Page 282 - Forensic Structural Engineering Handbook

8.22 CAUSES OF FAILURES

required localized placement of light-weight floor leveling material. However, place-
ment of more than 4 in of the light-weight topping increased the seismic mass substan-
tially, which in turn required reinforcement of the slab to resist the increased seismic
load demand. Strengthening of the slabs was accomplished by application of carbon
fiber reinforced polymers to the underside of the slab.
3. The undulations in floor level can be easily measured using various devices. Water- and
oil-based (manual and electronic) manometers are the most commonly used pieces of
equipment to measure floor levelness. Figure 8.16 shows a contour plot developed from
measurements taken by a manometer and illustrates the variation in floor level.
4. The presence of certain types of aggregate can sometimes react with the concrete caus-
ing superficial “pop-outs” when changes in concrete surface temperature occurs, or due
to alkali-silica reactions.
5. Improper placement of reinforcement can negatively impact the proper performance of
reinforced concrete. Mislocated reinforcement can decrease the intended strength of the
affected member: Exposed reinforcement or lack of adequate cover will allow corrosion
to occur over time and cause spalling. Exposed post-tensioning cables can corrode and,
in time, release the post-tensioning forces causing anchorage failure. Lack of adequate
cover on reinforcement, as demonstrated in the ground penetrating radar (GPR) scan of
a slab shown in Fig. 8.17, can also raise concerns about fire rating of the member.
Unexpected misalignments or dimensional variations in columns or load bearing walls,
likely caused by displaced formwork during construction, can raise concerns about the
location of reinforcement, as well as member capacity.
6. The effect of improperly placed reinforcement has been previously described but the
consequences of missing reinforcement can be disastrous. On August 8, 1993, a mag-
nitude 8.1 earthquake rocked the western Pacific island of Guam. Construction of two,
12-story towers of a resort hotel had been completed just 3 weeks prior to the earth-
quake. The inherent construction defect created by missing reinforcement in the beam-
column joint of the concrete special moment resisting frames caused one of the
buildings to collapse as seen in Fig. 8.18. A detailed evaluation determined that to avoid
the rebar congestion created by the special detailing requirements for a concrete
moment frame in a high seismic region, confinement reinforcement in the beam-column

FIGURE 8.16 Floor levelness survey plot.