CASE 512 - Sheet Piling, J.J. Cecilio
Inadequate dimensional control of welded and extruded "Y's".
Collection of Facts: ASTM A328 "Steel Sheetpiling" and A690 "High Strength
Low-Alloy Steel Piles and Sheetpiling" for use in marine environments depend
on specification A-6 for dimensions control and markings. The markings in
12.3 is quite adequate; however, the dimensional control is inadequate related
to the interlock dimension variation. This is especially true of riveted,
welded and extruded "Y's".
Solution: The specification for the specific application should specify not
only interlock strength, but also specific interlock dimensions with limited
variations. It is noted here that the dimensions vary up and down the length
of the sheetpiles and "Y's", especially the extruded ones. The weakest point
on these piles from available data appears to be at approximately 75% of the
length from the lead of hook-end as they pass through the dies or rolls.
Hence dimensional variation is important.
Additional data is available from the NAVFAC library in NRL Memorandum Report
3869 "Trident Cofferdam Analysis" by C. D. Beachem dated January 2, 1979.
CASE S13 - Excessive Welding for a Guyed Tower, E. L. Mifflin
Unnecessary cost of welds for vertical member splices.
Symptom: In one case, approximately 70% of the welded fabrication of a tower
was in the flange plates used to splice the vertical members.
Collection of Facts: Usually the vertical members and legs of a guyed tower
are solid round bars. The legs are compression members that must be very
accurately finished on the ends for true alignment in the field. The only
time the member is in tension is in the erection of the mast or when it is in
a cantilevered top. Many of the splice designs use a donut shaped plate
continuously welded to the solid round bar to connect one leg to another. The
main compressive force is transmitted through bearing of solid round ends.
These donut shaped plates tend to be very thick.
In one specific design, the plates were three (3) to four (4) inches thick.
The designer called for full penetration continuous welds. This was excessive
and amounted to seventy (70) percent of the shop welding of the tower. This
excessive amount of welding, coupled with the complex welding procedures for
the material used in design (90,000 psi yield heat treated alloy steel),
resulted in a very expensive structure. Continuous welding was required for a
fatigue resistant design, but full penetration welds were not needed.
Solution: This is a special case where it would have paid to economize on a
connection by careful design that would have considered or even specified
erection procedures and actual stress requirements. Of course, it is
necessary to consider the nature of the connection, the type of material, and
the service conditions in designing the weld. In the case cited, the design
was adequate but involved unnecessary expense.