Item #1 - ODOT FTP

lifegunbarrelcityUrban and Civil

Nov 26, 2013 (3 years and 6 months ago)

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2010

AASHTO BRIDGE COMMITTEE AGENDA ITEM: WAI
151



SBJECT:

Horizontally Curved Concrete Box Girder Bridges


TECHNICAL COMMITTEE:
T
-
10


X DESIGN CONSTRUCTION LRFR MANUAL OTHER


X REVISION ADDITION



X US

SI BOTH


DATE PREPARED:

2/1/200
8




DATE REVISED:
10
/21
/09


ITEM INFORMATION


Item #1


Add
t
he followings to Article

5.2 DEFINITIONS


Duct Stack


A vertical group of tendons in which
the space between individual
tendons is less than

1
-
1/2”.


Local Bending


The
lateral bending

caused by curved post
-
tensioning tendons on
the concrete

cover between the internal ducts and the inside face of the curved
element (usually web
s).


Local Shear


The lateral shear caused by curved post
-
tensioning tendons on the
concrete cover

between the internal ducts and the inside face of the curved element
(usually web
s
).


Regional Bending


Transverse bending of a concrete box girder web due

to
concentrated lateral

prestress forces resisted by the frame action of the box acting
as a whole


Stirrup

and Duct Ties


Reinforcement designed to prevent local
bending

and/or
shear failure of curved

p
ost
-
tensioned

tendons cast withi
n a web.


Wobble


The unintended deviation of a tendon duct or sheath from its specified
profile.


Delete because the word isn
’t used in this ballot item.



Item #2


Add
t
he followings to Article
5.3 NOTATION


d
eff

= one
-
half the effective length of the
failure plane in shear and tension
for
curved element
(in.)
(
5.10.4.3.1
)


h
c

=
clear s
pan of the web of concrete box girder bridges between the top and
bottom

slabs measured

along the axis of the webs
(in.)

(
C5.10.4.3.1
)
.


h
ds

=

height of a vertical group of ducts
(in.)

(
C5.10.4.3.1
)
.


M
end

=
moment at the ends of a hypothetical unreinforced concrete beam

consisting of the

cover concrete over the inside face of a stack of horizo
ntally
curved p
ost
-
tensioned

tendons

(in.
-
k)
(5.10.4.3.1
).


M
mid

=
moment at the midpoint of a hypothetical unreinforced concrete beam

consisting of the

cover concrete over the inside face of a stack of horizontally
curved prestress
post
-
ten
sioned

(in.
-
k)
(5.10.4.3.1
).


f
cr

=

design flexural cracking stress of the hypothetical unreinforced concrete

beam

consisting of the cover concrete over the inside face of a stack of
horizontally curved

p
ost
-
tensioned

tendons
(ksi)
(5.10.4.3.1
)


f
n

=

nominal flexu
ral cracking stress of the hypothetical unreinforced

concrete beam

consisting of the cover concrete over the inside face of a stack
of horizontally curved

p
ost
-
tensioned

tendons
(ksi)
(5.10.4.3.1
).


φ
duct

=
o
utside
diameter of p
ost
-
tensi
oning

duct

(in.) (5.10.4.3.1)


ψ
cont

=
g
irder
web continuity factor for evaluating regional bending (5.10.4.3.1)



Item #3


Add

the following Article


5.8.1.5 Webs of Curved Post
-
Tensioned Box

Girder Bridges


C
urved post
-
tensioned box girder

having both an overall clear height
h
c

in excess of
4
-
ft and F
u
-
in

greater than 4.0

shall be designed for



the combined

effects of global shear resulting from ve
rtical

shear and torsion



transverse web regional

bending resulting from lateral prestress force

and the
effects of dead load, live load and

transverse post
-
tensioning.




Item #4


Add the following to the Commentary of
C5.8.1.5

(
Shortened by SH;
chec
k w/D.
Goodyear

and J. Corven on what they felt would be appropriate here)


Transverse web bending is a function of the vertical loads, restoring effect of
the longitudinal prestressing, and any transverse prestressing.
Considering global
web shear and re
gional

web transverse bending separately will tend to

underestimate the amount of vertical

reinforcing steel required in the webs.

More
rigorous approac
hes that

consider

the interaction of these
combined
forces are

presented in
Menn

(
1990
)

a
nd
Nutt (2008)
.




Item #5
a

Revise the first paragraph of Article 5.10.4.3 as follows:

Reinforcement shall be used to confine curved

t
endons

if required by
Art
icl
e

5.
8.1.5
.

The reinforcement shall be proportioned to

ensure that the steel
stress

at service limit state does not

exceed 0.6
fy
, and the assumed value of
fy
shall not

exceed 60.0 ksi.
Unless a strut
-
and
-
tie analysis is done and indicates
otherwise,
s
pacing of the confinement

reinforcement shall not exceed either 3.0
times the

outside d
iameter of the duct or 24.0 in.

Delete the third paragraph of Article 5.10.4.3

per recommendation of D. Goodyear (
not
necessary;
deviation of webs into anchorage zones might be misinterpreted)
:


When a tendon curves in two planes, the in
-
plane and out
-
of
-
p
lane forces shall be
added together vectorially.


Item #5b

After the 3
rd

(last) paragraph

of Article 5.10.4.3
, i
nsert
a
new
paragraph as follows:


In
-
plane force effects are due to a change in direction of the tendon

within the
plane of curvature
.
Out
-
of
-
plane force

effects are due to the spreading of the wires or
st
r
ands within the duct.


After the 3
rd

(last) paragraph of Commentary to Article 5.10.4.3, insert a new paragraph
as follows:

Fig
.

C
-
1

shows a
n in
-
plane

deviation in the ver
tical curve
,
and

Fig
.
C
-
2

shows a
potential
in
-
plane
deviation in the horizontal curve
.
Out
-
of
-
plane force effects are shown
in
Fig. C
-
3

and can be affected by ducts

stacked vertically or stacked with a horizontal
offset
.


Move Figures
C5.10.4.3.1
-
1
,

C5.10.4.3.1
-
2
, C5.
1
0.4.3
.2
-
1

here

and renumber them
5.10.4.1
-
1, 5.10.4.1
-
2, 5.10.4.1
-
3
.


Figure C5.10.4.3
-
1 In
-
Plane Forces
in a
Soffit

Blister

[Note to M&M: pls
change “global” to
“regional”
; also,
PLEASE

show
an

overhang

so that the orientation is clear
]

Figure 5.10.4.3
-
2
In
-
Plane

Force Effects in
Curved Girders




Item #6
a


Revise the 1
st

and 2
nd

paragraph
s

of Article 5.10.4.3.1
as follows:


P
u

= the tendon force
factored as specified in Article 3.4.2
(kip)

(rest of paragraph is unchanged

The maximum deviation force shall be determined on the
basis that all the
tendons, including provisional tendons, are stressed.
In
-
plane force effects due to
tendons curved at the tendon anchorage are addressed in Article 5.10.9.


Delete the 2
nd

paragraph of Commentary
. Possibly replace with note on
subtra
cting the
‘concrete cylinder component’ (DG)
. Suggested Commentary not incorporated based on
D. Goodyear
’s comment.



Item #6b

Insert a new sub
-
article title
after the existing 2
nd

paragraph of Article 5.10.4.3.1
as
follows:

5.10.4.3.1a Shear Resistance
to Pull
-
out


Item #6c

Modify the
existing 3
rd

and 4
th

paragraph
s

of Article 5.10.4.3.1 as follows:

The shear resistance
per unit length

of the

concrete cover against pull
-
out by
deviati
on forces,

V
r
,

shall be taken as:

V
r

>
φ
V
n




(5.10.4.3.1
a
-
2)


in which:


ci
c
n
f
d
V
'
125
.
0




(5.10.4.3.1
-
3)


ci
eff
n
f
d
V
'
15
.
0





(5.10.4.3.1
a
-
3)


where:


V
n

=

nominal shear resistance of two shear

planes per unit length (kips/ft.)


φ

=

resistance factor

for shear specified in

Ar
ticle 5.5.4.2

0.75


d
e f f
=
one
-
half the effective length of the failure plane in shear and tension for
a
curved
element

(
5.10.4.3.1
)

(in.)


dc
=

minimum concrete cover over tendon

duct
, plus one
-
half of the duct diameter

(in.)


f

c
i

=

specified compressive strength of concrete

at time of initial loading or

post
-
tensioning



For single duct stack or for

s <

duct


4
duct
c
eff
Φ
d
d







(5.10.4.3.1
a
-
4)



For

s



duct

,
d
eff


is taken as the lesser of the follow
ing:


2
duct
w
eff
Φ
t
d







(5.10.4.3.1
a
-
5)

and

2
4




s
Φ
d
d
duct
c
eff




(5.10.4.3.1
a
-
6)


s
=

clear distance between tendon ducts in vertical direction (in.)

φ
duct

= outside diameter of prestress duct (in.)





[Note to M&M: pls delete equations

from beneath the figure]


Figure 5.10.4.3.1
a
-
1


Definition of d
eff

If the factored in
-
plane deviation force exceeds

the factored shear resistance of the
concrete cover,

as specified in Eq.
5.10.4.3.1
-
2
,

fully anchored tiebacks

web and duct
ties
hooked a
round the

outermost stirrup legs

to resist the in
-
plane

deviation forces
shall be provided in the form of

either non
-
prestressed or prestressed reinforcement.

Stirrup

and duct ties shall be designed using strut

and tie methods.
???



Item #
7


Revise
the
existing 3
rd

paragraph
and add new paragraph
s

in
Article C5.10.4.3.1a as
follows:


The two shear planes for which Eq.
3

5.10.4.3.1
-
3

gives
Vn
are as indicated in
Figure
C2

C5.10.4.3.1
-
2

for single and multiple tendons.


When more than one vertical group of

ducts

are located side by side in a
single web, all

possible shear and tension failure planes should

be
considered in
determining d
eff
.



A generic

stirrup

and duct tie detail is shown in Figure

C5
.10.4.3.1
a
-
1
. Small
diameter reinforcing bars

should be used for better development of these

bars.
There have been no reported web failures

when this detail has been used.

Nutt
(2008) provides
an example problem that

illustrates the design of a
stirrup

a
nd duct
tie system using strut and tie methods.



Figure C5.10.4.3.1
a
-
1

Typical Web and Duct Tie

Detail

Item #
8

Insert a new sub
-
article title
after the existing 4
th

paragraph of Article 5.10.4.3.1
as
follows:

5.10.4.3.1b Cra
cking of Cover Concrete


Revise
the 5
th

paragraph and insert a new paragraph
as follows:


Where stacked ducts are used in curved

girders, the moment resistance of th
e
concrete

cover, acting in flexure, shall be investigated

When the clear distance between ducts oriented in a vertical column is less
than 1
-
1/2” the ducts sh
all be considered stacked.
R
esistance to cracking shall be
investigated at the ends and mid
-
heigh
t of the unreinforced cover concrete. No more
than three ducts shall be allowed in any one stack.


The applied local moment per unit length at

the ends of the cover shall be
taken as:

.
(5.10.4.3.1
b
-
4)


and the applied local moment per unit length at

the
mid
-
height of the cover shall be
taken as:



Stirrup Tie,
Typ.


(5.10.4.3.1
b
-
5)


where

h
ds

=

the height of the duct stack as shown in

Figure C5.10.4.3.1
-
2
?


Tensile stresses in the unreinforced concrete

cover resulting from Equations
5.10.4.3.1
b
-
4 and

5.10.4.3.1
b
-
5 shall
be combined with the tensile

stresses from
regional bending of the web as

defined in Article 5.10.4.3.1c to evaluate the

potential
for cracking of the cover concrete. If

combined tensile stresses exceed the cracking

stresses given by Equation 5.10.4.3.1
b
-
6
, ducts

shall be restrained by
stirrup

and
duct tie

reinforcement.


f
cr

=
φ
f
n





(5.10.4.3.1
b
-
6)


where:


ci
n
f
f
'
24
.
0


φ

=
0.55


0.85


Item #
9


Add
C5.10.4.3.1b as follows:


When the vertical spacing between ducts

exceeds 1
-
1/2”, the concrete
between the ducts is

sufficient to mitigate the bending of the cover

concrete and no
flexural cracking check is

required. Experience has shown that a vertical

stack of
more than three ducts can result in

cracking of the cover concrete. Stacks of ducts

may
be placed one over the other provided there

is more than 1
-
1/2” spacing
between the upper

and lower ducts of the two stacks and the other

provisions of this
specification are satisfied.


Figure C5.10.4.3.1
-
5 illustrates the concept

of an unreinforced cover

concrete
beam to be

investigated for cracking.






Figure C5.10.4.3.1
-
5 Hypothetical Unreinforced

Concrete Cover Beam


The resistance factor has been determined based on past successful practice in
California.


Item #1
0

Insert a new sub
-
article title
between the existing 5
th

and 6
th

paragraphs of Article

5.10.4.3.1
as follows:

5.10.4.3.1c Regional Bending


Insert the following paragraph and r
evise

the 6
th

and 7
th

paragraphs
as follows:


The regional flexural effects of in
-
plane

forces may be taken as:


4




c
in
u
u
h
F
M




(5.10.4.3.1
-
7)


where:


ψ
cont

=

0.6 continuity factor for interior webs
;
0.7 continuity factor for
exterior webs


h
c

=

span of the web between the top and

bottom slabs measured along the
axis of

the web as shown in Figure C5,10.4.3.1
-

2b.


For curved girders, the
global

local

flexural

and shear
effects of out
-
of
-
plane
forces
as

described in Article 5.10.4.3.2

shall be

evaluated


Where curved ducts for tendons other than

those crossing at approximately 90°
are located so

that

the direction of the radial force from one

tendon is toward another,
confinement of the ducts

shall be provided by:




Spacing the ducts to ensure adequate

nominal shear resistance, as
specified in

Eq.
5.10.4.3.1
-
2
;




Providing confinement reinforcement to

resist the radial force; or




Specifying that each inner duct be grouted

before the adjacent outer duct is
stressed.



Item #1
1


Add
C5.10.4.3.1c as follows

(Note: DG disagrees with
ignoring stirrup
reinforcement)
:


When determining tensile
stresses for the

purpose of evaluating the potential
for cracking

of the cover concrete as specified in Article

5.10.4.3.1b, the effect of
regional bending shall

be combined with bending of the local concrete

cover beam.
It is recommended that the effect o
f

stirrups in resisting bending be ignored, and

that the ducts be considered as voids in the

transverse section of the webs. This
approach is

illustrated in the NCHRP 12
-
71 Example

Problem.


The wedging action of strands within the

duct due to vertical cur
vature of
the tendon can

exacerbate tendon pull out resulting from

horizontal curvature of
the tendon as described

in Articles 5.10.4.3.1a and 5.10.4.3.1b.




Item #1
2


Revise Article C5.10.4.3.
2
as follows

(JC asks if there should be some discussion of
ver
tically or offset
-
stacked tendons)
:


Out
-
of
-
plane forces in multi
-
strand, post
-
tensioning

tendons are caused by the
spreading of

the strands or wires within the duct, as shown in

Figure
C1

C5.10.4.3.2
-
1
.
Small out
-
of
-

plane

forces may be resisted by concre
te in shear;

otherwise, spiral
reinforcement is most effective to

resist out
-
of
-
plane forces.
In horizontally curved

bridges, out
-
of
-
plane forces due to the vertical

curvature of tendons should be added
to in
-
plane

forces resulting from horizontal curvatur
e

of the tendons



Item #1
2


Revise Article
5.10.5 External Tendon Supports

as follows:


Unless a vibration analysis indicates

otherwise, the unsupported length of external

tendons shall not exceed 25.0 ft.
External
t
endon

s
upports in curved concrete box

girders shall be

located as to prevent tendons from touching the

interior faces of
webs. When deviation saddles

are required for this purpose, they shall be

designed
in accordance with Article 5.10.9.3.7.



Item #1
3


Revise Article C5.10.5as follows

(modi
fied per D. Goodyear suggestion)
:


Deviation saddles in tightly curved bridges

may

be
considered as
continuous across
the soffit as

recommended by University of Texas research

Beaupre et. al. (1988).



Item #1
4


Revise Article
5.10.9.3.7 Deviation Sa
ddles

as follows:


Deviation saddles shall be designed using the

strut
-
and
-
tie model or using
m
ethods based on test

results.
A load factor of 1.7 shall be used with

the maximum deviation force. If using a method

based on test results, resistance
factors of

0.90

shall be used for direct tension and 0.85 shall be

used for shear.


Revise the last sentence of the Commentary to Article 5.10.9.3.7 as follows

per DG
suggestion
:


Design and detailing guidelines
are

presented in Beaupre et al. (1988)
should
result i
n safe and serviceable design
.


Item #1
5


Revise Article
5.13.2.2 Diaphragms

as follows:


Unless otherwise specified, diaphragms shall

be provided at abutments, piers, and
hinge joints to

resist lateral forces and transmit loads to points of

support.



Int
ermediate diaphragms may be used between

beams in curved systems or where
necessary to

provide torsional resistance and to support the deck

at points of discontinuity
or at right angle points of

discontinuity or at angle points in girders.


For spread box
beams
and for curved box

girders
having an inside radius less than
800 ft.,

intermediate diaphragms shall be used.



Diaphragms should be designed by the strut
-
and
-

tie method, where applicable.


In bridges with post tensioned diaphragms, the

diaphragm ten
dons must be
effectively tied into the

diaphragms with bonded non
-
prestressed

reinforcement to resist
tendon forces at the corners

of openings in the diaphragms.



Item #1
6


Revise Article
C5.13.2.2

as follows:


In certain types of construction, end

diaphr
agms may be replaced by an edge
beam or a

straightened strip of slab made to act as a vertical

frame with the beam ends.
Such types are low I
-
beams

and double
-
T beams. These frames should

be designed for
wheel loads.


The diaphragms should be essentially solid,

except for access openings and utility
holes where

required.


For curved bridges, the need for and the required

spacing of diaphragms depends
on the radius of

curvature and the proportions of the webs and

flanges
.
Some references
have

found that interior

diaphragms contributed very little to the global

behavior of
concrete box girder bridges

(Nutt 2008, UT Austin xxxx)
.



Item #17


Add the following references:


Podolny, J. W. and J.M. Mueller, 1982.
C
onstruction and Design of Prestressed
Concrete segmental Bridges,
Wiley, New your, NY.


Menn, C. 1990
Prestressed Concrete Bridges.
Birkhauser Verlag, Basel, Switzerland


Nutt, R. 2008.
“Design Specifications and Commentary for Horizontally Curved
Con
crete Box
-
Girder Highway Bridges.”
NCHRP Report 620.
Transportation Research
Board, Washington, DC.


OTHER AFFECTED ARTICLES


None

BACKGROUND


For the check of regional stresses, the researchers originally proposed
ci
n
f
f
'
16
.
0


and

= 0.55.
A table of
20

existing curved cast
-
in
-
place post
-
tensioned box girders in
California was compiled with information on

radius, post
-
tensioning force,
existing
stirrups

and duct ties.

A third of these structures were visited to confirm that ther
e were
no signs of existing stress. Then, calculations were done using the methodology as
proposed by the researchers.
S
tresses in
5
successful
bridges
without duct ties
greatly
exceeded

the capacity
ci
n
f
f
'
16
.
0


meaning that

phi should have a

value > 1.0.

Instead,
the appropriate value for phi was derived by comparing
ci
n
f
f
'
24
.
0

to
stresses in 5 successful bridges without duct ties
.

It was found that there was a reserve of
approximately 15%. Thus


= 0.85 is being recommended.



ANTICIPATED EFFECT ON BRIDGES


Mr. Nutt presented his work at the Western Bridge Engineers
’ Seminar in Sacramento
Sept. 2009. When asked of the anticipated effect on bridges, he responded minimal.
This differed from chec
ks done by several engineers at Caltrans and so the above changes
were made for implementation, still using the proposed methodology.


REFERENCES


NCHRP
Report 620



Design Specifications and Commentary for Horizontally Curved
Concrete Box Girder
Highway Bridges

________________________________________________________________________

Lead State: WA, TX, CA

Industry:

FHWA: