simplified analytical model of a covered burr- arch-truss ... · • bottom chord splice joints •...

Civil and Construction Engineering Iowa State University

Fouad Fanous, Douglas Rammer & Terry Wipf

Iowa State University, Bridge Engineering Center

Forest Product Laboratory

Simplified Analytical Model of a Covered Burr-

Arch-Truss Timber Bridge


Objective • Develop a simple analytical model

• Approximately predicts behavior

• Assist in load rating calculations

• Include as built characteristics – eccentric connections, splice joints, material properties, etc.

Finite Element Analysis • Development of 2-D and 3-D finite element models for each bridge

Selected Bridges • Indiana & Vermont

• Burr-Arch and Queen-Post Truss Bridges

Recommendation • From comparison of displacement and strain values of field and analytical – recommend

appropriate modeling approach

Study Scope

Bridge Descriptions

3

Zacke Cox Bridge

Views of the Zacke Cox Bridge


Elevation View


Views of the

Zacke Cox Bridge – Cont.

Glulam Floor Beam

Double Bottom

Chord

Top Diagonal Bracing

South

Truss

North

Truss

Concentric

Arches

Cross Sectional View

Plan View

Bridge Descriptions

6

• Bottom Chord Splice Joints

• Single headed hook fishplate and iron shoe splice joint (Marston (2006))

• Total of 4 splice joints – 2 within each truss element

• Field Measured Timber Dimensions

Structural

Member

Base length

(in)

Height Length

(in)

Bottom Chord 5.5 11.5

Floor Beam 10.5 13.8

Verticals 7.5 9.5

Diagonals 7.5 7.5

Arch 4.75 9.5

Top Chord 7.5 9.8

Centerline of Bolts

Bridge Schematics – Strain Gage Locations

Strain sensor locations Strain sensor details

South Truss


1112

4785 6084

1882

4815 1967

Strain Gage Locations – Near the bottom

Splice

Top View

Bottom View

Finite Element Analysis

9

ANSYS - Software

• Boundary Conditions

• Truss

• Arch

• Truck Loading

• Small Truck

Back

Axle

Front

Axle Back

Axle

% Front

Axle

% Front

Axle

Different modeling

2-D model Ignoring joint eccentricities

2-D model considering joint eccentricities

2-D model Including joint eccentricities

and splice joints

2-D model Including joint eccentricities

splice joints and the as built top chord

Results

11

Small Truck

• Deflection in the Vicinity of the Splice Joint

• Analytical deflected shapes

• Discontinuous member – top (tension) and bottom (compression)

• Continuous member – top (compression) and bottom (tension)

• Deflection and Deformation

Analysis using STAAD Software

Measured and Calculated Deflection

Front Axel 44.75 kN Front Axel 68.7 kN

Distance 3.07 m

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0 10 20 30 40 50 60

Dis

pla

ce

me

nt

(in

.)

Distance of front axle from west abutment (ft)

Measured Displacement N-Truss

Measured Displacement S-Truss

Analytical Displacement

Average Measured Displacement N & S Trusses

Deformed Shape

Back

axle

Front

axle

Splice joint 1

Splice joint 2

Concave

Downward

Strain Comparison

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

20.00

40.00

60.00

80.00

0.00 20.00 40.00 60.00 80.00

Mic

ro s

trai

n

Distance of front axle from end of abutment (ft)

Gage No.4821

Gage No. 4781

Staad 3-D gage no. 4781


Diagonal member - Gages 4821 and 4781

-150.00

-100.00

-50.00

0.00

50.00

100.00

0.00 20.00 40.00 60.00 80.00

Mic

ro s

trai

n


Field test gage no. 4692

Field test gage no. 4782



Strain Comparison –Cont.

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.00 20.00 40.00 60.00 80.00

Mic

ro s

trai

n


Field test gage B1112

Field test gage B1882

Staad 3-D gage No. B1112

Staad 3-D gage No. B1882

Splice Joint – Gages B1112 and B1882

Strain Comparison –Cont.

Source of Discrepancies

Data Collection Method

Modeling

Member Conditions

Material Properties


Source of Discrepancies – Cont.

Load Distribution

Geometric Irregularities

1. Out of plan

2. Sag of the bottom Chord

3. Connections

Summary & Conclusions

• 2-D and 3-D Analysis

• ANSYS & STAAD

• Analytical Vs. Field Test Results

• Factors affecting Analysis Accuracy

• Splice Joints

• Member Conditions

• Joint Eccentricities

Acknowledgement

Douglas Wood

Allison Lunde

Graduate Students


This study is part of the Research, Technology and Education portion of the National Historic

Covered Bridge Preservation (NHCBP) Program administered by the Federal Highway

Administration. The NHCBP program includes preservation, rehabilitation and restoration of

covered bridges that are listed or are eligible for listing on the National Register of Historic Places;

research for better means of restoring, and protecting these bridges, development of educational

aids; and technology transfer to disseminate information on covered bridges in order to preserve

the Nation's cultural heritage.

This study is conducted under a joint agreement between the Federal Highway Administration -

Turner Fairbank Highway Research Center, and the Forest Service - Forest Products Laboratory.

Federal Highway Administration Program Manager - Sheila Rimal Duwadi, P.E.

Forest Products Laboratory Program Manager - Michael A. Ritter, P. E.

Thanks

Questions

–?


simplified analytical model of a covered burr- arch-truss ... · • bottom chord splice joints •...

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