Steel Construction
Steel frame construction is one of the most durable forms of construction for buildings. Unlike residential construction where the design is primarily governed by building codes, most steel structures are designed by engineers or other professionals to support the live and dead loads for the structure’s intended purpose. Wind and seismic loadings are also accounted for in the design.
In residential construction, drawings normally consist of floor plans and elevations. Sizing of structural members is left up to the builder or the lumber yard supplying the materials. In commercial or industrial steel frame construction, the sizing of the structural members is done by the design professional and indicated on the drawings. There may also be fabrication drawings showing all of the connection details in addition to the sizes.
Failures in steel structures are therefore not common occurrences. When failures do occur, they are normally the result of an overload condition on a portion or all of the structure. Investigation of failures in steel structures consists of first identifying the location where the failure originated, and then the mode of failure. Once the initial failure location is identified, the cause of the failure can then be determined.
Common causes of overloads causing structural failures are floor loads that exceed the design capacity of the floor, attaching equipment to structural members in locations not designed for the equipment load, and making building additions or alterations to the structure. Tornadoes and snow loads that exceed the expected loading are typical weather events that can cause failures in steel frame buildings. On buildings with flat roofs, plugged roof drains can cause excessive water to accumulate and lead to collapse.
Fires are another cause of failures in steel-framed structures. If the steel framing is not protected by fire-resistant materials, heat from the fire will raise the temperature of the steel above its yield point. Once the steel yields, it will no longer be able to carry the loads it was designed for, and deformation or collapse of the structure will occur. Fire-damaged members are not normally repairable and usually need to be replaced when the structure is repaired.
Design, fabrication, and erection drawing are the most useful tools available when analyzing a structural steel failure. Using the drawings, Donan Engineering’s professional team of investigators can determine if the structure was constructed in accordance with the original plans. More importantly, the plans can be used to determine if the current or previous owner had made alterations to the structure that have compromised the ability of the structure to serve its intended purpose. Building owners occasionally remove, cut, or damage structural members that are a vital part of a building’s structural integrity without recognizing the significance of these actions.
If drawings are not available, there are other indications of alterations that our professional engineers are trained to recognize. Among these are welds and torch cuts that have been left unpainted, primed or unprimed steel, and different colored finish coats of paint.
In older steel frame construction, corrosion can be a cause of structural failures. Long-term exposure to moisture from leaks in the building envelope, such as in the roof or window openings, causes the steel members to rust. When steel rusts, it loses cross-sectional area, making it thinner and unable to carry the design loads. Often, the corrosion is in small concentrated areas that behave in the same manner as if the steel were cut. In addition to exterior sources, water causing the corrosion may come from leaks in interior plumbing or heating systems.
In industrial buildings, corrosion can be caused by chemicals. The chemicals may be leaks from improperly stored containers or leaks and/or spills from a manufacturing process.
Steel frame construction can be damaged by explosions that are within or without the structure. In this case, the cause of the structural damage can be from the blast shock waves or from flying debris. These are usually significant events where the cause of the structural failure is known. In such scenarios, Donan Engineering can determine the extent of the structural damages and recommend a method of repair.
Another common type of steel frame construction is the manufactured steel buildings sometimes known as “pre-engineered steel frame” buildings. Manufactured buildings are most vulnerable to damage while they are being erected. Until all of the building is completed, the structure can be unstable and easily damaged by wind, unbalanced construction loads, improper storage of construction materials and impact from construction machinery. Collapse due to wind is the most common type of failure. Because these building are unstable during construction, they must be braced during erection. The most common bracing method is with cables from the tops of columns to the base of a column on the adjacent row or line of columns. To be effective, the bracing must be in the form of an “X” in both the lateral and longitudinal directions. The cables must also be tight with no appreciable slack. Wind failures are most often caused by the lack of temporary bracing or insufficient bracing.
Manufactured steel frame buildings are also vulnerable to the all of the same structural failures as a conventional steel frame building. What makes them more susceptible to these same failures is the manner in which they are designed and constructed. In conventional steel framing, the steel members are selected from a finite number of sizes and cross sections that are either rolled shapes or built-up welded sections. When the designer sizes the member for the load, he or she chooses the smallest section that meets the design load. This usually results in a member that is larger than needed and thus has a built-in factor of safety.
Manufactured steel buildings are often built with light-gauge sheet metal formed to a particular shape and cross-sections that are specific to the purpose, such as roof purlins and siding girts. By choosing the material thickness and special shapes, the most economical use of steel with the least weight is used. This can result in shapes that meet the design load requirements, but that have little extra in the way of a factor of safety.
Manufactured building frames are typically fabricated with a variable cross section from top to bottom of columns and across the width of the building. This method of design recognizes that the normal building loads are not uniform across the entire member; however, if there is some unusual loading in a lighter area, buckling or other failure can occur. The end frames on manufactured buildings are usually designed for one-half the roof load as the interior frames unless the manufacturer is informed of possible future additions. If the building is subsequently added onto without reinforcing or moving the frame to the end of the addition, it will be undersized and subject to failure.
Building owners often hang loads from the roof structure or mount equipment on the building columns. In conventional steel framing, because of the inherent over-sizing of the steel members, structural problems are not necessarily caused; in a manufactured steel frame building, overloading will almost always be the cause of buckling, bending, or other failure unless the manufacturer has designed the steel for a specific load (other than the lighting, piping, and HVAC equipment which are normally accounted for in the design).
Steel frame structures are able to withstand normal wind force if they are properly built and maintained. Tornadoes and falling trees will cause damages, especially to the sheet metal siding and roofing of manufactured steel buildings. Puncture damage is common from flying debris because of the thin gauge of the materials. Roof damage from wind is not normally from the roof being blown off, but rather by uplift from the negative pressure of high winds blowing across the surface or created by the geometry of high and low roofs, adjacent structures, etc.
Standing seam roofs are becoming increasingly common on manufactured steel buildings. These roofs use concealed clips to hold down the roofing sheets. The sheets are seamed together with cap strips or mechanically interlocked by rolling the edges together with a power roller. Each manufacturer may have its own proprietary standing seam design. The seams are not designed to be unfastened without damaging the seam.
Tornado-force winds can cause the concealed fasteners to fail and the roof to uplift even though the roof remains in one piece. Several unique features of manufactured steel buildings present challenges if the buildings require repairs. The buildings are typically insulated with fiberglass batts enclosed in plastic film. The insulation is furnished in rolls and is installed over the purlins and girts before the roofing and siding are installed. The roofing and siding are then installed over the insulation and fastened through the insulation. Correct replacement of insulation that cannot be repaired in place requires the removal and replacement of the roofing and siding as well as the insulation itself.
Standing seam roofs are installed from one side of the roof to the other. Individual roof panel replacement is done in the opposite direction across the roof (last on, first off) until the damaged panel is reached. Repair panels can sometime be inserted in the middle of the roof by making a field splice down the middle of adjacent panels. This usually results in a less-than-satisfactory repair which will be subject to future leaks and therefore is generally not recommended.
Donan Engineering has trained professionals that are familiar with all types of steel frame construction. We are able to make accurate, well-informed decisions regarding the cause of damage to steel-framed structures and the appropriate methods of repair.