prestressed beam depth of neutral axis
prestressed beam depth of neutral axis
(OP)
I am currently doing a strength design hand check for a 72" Bulb-T. While doing the check for rectangular-vs-T Beam action, I am getting "odd" results for the depth of the neutral axis "c-value" (using equations in LRFD 5.7.3.1.
Using my numbers I get that the rectangular section "c" is more than the depth for T-Beam action, which is counter to what I would expect.
Here are my values:
Aps=7.956 (52 strands)
fpu=270ksi
f`c = 4.35ksi
b1 = 0.832
bf=75in
bw=48in
k=0.28
dp=58.4in
When checking for rectangular beam…..I get c=8.9 in
When checking for T-beam action………I get c=8.5 in
I understand it is not a big difference, it just bugs me that it does not give me results like I expect.
Does this seem odd to you also?
Do you know why I am getting these results?
Thanks for any help that you can provide.
Using my numbers I get that the rectangular section "c" is more than the depth for T-Beam action, which is counter to what I would expect.
Here are my values:
Aps=7.956 (52 strands)
fpu=270ksi
f`c = 4.35ksi
b1 = 0.832
bf=75in
bw=48in
k=0.28
dp=58.4in
When checking for rectangular beam…..I get c=8.9 in
When checking for T-beam action………I get c=8.5 in
I understand it is not a big difference, it just bugs me that it does not give me results like I expect.
Does this seem odd to you also?
Do you know why I am getting these results?
Thanks for any help that you can provide.





RE: prestressed beam depth of neutral axis
RE: prestressed beam depth of neutral axis
Slab thickness = 8in
the "web" is the flange of the Bulb-T = 48", while the "web" is the effective width governed by beam spacing
dp = (beam depth + deck + haunch)-(ycg) therefore..
db = (72" + 8" + 0.5") - (22.1") = 58.4" (falls in draped strand area)
Any other thoughts?
RE: prestressed beam depth of neutral axis
line 3 should read....
the "web" is the flange of the Bulb-T = 48", while the "flange" is the effective width governed by beam spacing
RE: prestressed beam depth of neutral axis
RE: prestressed beam depth of neutral axis
The precast supplier drapes (aka harps) the strand to minimize the prestressed enduced moment near the ends of the beam in an effort to satisfy the service design criterea.
The point that we are looking at is well with the harped area, resulting in the elevated c.o.g. of the strand.
RE: prestressed beam depth of neutral axis
These strands will have significantly different strain during strength limit than the strands close to the tension face of the beam. I believe PCI also suggests similar recommendation.
Also, since your section is close to the end of the beam, you should consider reduced stress in the strands due to lack of development length.
RE: prestressed beam depth of neutral axis
yc=4.95" for a dp=76"
I still get a rectangular section "c" of 9"
and a T-Beam "c" of 8.7"......
I have went on and it really does not make a difference in the end, just does not feel right
RE: prestressed beam depth of neutral axis
RE: prestressed beam depth of neutral axis
in either case, "c" - depth to neutral axis and
"a" depth of Whitney stress block
would be proportionally the same....and the "a" for the T-Beam would be less than that of the rectangular section