Lightweight Concrete for High Strength - Expanded Shale & Clay

l
d
3
4.5
= 0 .33 f
si
d
b
+ ( f f ) d
' su
−
se b '
f
f
ci
c
( A.22)
A.11.9 Buckner, 1994
Buckner noted that there was an interaction between transfer length, effective prestress
and design stress. Buckner recommended using a procedure similar to that of Hanson and Kaar.
He also theorized that the development length expression should be based also on ultimate strain
and suggested Equations A.23 and A.24.
l
d
f
si
d
b
= lt
+ l
fb
= + λ ( f
3
*
su
−
f
se
) d
b
( A.23)
1.0
≤ [ λ = (0.6 + 40ε
)] ≤ 2.0
( A.24)
ps
where
d b = diameter of prestressing strand
f pt = stress in prestressing strand just prior to strand release
f su *= stress in prestressed reinforcement at nominal strength of member
f se = effective prestressing stress after losses
l fb = flexural bond length
l t = transfer length of prestressing strand
ε ps = strain corresponding with f su *
A.11.10 FHWA Study, 1996
In addition to the numerous studies aforementioned, the FHWA also published findings
from their own study addressing the bond of prestressing strand in NWC and HPC. The FHWA
study addressed variables as specified previously in A.8.12. Based on FHWA results and the
results of other research programs, the restriction on the use of 0.6-inch strand was lifted. The
FHWA proposed Equations A.25 and A.26 for “best-fit” or “mean” and to provide a “95%
confidence interval,” respectively.
A-22