Page 284 - Forensic Structural Engineering Handbook

8.24 CAUSES OF FAILURES

Consequences of Thermal Loading in a Parking Garage. A five-story parking garage
demonstrates the damaging consequences of thermal loading. The structure is rectan-
gular in plan with dimensions of approximately 200 by 150 ft. It is a reinforced con-
crete structure with cast-in-place columns that support a 2-in-thick topping slab on
12-in-deep double-T precast beams framing into precast concrete girders that span
between the cast-in-place columns. A cross section of the parking garage is shown in
Fig. 8.19. The bottom of the bearing ends of the precast girder have an embedded
3/8–in-thick steel plate that bears on a similarly embedded 3/8-in thick steel plate on
top of the cast-in-place columns. Severe cracking of the concrete columns at the upper-
most level of the parking garage was observed as shown in Fig. 8.20. The cracking was
consistently worse at the columns on gridline 5. In addition, cracking of the topping
slab was also observed at all interfaces between adjacent double T-beams and precast
girders to double-T beams.
It was also noted that the steel bearing plates on gridline 5 on the columns and girders
on gridline 5 had been welded to each other and likely placed as a temporary measure dur-
ing construction. This cracking of the concrete columns could have been avoided if the two
bearing plates had not been welded together. This fillet weld was not required according to
the design drawings. Further evaluation revealed that cracking of columns occurred pri-
marily due to the constraint on the thermal expansion and contraction of the topping slab at
the roof level that was exposed to outside temperature changes. It was further compounded
by water penetration into concrete through cracks in the topping slab along the interfaces
of precast elements.

Steel Structures
Steel framing is often concealed from view by building finishes or fire proofing, making
the determination of causes of defects more challenging. A significant portion of steel
framing is fabricated in the shop where better quality control is achieved. Despite that, field
fit-up challenges produce defects which may compromise the structural member’s ability
to carry its intended design load. Some of the defects encountered in steel framing are dis-
cussed below:
1. Poor connection detailing and field fit-up issues are the most common source of oversized
holes, torched steel members, and bent and missing bolts. These conditions create local-
ized brittleness and stress concentration points which can result in under-performance of
the connection.
2. Long spans using steel floor framing can create uncomfortable floor vibrations. Review
of a floor framing system’s susceptibility to floor vibrations should be performed dur-
ing the design process. The inherently low value of damping in steel framing further
accentuates the perception of floor vibrations.
3. The 1994 Northridge earthquake exposed the vulnerability of steel moment frame con-
nections to failure during seismic loading. A large number of buildings with steel
moment frames were discovered to have suffered weld failure in their moment connec-
tions. Extensive research undertaken subsequent to the Northridge earthquake revealed
many shortcomings, some in the construction process (e.g., poor workmanship and fail-
ure to remove the back-up bars) and some in the fundamental understanding of the sen-
sitivity of AWS-compliant welds to minor imperfections under high-stress conditions.
Many of these shortcomings have been since rectified. Also, less sensitive and better
behaved prequalified moment resisting frame connections have been developed in the
aftermath of the Northridge earthquake, such as the Reduced Beam Section shown in
Fig. 8.21.