bent BracingA portal frame is a I-shaped sub-frame consisting of two columns and a rafter placed between the two adjacent mainframe columns in a bay. Because of their high cost, portal frames are usually used only after considering the other forms of bracing. When used in a bay with a commercial rollup door, a portal frame will require an exception to the general rule of requiring 2' above a rollup door to store the door when it is up. Because of the height of the portal rafter, 3' might be needed above the rollup door, requiring the height of the building to be increased.
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