Page 180 - Forensic Structural Engineering Handbook

THE FIRST STEPS AFTER A FAILURE 5.13

moved to a safe location. Care should be exercised during the move to minimize disturbing
the set of the concrete.
Whether the cylinders are on-site or off-site, a decision must be made quickly on their
disposition. One option is to test the cylinders using the same protocol called for in the pro-
ject specifications, and using the same testing laboratory. However, consideration should
be given to altering the protocol, depending on the nature of the case and the number of
cylinders available. For example, it may be advisable to test certain cylinders immediately,
to determine the concrete strength on the day of the collapse; or it may be advisable to send
some or all of the cylinders to an independent laboratory. It may also be prudent for the
investigating engineer to witness the testing of the cylinders.
Testing laboratories normally dispose of cylinders shortly after testing them. However,
consideration should be given to retaining the tested cylinders either for their own eviden-
tiary sake or for additional testing, such as petrographic examination.

Snow and Ice Accumulation
The weight of snow actually on a roof at the time of its collapse is an extremely valuable
piece of information. When compared against the code-prescribed design loads for the
structure and the design criteria, it can provide an indication of whether the failure was due
to an error in design or construction, or due to an unforeseeable overload.
Depending on climactic conditions, the amount of snow and ice may change dramati-
cally in the days—or even hours—following a collapse. Additional precipitation may
increase the weight, winds may sweep off snow or add to drifted areas, and melting may
reduce the weight. To obtain the most reliable measurement, it is therefore important that
the snow depth and weight be measured as quickly as possible.
For several reasons, however, it may be difficult or impossible to accurately measure
the snow weight. Measurements of snow weight in collapsed areas may not be representa-
tive of conditions present prior to the collapse, since the depth of snow may have been rad-
ically altered by the collapse itself. Measurements in adjacent uncollapsed areas of the
same roof are potentially the most meaningful, if the thermal characteristics of the roof and
the temperature below it match that of the collapse area. Care must be taken that the con-
figuration of adjacent roofs and other obstructions does not materially affect the expected
drift heights or accumulations due to sliding snow. Safety of the personnel measuring the
snow weight should always be a concern. Metal roofs may be extremely slippery, even
with mild slopes, and the stability of the remaining structure may be questionable.
If it is not possible to measure the snow weight on the roof itself, the next best alterna-
tive is to measure it on nearby roofs with similar wind exposures. The geometric conditions
of these roofs should be carefully documented, especially with regard to conditions that
simulate or differ from the subject roof.
The physical measurement of the snow weight needs to be done carefully. One way is
using the inverted container method:
1. Press the inverted container down into the snow until the roof is firmly contacted.
2. Remove the snow and ice on all sides of the container.
3. If it is found that the ice has prevented the container from reaching the roof, carefully
remove the ice around the perimeter of the container. If ribs prevent uniform contact
with the roof, such as on metal panel roofs, notch the container to fit over the ribs.
Another alternative for ribbed roofs is to use a container that fits between the ribs, and
to locate it appropriately.
4. Slide a thin sheet, possibly metal, under the container to help in retaining the contents
while the container is flipped to its normal position.