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Louisiana Transportation Research Center

Sponsored jointly by the Louisiana Department ofTransportation and Developmentand Louisiana State University

4101 Gourrier AvenueBaton Rouge, LA 70808-4443www.ltrc.lsu.edu

Continuity Diaphragm for Skewed Continuous Span

Precast Prestressed Concrete Girder Bridges

I n t r o d u c t i o n

O b j e c t i v e s

Scope

LTRCProject No. 01-1ST

Principal Investigator: Aziz Saber, Ph.D., P.E.

Louisiana Tech University

LTRC Contact: Walid Alaywan, P.E.

Phone (225) 767-9106

March 2004

Technical Summary

Report 383

M ost highway bridges ar e built a s cast-in-place

reinforced concrete sla bs an d pr estressed concrete

girders. The shear connectors on th e top of th e gird-

ers as sur e composite action between t he slabs an d

girders . The design guidelines for bridges in

AASHTO Section 8.12 indicat e th at diaphr agms

should be installed for T-girder s pans and may be

omitted where structural analysis shows adequate

strength. Furthermore, the effects of diaphragms are

not a ccoun ted for in pr oport ioning t he girders.

Therefore, the u se of diaphr agms sh ould be investi-

gated.

Continuity diaphragms used in prestressed girder

bridges on skewed bents cause difficulties in detail-

ing and const ruction. Details for bridges with sm all

skew an gles (

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Resea rch App r oach

C o n c l u s i o n s

NOTICE: This technical summary is disseminated under

the sponsorship of the Louisiana Department of

Transportation and Development in the interest of infor-

mation exchange. The summary provides a synopsis of

the projects final report. The summary does not estab-

lish policies or regulations, nor does it imply DOTD

endorsement of the conclusions or recommendations.

sta tes. The ana lysis concentr at ed on th e effects of

th e following para meter s: girder type, bridge skew

an gle, girder spacing, span length, and diaph ra gm

condition.

F inite element m odeling is among the most popularmet hods of an alysis. Significant ad van ces in com-

put er t echnology allow complicat ed models t o be con-

str ucted and ana lyzed. The finite element model

used in t his investigation simu lated t he beha vior of

skewed continuous span bridges. The girders were

modeled using Tridimensional Elements available in

GT STRUDL. Plat e Elements were used for the

bridge deck. Pr ismat ic members were used to model

th e continuity diaphr agms an d the conn ection

between the deck plat e element s and t he girder ele-

ments .

The resear ch encompassed a review of literat ur e and

current use of contin uity diaphr agms. A technology

and practice state-of-the-art review was conducted

th rough a literat ur e search and su rvey of bridge

engineers from other states. The litera tu re search

was conducted first . Design procedures , const ru ction

pra ctices, maint enan ce, an d st ates limit of pra ctice

were t he focus of th is review.

A survey was designed an d ma iled to out -of-sta tebridge engineers, and th e results of th e litera tu re

search were combined with the following bridge

para meters: mat erial properties, relative dimensions

of girders and slabs, bridge geometry, skew angle,

girder spacing, span length, num ber of spans , type of

loading, locat ion an d st iffness of diaphr agms, an d

location of sup ports . The design of th e survey was

based on modern m eth odologies of surveying and

sensing.

The bridge param eters considered in th e ana lysisincluded sk ew angle, length of th e bridge span, beam

spacing, the ratio of beam spacing to span (aspect

ra tio), an d t he r at io of the girders st iffness to slab

stiffness. The effects of diaph ra gms on moment s

from tr uck an d lane loading on cont inuous slab an d

girder bridges were studied.

The para meters t hat may affect th e load distribut ion

of a br idge can be divided int o five main categories:

mat erial properties of the slabs an d girders, relative

dimensions of the girders and slabs, geometr y of the

bridge, type of loading on th e bridge, location an d

stiffness of diaphra gms, an d t he locat ion of supports.

A Design of Experiments (DOE) was conducted to

identify the factors th at contribut e the most to the

str uctur al performa nce of contin uity diaphr agms.

The DOE was based on Taguchi and var ious other

techniques. DOE reduced the number of par amet ricstudies (num erical or ph ysical) needed to reach con -

vincing conclusions a bout a given pr oducts per form-

an ce. DOE also helped discover an inter action

among these factors. Based on th e DOE an alysis, a

determ inistic finite element a na lysis th at used elabo-

ra te t hr ee-dimensional models of skewed bridges

including slab, girders, supports and diaphragms,

was performed using GT STRUDL. A mu lti-scale

an alysis was condu cted to allow th e use of a refined

model of the continuity diaphr agm while run ning

cost-effective finite element analyses.

T he resu lts of finite element an alyses performed in

this st udy indicated th at th e effects of continuity

diaphragms in skewed continuous span precast pre-

str essed concrete girder s were minor. However, in

order to implement t he findings, it is r ecommended

tha t laboratory tests an d field measu rement s be con-

sidered to verify th e th eoretical results.

Note : This study dealt with the first a nd second

par ts of th e objective, but t he resu lts did not warra nt

addressing the th ird and fourt h par ts . Furt hermore,

the r esults ma y not be conclusive an d implementa ble

since the stu dy encompassed only the t heoretical

aspect. Laborat ory test s an d field measur ements will

be initiated in t he n ext fiscal year. If those results do

not confirm th e findings from th is study, the third

and fourth parts of the objective will then be

addressed.