Page 402 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.33

ASTM A490 for example, are susceptible to stress corrosion cracking if their yield strength
or hardness is too high. Cracking of these types of bolts under adverse environmental con-
ditions can lead to significant structural failure. The ASTM specifications for high-strength
bolts limit bolt hardness to avoid just such failures.
While structures are designed for service, they must usually go through an erection process
during which they experience types of loading that they will not see in service. Failure during
erection is most often due to inadequate care in the calculation of stresses on the members
erected or the supports used in the process. Many failures are due to poorly designed tempo-
rary welds, lifting lugs, and restraining members. Too often lifting structures are not so much
designed as simply built with the most cursory design in the expectation that their apparent size
will render them adequate. The materials used in these structures are often of unknown origin
or properties. Experience shows that erection failures can result from such an approach. Proper
design for erection and proper attention to welding or bolting of these “temporary” structures
as well as control of their composition and properties are mandatory for failure prevention.
A somewhat more general problem that can lead to structural failures is lack of proper
review of shop drawings and erection or assembly plans. For a variety of reasons, the
design of the structure may be changed between the time drawings are submitted to the fab-
ricator and the structural components are actually built. These changes are usually, but not
always, reflected in shop drawings; therefore a careful review of the sequence of design
drawings, shop drawings, erection plans, and the as-built structure is sometimes necessary
to trace why a structure has failed. At least one major structural failure, the Hyatt-Regency
Skywalk in Kansas City, has been attributed to failure of a connection, the design of which
was changed during shop fabrication. These drawings and plans should also be checked for
tolerances and interferences. In one nuclear power facility, the seismic pipe restraints on
the cooling water piping for the reactor were placed in locations not as shown in the origi-
nal design because of interference with equipment not shown on the drawings. The mis-
placement of these restraints invalidated the computer analysis of earthquake resistance of
the facility, forcing a multimillion-dollar reanalysis and retrofit.

Service and Repair Errors
Repair of structures already in place, while sometimes necessary, is another potential source
of structural failures. There is apparently a perception on the part of many engineers that
repairs to correct corrosion problems or in-service cracking, or modifications to improve
access, or to install new capacity, are of such a minor nature that no special care is required.
Like temporary welds used to assist in erection, they are often not subjected to any particular
design process, are placed in the wrong location, are executed carelessly, or are of inadequate
strength. In some cases, the materials employed are not similar to, or compatible with, the
materials and processes used in the original fabrication. For older structures made of steels
having low weldability, welded repairs or additions are particularly troublesome.
Examples of this type are hangers for pipes or cables added to bridge members, welds
made to steels of poor weldability to replace corrosion-damaged members, weld plugging
of misdrilled or punched holes, and replacement bolts not properly tightened. In many of
these cases, fatigue cracking from the stress concentrations not considered in the design
process and/or cracking from an erroneous welding process occurs. Weld cracking also
occurs because the repair weld is made in a now fairly rigid structure, which requires greater
care and skill in welding and higher welding preheat than the original shop weld. When the
repair is made in the field, neither of these requirements is likely to be met. Indeed, repair
welding is often made without a well-thought-out weld procedure even though the weld con-
figuration of the repair is seldom anything like a common or standard one and often nothing
like the original one in the structure.