Cable Bracing for Steel Buildings
40 x 60 Steel Building
Metal Buildings Need Bracing
Most metal buildings require cable braces (X braces), or steel rod bracing, or some kind of X bracing. This is often because the capacity of the light gauge steel panels in shear is insufficient to transfer wind and seismic loads to the foundation.
Longer buildings can have sufficient shear capacity if properly designed. Buildings with many openings (e.g. equipment storage buildings) will require bracing.
Kinds of Bracing for Steel Buildings
A pre-engineered metal building (PEMB) will be drop-shipped with bracing included. Inventory your shipping statement. The cables will be itemized there. The most popular cables are aircraft cable (also called 7x19 wire rope.) These cables have a very high tensile capacity and are easy to install. Woven wire cables like these must be galvanized (GALV) or stainless steel (SST) material.
The second most common brace material in walls is round bar. In large buildings, half-inch to three-quarter-inch bars are not uncommon. The taller the eave of the building, the greater the magnification of loads into the cable, and the larger diameter required.
Steel Angles Brace against Heavy Loads
Angle Iron Bracing
The least common section used in bracing steel buildings is angle iron. Angle iron is hot-rolled to form a 90-degree bend. Because of the cross section, it is called an "L". For example, a common section is an L3x3x¼ (say "L three by three by a quarter"). Each leg is 3" and the thickness is one-quarter inch. L sections are used to brace against very heavy design loads.
Many large L section X brace seismic retrofits are visible in the buildings of San Francisco. Dine in a brick restaurant down by the piers and you will see these X braces.
Buildings with regular (not occasional) human occupancy and structures with important use or accessory to an important building (like a hospital or fire station) will have magnified design loads. High seismic areas like San Francisco also boast heavy sections to resist seismic forces entering the structure through the ground movement. I have seen brick walls braced with double L8x8x½. An advantage of heavy bracing like this is that it resists loads in tension, and in compression. This is also a requirement of the International Building Code and the California Building Code.
These are the three main types of wall cross-bracing materials.
Steel Building Wall Brace
Roof Bracing for Metal Buildings
Roof bracing can be formed with cables or rods, as described for walls, above. Often, the designer will specify the same size in roof and walls, if there is not a large difference in material cost. The savings for bulk purchases often overcome the difference in cost by size. The result is a slightly higher factor of safety in the building.
Occasionally, in roof bracing, a flat bar will replace other sections. This is typically to prevent landing places for birds and to keep the roof flat.
For milking parlors or grade AA facilities, designs must prevent birds from nesting or otherwise having position to deposit waste on surfaces and animals that need to be clean for milking or laying. Flat cable braces lay on top of roof purlins. The metal sheeting fits snugly on top of the purlins. A round bar would create some forming of the corrugated or ribbed panels. Flat bar does not exhibit this problem.
When occasional bird droppings are not a major concern, cable braces are easily installed between the webs of wide flange W beams (popularly called I-beams). Hillside washers provide easy connection to looped and crimped cables.
Shear Capacity of R-panel and Other Light Gauge Steel Sheathing
A long wall without penetrations for doors or permanent openings provides about 135 pounds per foot (plf) of shear capacity. This requires #14 screws affixed at 6" on sheet edges and sheet overlaps, and 12" on center at purlins and girts in the field of the panels. For 135 plf capacity, girts should be 5' o.c. or better. Spacing girts farther apart reduces the shear capacity.
Many panel types provide greater than 135 plf. Each type of light gauge steel panel provides different strength. You must check with the manufacturer. Most post shear and span tables on their websites. Look for engineer's specifications or load tables. There is no industry standard nomenclature for these data sheets. You may need to click around a bit to find the load tables you need.
Cable bracing or other cross bracing in these walls provides a redundant force resisting system. If the screws tear through the metal sheets, the cables will take the load. Most likely, the wall panels and braces will work together to resist the loads.
Connections in Wind Walls
Pre-engineered metal buildings (PEMBs) often have end walls of light gauge steel ("C" purlins). These columns and rafters transfer wind loads to an adjacent frame via roof braces and wall braces. To connect the cable braces, the thickness of the C is reinforced with a rectangular piece of metal. Typically, columns are 8" C's and the thickness is .057" or .075" (16 GA or 14 GA). The reinforcement will be 3/16" or 1/4".
These cable end connections should be located very close to base plate and haunch connections. Loads should transfer only minimally through the members in wind walls.
Hillside Washer for Cross Brace Connection to Web
Cross Bracing Connection to Wide Flange Beams
Typically, connections to W beam columns or rafters are made using hillside washers and a short-slotted hole through the web. As shown, the washer provides a smooth edge to prevent cables from fraying.
Cables should be ASTM 1023 standard, to ensure quality. However, the connection itself must also be designed and installed for long service life. Even in enclosed buildings, cable should be galvanized (GALV) or stainless steel (SST).
Cross Bracing Connection to Pipe Columns
Pipe columns require tabs to connect cable braces. The tabs are punched and welded to the columns in the shop. In the field, a U-joint is bolted through the tab hole. Cables are looped around the U-joint and crimped. Or, the hole in the plate is smoothed and the cable passes directly through the hole.
Cables are tightened using turnbuckles spliced into the span. These are spliced in off-center so that both eye bolts are not in direct contact. Another method uses a flat plate with eye bolt connectors at 4 equally spaced distances. The flat plate connection eliminates cable fraying by rubbing during years of deflecting under wind loads.
What are Alternatives to Cross Bracing?
There are two more ways to resist wind forces hitting the building parallel to the ridge. The most common is a cantilevered column system. Poles are embedded into the earth in pier footings. The depth of the footing resists the overturning force of wind and seismic forces.
The second way is a moment resisting beam. A strong connection is fabricated and installed at each end of the beam. This connects columns of two frames. The strength of the connection resists the bending force created by wind pushing the end wall (a moment force.) This is sometimes called a portal beam.
Roof-only buildings and shops requiring frequent access are limited by X-braces. They block the bay in which they are installed. So, steel buildings of this type are built using cantilevered columns or moment resisting beams.