bridge seismic design. automated seismic design of bridges aashto lrfd seismic design - csibrigde

7/15/2019 Bridge Seismic Design. Automated Seismic Design of Bridges AASHTO LRFD Seismic Design - CSIBrigde

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ISO BRG083110M3Berkeley, California, USA

Version 15August 2010

CSiBridge

Bridge Seismic DesignAutomated Seismic Design of Bridges

AASHTO Guide Specification forLRFD Seismic Bridge Design


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DISCLAIMER

CONSIDERABLE TIME, EFFORT AND EXPENSE HAVE GONE INTO THE

DEVELOPMENT AND TESTING OF THIS SOFTWARE. HOWEVER, THE USER

ACCEPTS AND UNDERSTANDS THAT NO WARRANTY IS EXPRESSED OR

IMPLIED BY THE DEVELOPERS OR THE DISTRIBUTORS ON THE ACCURACY

OR THE RELIABILITY OF THIS PRODUCT.

THIS PRODUCT IS A PRACTICAL AND POWERFUL TOOL FOR STRUCTURAL

DESIGN. HOWEVER, THE USER MUST EXPLICITLY UNDERSTAND THE BASIC

ASSUMPTIONS OF THE SOFTWARE MODELING, ANALYSIS, AND DESIGN

ALGORITHMS AND COMPENSATE FOR THE ASPECTS THAT ARE NOT

ADDRESSED.

THE INFORMATION PRODUCED BY THE SOFTWARE MUST BE CHECKED BYA QUALIFIED AND EXPERIENCED ENGINEER. THE ENGINEER MUSTINDEPENDENTLY VERIFY THE RESULTS AND TAKE PROFESSIONALRESPONSIBILITY FOR THE INFORMATION THAT IS USED.


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i

Contents

Bridge Seismic Design

Foreword

Step 1 Create the Bridge Model

1.1 Example Model 1-1

1.2 Description of the Example Bridge 1-2

1.3 Bridge Layout Line 1-4

1.4 Frame Section Property Definitions 1-4

1.4.1 Bent Cap Beam 1-5

1.4.2 Bent Column Properties 1-5

1.4.3 I-Girders Properties 1-6

1.4.4 Pile Properties 1-7

1.5 Bridge Deck Section 1-8

1.6 Bent Data 1-8

1.7 Bridge Object Definition 1-10

1.7.1 Abutment Property Assignments 1-11

1.7.2 Abutment Geometry 1-13


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CSiBridge Seismic Design

ii

1.7.3 Bent Property Assignments 1-14

1.7.4 Bent Geometry 1-151.8 Equivalent Pile Formulation 1-16

1.9 Bent Foundation Modeling 1-16

1.10 Mass Source 1-17

Step 2 Ground Motion Hazard and Seismic Design Request

2.1 Overview 2-1

2.2 AASHTO and USGS Hazard Maps 2-1

2.3 Seismic Design Request 2-3

2.4 Perform Seismic Design 2-7

2.5 Auto Load Patterns 2-7

2.6 Auto Load Cases 2-8

Step 3 Dead Load Analysis and Cracked Section Properties

Step 4 Response Spectrum and Demand Displacements

4.1 Overview 4-1

4.2 Response Spectrum Load Cases 4-1

4.3 Response Spectrum Results 4-4

Step 5 Determine Plastic Hinge Properties and Assignments

5.1 Overview 5-1

5.2 Plastic Hinge Lengths 5-1

5.3 Nonlinear Hinge Properties 5-3

5.4 Nonlinear Material Property Definitions 5-6

5.4.1 Nonlinear Material Properties Definitions

For Concrete 5-6

5.4.2 Nonlinear Material Properties Definitions

For Steel 5-8

5.5 Plastic Hinge Options 5-9


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Contents

iii

Step 6 Capacity Displacement Analyses

6.1 Displacement Capacities for SDC B and C 6-2

6.2 Displacement Capacities for SDC D 6-3

6.3 Pushover Results 6-6

Step 7 Demand/Capacity Ratios

Step 8 Review Output and Create Report

8.1 Design 01 D/C Ratios 8-2

8.2 Design 02 Bent Column Force Demand 8-2

8.3 Design 03 Bent Column Idealized MomentCapacity 8-2

8.4 Design 04 Bent Column Cracked Section

Properties 8-3

8.5 Design 05 Support Bearing Demands Forces 8-3

8.6 Design 06 Support Bearing Demand

Displacements 8-4

8.7 Design 07 Support Length Demands 8-5

8.8 Create Report 8-5

References


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iv

List of Figures

Figure 1-1 3D View of Example Model 1-1Figure 1-2 Example Bridge Elevation 1-2Figure 1-3 Example Bridge Plan 1-3Figure 1-4 Example Bridge BENT1 Elevation 1-3Figure 1-5 3D Bridge Layout Line Data 1-4Figure 1-6 3D Cap Beam Section Property Definition 1-5Figure 1-7 Bent Column Property Definition 1-6

Figure 1-8 Precast I-Girder Properties 1-6Figure 1-9 Pile Properties 1-7Figure 1-10 Bridge Deck Section Properties 1-8Figure 1-11 Bridge Bent Data 1-9Figure 1-12 Bent Column Data 1-9Figure 1-13 Bent Column Base Restraint Definitions 1-10Figure 1-14 Bridge Object Data form 1-11Figure 1-15 Abutment Property Definitions 1-12Figure 1-16 Abutment Bearing Properties 1-13Figure 1-17 Abutment Bearing Geometry 1-13Figure 1-18 Bent Assignments form 1-14Figure 1-19 Bent Bearing Data 1-15Figure 1-20 Bent Support Geometry 1-15Figure 1-21 Equivalent Pile Properties 1-16Figure 1-22 View of Bent Foundations 1-17Figure 1-23 Bent Column Base Connectivity 1-17Figure 1-24 Mass Source Definition 1-18


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Contents

v

Figure 2-1 AASHTO/USGS Hazard Maps used to determine

the Demand Response Spectrum 2-2

Figure 2-2 Response Spectrum Function Data form 2-2Figure 2-3 Bridge Design Request form 2-3Figure 2-4 Seismic Design Parameters form 2-4Figure 2-5 Perform Seismic Design 2-7Figure 2-6 Auto Load Patterns 2-7Figure 2-7 Auto Load Cases 2-8Figure 3-1 Auto Stage Construction Load Case used to apply

Cracked Section Property Modifiers 3-2Figure 4-1 U1 Direction Response Spectrum Load

Case form 4-2Figure 4-2 ABS Response Spectrum Load Case form 4-3Figure 4-3 BENT1 Displacements for the three

Auto-Defined Response Spectrum Load cases 4-3Figure 4-4 Modal Load Case Definition 4-4

Figure 5-1 Hinge Locations 5-2Figure 5-2 Hinge Locations 5-3Figure 5-3 Moment Curvature Diagram 5-4Figure 5-4 Auto Hinge Assignment Data 5-5Figure 5-5 Sample Hinge Data form 5-5Figure 5-6 Nonlinear Material Data form for Concrete 5-6Figure 5-7 Nonlinear Stress-Strain curves for Confined

and Unconfined Concrete 5-7Figure 5-8 Concrete Model - Mander Confined 5-7Figure 5-9 Nonlinear Material Data form for steel 5-8Figure 5-10 Nonlinear Stress-Strain Plot for steel 5-9Figure 5-11 Plastic Hing Fiber option 5-10


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Figure 5-12 Section Designer options 5-10

Figure 6-1 Rectangular Beam Design 6-1Figure 6-2 Design Requirements for SDC 6-2

Figure 6-2 BENT1 Transverse Pushover Load Case 6-4Figure 6-3 BENT1 Application of Property Modifiers

and Dead Loads to BENT1 6-5Figure 6-4 BENT1 Pushover Load Pattern for the

Transverse Direction 6-6

Figure 6-5 Display of BENT1 Pushover Curves 6-7Figure 7-1 D/C Displacement Ratios 7-1


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vii

Foreword

Over the past thirty-five years, Computer and Structures, Inc, has introduced

new and innovative ways to model complex structures. CSiBridge, the latest

innovation, is the ultimate integrated tool for modeling, analysis, and design of

bridge structures. The ease with which all of these tasks can be accomplished

makes CSiBridge the most versatile and productive bridge design package in

the industry.

Automated seismic design, one of CSiBridges many features, incorporates the

recently adopted AASHTO Guide Specification for LRFD Seismic Bridge

Design. CSiBridge allows engineers to define specific seismic design pa-rameters that are then applied to the bridge model during an automated cycle of

analysis through design.

Now, users can automate the response spectrum and pushover analyses. Fur-

thermore, the CSiBridge program will determine the demand and capacity dis-

placements and report the demand/capacity ratios for the Earthquake Resisting

System (ERS). All of this is accomplished in eight simple steps outlined as fol-

lows:

1. Create the Bridge Model2. Evaluate the Ground Motion Hazard and the Seismic Design Request3. Complete the Dead Load Analysis and evaluate the Cracked Section Properties4. Identify Response Spectrum and Demand Displacements


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viii Foreword

5. Determine Plastic Hinge Properties and Assignments6. Complete Capacity Displacement Analysis7. Evaluate Demand/Capacity Ratios8. Review Output and Create ReportA detailed explanation of each of the steps is presented in the chapters that fol-

low. The example bridge model shown in the figure illustrates the CSiBridge

Automated Seismic Design features.

Schematic of the Eight Steps in the

Automated Seismic Design of Bridges using CSiBridge


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Example Model 1 - 1

STEP 1Create the Bridge Model

1.1 Example ModelThis chapter describes the first step in the process required to complete a Seis-

mic Design Request for a bridge structure using CSiBridge. It is assumed the

user is familiar with the requirements in the program related to creating a

Linked Bridge Object. Only select features of the model development are in-

cluded in this chapter. The CSiBridge model used throughout this manual isavailable and includes all of the input parameters.

Figure 1-1 3D View of Example Model


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1 - 2 Description of the Example Bridge

As described in the AASHTO Guide Specifications for LRFD Seismic Bridge

Design, the seismic design strategy for this bridge is Type 1 Design a ductilesubstructure with an essentially elastic superstructure. This implies that the de-

sign must include plastic hinging in the columns.

1.2 Description of the Example BridgeThe example bridge is a three-span concrete I-girder bridge with the following

features:

Piles: 14-inch-diameter steel pipe pile filled with concrete. The concrete is re-

inforced with six #5 vertical bars with three #4 spirals having a 3-inch pitch.

Pile Cap: The bent columns are connected monolithically to a concrete pile capthat is supported by nine piles each. The pile caps are 13-0 x 13-0 x 4-0

Bents: There are two interior bents with three 36-inch-diameter columns.

Deck: The deck consists of five 3-3-deep precast I-girders that support an

8-inch-thick deck and a wearing surface (35 psf). The deck width is 35'-10"

from the edge-of-deck to edge-of-deck.

Spans: Three spans of approximately 60-0.

The abutments are assumed to be free in both the longitudinal and transverse

directions.

Figure 1-2 Example Bridge Elevation


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STEP 1 - Create the Bridge Model

Description of the Example Bridge 1 - 3

Figure 1-3 Example Bridge Plan

24

'-10"

24

'-10"

Figure 1-4 Example Bridge BENT1 Elevation


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1 - 4 Bridge Layout Line

1.3 Bridge Layout LineThe example model has three spans of approximately 60 feet each. The layout

line is defined using the Layout > Layout Line > New command and the

Bridge Layout Line Data form shown in Figure 1-5. The layout line is straight,

with no variation in elevation. The actual length of the layout line is 178.42 ft.

Figure 1-5 3D Bridge Layout Line Data

1.4 Frame Section Property DefinitionsFour frame section properties must be described by the user to develop the ex-

ample model. The four types of frame elements used in the example model

consist of a pile, bent cap beam, bent column, and precast concrete I-Girder.

The section property definition for each of the elements is given in the subsec-

tions that follow.


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Frame Section Property Definitions 1 - 5

1.4.1 Bent Cap BeamThe bent cap beams were defined using the Components > Type > Frame

Properties > Expand arrow command. The Add New Property button >

Frame Section Property Type: Concrete > Rectangular was used to add the fol-

lowing concrete rectangle:

Figure 1-6 3D Cap Beam Section Property Definition

The material property used was 4000 psi. Note that the units shown in Figure

1-6 are in inches. (To check this, hold down the Shift key and double click in

the Depth or Width edit box. This will display the CSiBridge Calculator.)

1.4.2 Bent Column PropertiesThe bent columns were defined using the Section Designer option that can be

accessed using the Components > Type > Frame Properties > New > Other

> Section Designer command. The size and quantity of both the vertical and

confinement reinforcing steel were defined using the form shown in Figure 1-7.

Further discussion of the column section properties as they pertain to the plas-

tic hinge definitions is provide in Step 5.


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1 - 6 Frame Section Property Definitions

Figure 1-7 Bent Column Property Definition

1.4.3 I-Girder PropertiesThe I-Girder properties were

input using inch units, as

shown in Figure 1-8. (Again,

check this by holding down

the Shift key and double

clicking in a dimension edit

box to display the CSiBridge

Calculator.)

Figure 1-8 Precast I-Girder

Properties


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Frame Section Property Definitions 1 - 7

1.4.4 Pile PropertiesThe piles were defined as 14inch-diameter concrete piles with six #9 vertical

bars (Components > Type > Frame Properties > New > Concrete > Circu-

lar command). The outer steel casings of the pile were found to increase in the

flexural stiffness of the piles by a factor of 2.353. This value was applied as a

property modifier to the pile section property. The pile will be added to the

bridge model as Equivalent Cantilever piles, as shown in Figure 1-9 and as

described in subsequent Section 1.8. Using this method, the pile is replaced by

a beam that has equivalent stiffness properties to that of the pile with the sur-

rounding soil.

Figure 1-9 Pile Properties


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1 - 8 Bridge Deck Section

1.5 Bridge Deck SectionThe bridge deck section is 38.833 feet wide with a total of five

I-girders, as shown in Figure 1-10 (Components > Superstructure Type >

Deck Section > New command). The parapets as well as the wearing surface

are not part of the bridge deck structural definition but will be added to the

bridge model as superimposed dead loads (SDEAD).

Figure 1-10 Bridge Deck Section Properties

1.6 Bent DataThe bents for the subject model have three columns, each with a cap beam

width of 38.25 feet. The Bridge Bent data form shown in Figure 1-11 is used to

input the number of columns and the cap beam width. Since multiple columns

are specified, the location, height and support condition for each column needs

to be specified using the Bent Column Data form, which is accessed using the

Components > Substructure Item > Bents > New command.


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Bent Data 1 - 9

Figure 1-11 Bridge Bent Data

After the Modify/Show Column Data button is used, the Bent Column Data

form shown in Figure 1-12 can be used to define the type, location, height, an-

gle and boundary conditions for each bent column.

Figure 1-12 Bent Column Data

An important part of this example model is the inclusion of the foundation ele-ments. Although the foundations can be represented as Fixed, Pinned, or

Spring-Support restraints at the base of the columns, these have been explicitly

modeled in this example. It is important to note that when foundation objects


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1 - 10 Bridge Object Definition

are part of the bridge model, the base of the bent column must not be re-

strained, but instead, connected to the foundation elements. Restraining thebase of the columns in the Bent Column Data form using Fixed or Pinned re-

straints would prevent the bridge loads from reaching the foundation. In this

example, a foundation spring (BFSP1) having no stiffness in any direction is

used as the Base Support data. After the foundations have been modeled and

connected to the bent column bases, support of the bent columns will be

achieved. The Foundation Spring Data form is shown in Figure 1-13. Access

this form by clicking the Foundation Spring Properties button on the Bridge

Bent Column Data form and then the Add New Foundation Spring button on

the Define Bridge Foundation Springs form, or by using the Components >

Substructure Item > Foundation Springs > New command.

Figure 1-13 Bent Column Base Restraint Definitions

1.7 Bridge Object DefinitionThe Bridge Object Data form (click the Bridge > Bridge Object > New com-mand) is used to define the location and bearing property assignments of the

abutments and bearings. The seismic response of the bridge model will depend

on the Earthquake Resisting System (ERS). The user can define the types of


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Bridge Object Definition 1 - 11

support conditions at the abutments and bents. The ERS will depend on the

types of supports used at the abutments and bents and the bearing propertiesthat are used for each. If a bearing has a restrained DOF, it will provide a load

path that will act as part of the bridge ERS. Abutments can be defined using

bents as supports (this feature was not used in the subject example).

Figure 1-14 Bridge Object Data form

The span data is used to define the span lengths and bent locations. Cross dia-

phragms also can be included in a bridge model using the Modify/Show As-

signments > In Span Cross Diaphragms command and Modify/Show button.

No cross diaphragms were used as part of the example model.

1.7.1 Abutment Property AssignmentsBoth the start and end abutment assignments are defined using the Bridge Ob-

ject Abutment Assignment form shown in Figure 1-15 (Bridge > Bridge Ob-

ject > Supports > Abutments). The abutment bearing direction can be as-

signed a bearing angle if skewed abutments are needed. Diaphragms can be

added to the abutment as well. Abutments can be modeled using bents by se-

lecting Bent Property in the Substructure Assignment area of the Bridge Ob-


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ject Abutment Assignment form. After that selection has been, an option is

available to select the appropriate property definition from a list of previouslydefined bent properties.

Figure 1-15 Abutment Property Definitions

The substructure location is critical because CSiBridge accounts for the super-

structure/substructure kinematics. The ends of the bridge deck will have a ten-

dency to rotate due to gravity loading. If the abutment bearings are restrained

against translation at both ends of a bridge, outward reactions on the bearings

and deck moments can be induced as a result of these restraints. The amount ofoutward thrust and the moment in the deck are a function of the amount of ro-

tation and distance from the deck neutral axis to the top of abutment bearings.

Therefore, the user should pay special attention to the substructure and bearing

elevations as well as the bearing restraint properties. The user also must keep in

mind that the seismic resisting load path is dependent on the restraint properties

of the bearing at both abutments and bents.

For this example, only the vertical translation of the abutment bearings was set

to Fixed. All other abutment bearing components were set to Free since the

abutment restraint was assumed to be free in the longitudinal and transverse di-

rections. See Figure 1-16 (display this form by clicking the + plus beside the

Bearing Property drop-down list on the Bridge Object Abutment Assignmentsform and the Add New Bridge Bearing or Modify/Show Bridge Bearing but-

ton on the Define Bridge Bearings form).


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Figure 1-16 Abutment Bearing Properties

To help visualize the abutment geometry, the graphic shown in Figure 1-17 in-

cludes the values in the example model to define the location of the abutment

bearings and substructure. It should also be noted that the CSiBridge program

automatically includes the BFXSS Rigid Link when the bridge object is up-

dated.

1.7.2 Abutment GeometryFigure 1-17 also shows the location of the BRG1 action point. This is the loca-

tion where the bearing will translate or rotate depending on the bearing defini-

tions.

Figure 1-17 Abutment Bearing Geometry


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1.7.3 Bent Property AssignmentsThe bent property assignments are made using the Bridge Object Bent As-

signment form, shown in Figure 1-18 (Bridge > Bridge Object > Supports >

Bents command). Similar to the abutment property assignments, the bent prop-

erty assignments will include the bent directions, bearing properties, and sub-

structure locations.

Figure 1-18 Bent Assignments form

For this example model, the bearing properties at the bents have fixed transla-

tion restraints in all directions but free restraints for all rotational directions.

See Figure 1-19 (click the + plus beside the Bearing Property drop-down list;

click the Modify/Show Bridge Bearing button on the Define Bridge Bearings

form).


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Figure 1-19 Bent Bearing Data

1.7.4 Bent GeometryThe bent geometry is shown in Figure 1-20 for the input values used to define

the bearing and substructure elevations from the Bridge Object Bent Assign-

ment form (Figure 1-18).

Figure 1-20 Bent Support Geometry


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1 - 16 Equivalent Pile Formulation

Note that the BRG2 connects to the center of the cap beam. The substructure

elevation is used to define the top of the cap beam. The action point of BRG2is at Elevation -49.0.

1.8 Equivalent Pile FormulationAlthough it is not required to include explicit foundation elements (foundations

can be modeled as fixed, pinned or partially fixed restraints at the base of the

columns), these were included as part of the example model. Foundations can

be modeled in many ways. Equivalent length piles were used with an equiva-

lent length of 5.1 feet to model the pile surrounded by soil, as described in Sec-

tion 1.4.4. The equivalent lengths were established using the equations shown

in Figure 1-21.

Point of fixity

3K EI T

K EI T

2K EI T

f = yield calculated from an average

spt blow countN.

T = 5.1 feet; this effective length is

used in modeling the bridge foundation.

1 5

EI fPoint of fixity

3K EI T

K EI T

2K EI T

f = yield calculated from an average

spt blow countN.

T = 5.1 feet; this effective length is

used in modeling the bridge foundation.

1 5

EI f

Figure 1-21 Equivalent Pile Properties

After the lengths of the piles were known, the piles were connected to an area

object representing the pile cap. The cap was meshed at the top of the pile loca-

tions. The completed pile cap appears in Figure 1-22, which is shown using a

3D extruded view.

1.9 Bent Foundation ModelingThe next and critical step in the model definition is to connect the foundation to

the base of the bent columns. For this example, joint constraints were used asillustrated in Figure 1-23. This method of connecting the column base to the

foundation preserves connectivity even when updating the linked bridge model.


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Mass Source 1 - 17

Figure 1-22 View of Bent Foundations

Figure 1-23 Bent Column Base Connectivity

1.10 Mass SourceThe Mass Source definition (Advanced > Define > Mass Source) is used to

define the mass and loads to be included in the modal and response spectrum

load cases. In this example, the combined weight of the parapets and wearing

surface was approximated as 2.0 kips per linear foot acting along the bridge

deck. A load pattern was added as a superimposed type with the name SDEAD

Column-to-Foundation Connection


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1 - 18 Mass Source

(Loads > Load Patterns). Using the option, From Element and Additional

Masses and Loads, the program will calculate the self weight of the bridgestructure and include that mass along with the mass derived from the SDEAD

load assignment.

Figure 1-24 Mass Source Definition


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Overview 2 - 1

STEP 2Ground Motion Hazard and Seismic Design Request

2.1 OverviewThe ground motion hazard (response spectrum) can be determined by CSi-

Bridge by defining the bridge location using the latitude and longitude or the

postal zone. As an alternative, the user can input any user defined response

spectrum file. The site effects (soil site classifications) also are considered and

are part of the user input data.

2.2 AASHTO and USGS Hazard MapsThe recently adopted AASHTO Guide Specification for the LRFD Seismic

Bridge Design incorporates hazard maps based on a 1000-year return period.

When the user defines the bridge location by Latitude and Longitude, CSi-

Bridge creates the appropriate response spectra curve as follows:


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2 - 2 AASHTO and USGS Hazard Maps

Figure 2-1 AASHTO/USGS Hazard Maps used to determine the Demand Response Spectrum

Figure 2-2 Response Spectrum Function Data form

From the Response Spectrum Data form (Loads > Functions > Type > Re-

sponse Spectrum > New > NCHRP 20-07), the values for SDS

and SD1

are de-


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STEP 2 - Ground Motion Hazard and Seismic Design Request

Seismic Design Request 2 - 3

termined by CSiBridge and reported. The SD1

value is used to determine the

Seismic Design Category (SDC). The SDC is used to determine the analysisand design requirements to be applied to the bridge. For example, if the SDC is

A, no capacity displacement calculation is performed. If the SDC is B or C,

CSiBridge uses an implicit formula (see Section 4.8 of the AASHTO Seismic

Guide Specification). If the SDC is D, CSiBridge uses a nonlinear pushover

analysis to determine the capacity displacements.

2.3 Seismic Design RequestThe Design/Rating > Seismic

Design > Design Request >

Add New Request command

accesses a form that can be used

to specify the name and design

request parameters for a Seis-

mic Design Request. The form

is shown in Figure 2-3.

Figure 2-3 Bridge Design

Request form

For this example, clicking the Modify/Show button will display the Substruc-

ture Seismic Design Request Parameters form, shown in Figure 2-4. A briefdescription of the parameters on that form follows.

Item Substructure Seismic Design Request Parameter

1 Response Spec-trum Function -Horizontal

After a response spectrum function has been defined (see Section 2.2),the name of the response spectrum to be used for a specific SeismicDesign Request should be selected here.

2 Response Spec-trum Function -Vertical

After a response spectrum function has been defined (see Section 2.2),the name of the vertical response spectrum to be used for a specificSeismic Design Request should be selected here. None should beselected if no vertical response spectrum is to be included in the seismicdesign request

3 Seismic DesignCategory (SDC)Option

The user can choose to have the SDC be selected by the program (i.e.,Programmed Determined), or the user can impose a value for the SDC(i.e., User Defined). To impose a value, select it from Item 4, the Seis-mic Design Category.


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2 - 4 Seismic Design Request

Figure 2-4 Seismic Design Parameters form


4 Seismic DesignCategory

If the user has opted to specify the Seismic Design Category in Item 3,the user must specify the Seismic Design Category here as B, C or D.

5 Bent Displace-ment DemandFactor

This is a scale factor. The bent displacement demands obtained fromthe response-spectrum analysis are multiplied by this factor. It can beused to modify the displacement demand due to a damping valueother than 5%, or to magnify the demand for short-period structures.This factor will be applied to all bents in both the longitudinal andtransverse directions.

6 Gravity LoadCase Option

The user can specify which gravity load case is used to determine thecracked section properties for the bent columns. The choices includeAuto-Entire Structure, Auto This Bridge Object, or User Defined. As adefault, all Dead and Super Dead loads are included in the Auto-EntireStructure gravity load case.

7 Gravity LoadCase

If the User Option is selected for Item 6 Gravity Load Case Option, thegravity load case name must be selected here.


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Seismic Design Request 2 - 5


8 AdditionalGroup

If the Auto-This Bridge Object option is selected for Item 6 Gravity LoadCase Option, an additional group can be included in the gravity loadcase. This item is required only when the gravity load case is programdetermined. It may include pile foundations and other auxiliary struc-tures.

9 Include P-Delta If P-Delta Effects are to be included, the user needs to specify yes here.P-Delta effects will cause a more abrupt drop in the pushover curveresults if an idealized bilinear hinge has been assigned to the bent col-umns. It is recommended that an initial Seismic Design Request beperformed before including the P-Delta effects to help the user under-stand the nonlinear behavior of the bents.

10 CrackedProperty

Option

The cracked section properties for the bent columns can be automati-cally determined by the program or they can be user defined. If pro-

gram determined, the automatic gravity load case will be run iteratively.Section Designer will use the calculated axial force at the top and bot-tom on the column to determine the cracked moments of inertia in thepositive and negative transverse and longitudinal directions. The aver-age of the top and bottom column cracked properties will be applied asnamed property modifier sets and the analysis will be re-run to makesure the cracked-modified model converges to within the specifiedtolerance.

11 ConvergenceTolerance

This value sets the relative convergence tolerance for the bent-columncracked-property iteration. This item is required only when the cracked-property calculation is program determined.

12 MaximumNumber ofIterations

This value sets the maximum number of iterations allowed for the bent-column cracked-property iteration. The first run is considered to be thezero-th iteration. Usually only one iteration is needed. This item is re-

quired only when the cracked-property calculation is program deter-mined.

13 AcceptUnconvergedResultsConvergence

Specifies if the seismic design should or should not continue if the bent-column cracked-property iteration fails to converge. This item is re-quired only when the cracked-property calculation is program deter-mined.

14 Modal LoadCase Option

Specifies if the modal load case is to be determined by program orspecified by the user. The modal load case is used as the basis of theresponse-spectrum load case that represents the seismic design. If pro-gram determined, the modal load case will use the stiffness at the endof the auto-gravity load case that includes the cracked property effects.If user-defined, the user can control the initial stiffness, Eigen vs. Ritz,and other modal parameters by selecting user defined for Item 15 Mo-dal Load Case.

15 Modal LoadCase

The name of an existing modal load case to be used as the basis of theresponse-spectrum load case. This item is required only if Item 14 Mo-dal Load Case Option is user-defined.


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2 - 6 Seismic Design Request


16 ResponseSpectrum LoadCase Option

Specifies if the response-spectrum load case is to be determined byprogram or specified by the user. The response-spectrum load caserepresents the seismic demand. If program determined, this load casewill use the given response-spectrum function and modal load case.Acceleration load will be applied in the longitudinal and transversedirections of the bridge object, and combined using the 100% + 30%rule. If user-defined, the user can control the loading or select SRSS asthe method to account for directional combinations.

17 ResponseSpectrum LoadCase

The name of an existing response-spectrum load case that representsthe seismic demand. This item is required only if the response-spectrumload case option is user-defined.

18 ResponseSpectrum Angle

Option

Specifies if the angle of loading in the response-spectrum load case is tobe determined by program or specified by the user. If program deter-

mined, the longitudinal (U1) loading direction is chosen to be from thestart abutment to the end abutment, both points located on the refer-ence line of the bridge object. This item is required only if the response-spectrum load case option is user-defined.

19 ResponseSpectrumAngle

Angle (degree, from global X) that defines the direction of the responsespectrum load case. This item is required only if the response spectrumload case is user-defined.

20 DirectionalCombination

The type of directional combination for the response spectrum analysis

21 DirectionalScale Factor

For absolute directional combination this is the scale factor used for thesecondary directions when taking the absolute sum. This is typically 0.3if a 100/30 rule is to be applied. For CQC3 directional combination, thisis the scale factor applied to the response spectrum function in the

second horizontal direction. This is typically greater than 0.5. For theSRSS directional combination the directional scale factor is normally 1.0.

22 FoundationGroup

If foundations are included and explicitly modeled, then the foundationobjects need to be assigned to a group and that group needs to beidentified here. This way the foundation objects will be included in thepushover load case. This item is required only if the seismic design cate-gory is D.

23 PushoverTargetDisplacementRatio

The target displacement is defined as the target ratio of Capacity/Demand for the pushover analyses. This item is required only if theseismic design category is D.

24 Bent FailureCriterion

The criteria to determine the bent failure. means the bent fails when the pushover curve slope becomes negative.

This item is required only if the seismic design category is D.


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Perform Seismic Design 2 - 7

2.4 Perform Seismic DesignIt is not necessary to execute an analysis of the bridge model before running

the Seismic Design Request. To start the Bridge Seismic Design Request, use

the Design/Rating > Seismic Design > Run Seismic command. The Perform

Bridge Design form, which is shown in Figure 2-5, will be displayed. The De-

sign Now button will start the seismic design process.

Figure 2-5 Perform Seismic Design

2.5 Auto Load PatternsAfter the Bridge Seismic Design has been run, the user can review the load pat-

tern and load cases that CSiBridge has automatically generated by accessing

the Define Load Patterns form show in Figure 2-6 (Loads > Load Patterns

command).

Figure 2-6 Auto Load Patterns


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2 - 8 Auto Load Cases

2.6 Auto Load CasesThe reason for each of the auto load cases is explained in Step 7.

Figure 2-7 Auto Load Cases


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Dead Load Analysis and Cracked Section Properties 3 - 1

Step 3Dead Load Analysis and Cracked Section Properties

As shown in the schematic included in the Foreword, the third step begins with

the dead load analysis of the entire bridge model. The results of the dead load

analysis are then used to verify the analytical model followed by the determina-

tion of the cracked section properties that are then applied to the bent columns as

frame section property modifiers. The reduced stiffnesses of the bent columns

will affect the response spectrum and pushover analyses. The frame section prop-

erty modifiers are defined separately for each of the bent and abutment columns

as a named property set. The user can use the Section Designer program to ob-serve the moment-curvatures and I,

crackedproperties for the various cross-sections

(see also Step 5).

Auto load patterns and auto load cases are produced by the program. The load

case, which has the default name, _GRAV_SDReq1, is automatically developed

by CSiBridge as a single stage construction load case and is used to apply the

cracked section property modifiers to the columns. Figure 3-1 shows the Load

Case Data form for the _GRAV_SDReq1 load case (Analysis > Load Cases >

Type > All > New > Highlight _GRAV_SDReq1 > Modify/Show Load Case).

The auto load cases are not modifiable.


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3 - 2 Dead Load Analysis and Cracked Section Properties

Figure 3-1 Auto Stage Construction Load Case used to applyCracked Section Property Modifiers

As an option, the user can overwrite the cracked section property determined by

the program and instead, apply a user defined value. See Step 2 for the user op-

tions available in the Seismic Design Request.


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Overview 4 - 1

Step 4

Response Spectrum and Demand Displacements

4.1 OverviewThe seismic response of the entire bridge structure is analyzed by CSiBridge us-

ing the response spectrum function defined in Step 2. The number of modes used

by CSiBridge is automated and depends on the number of bridge spans. The user

should check the total mass participation to ensure that an adequate number of

modes are included in the modal analysis. The response spectrum displacementsare used by CSiBridge as the displacement demands as defined in Section 4.4 of

the AASHTO Seismic Guide Specification.

4.2 Response Spectrum Load CasesThree response spectrum load cases are automatically produced by CSiBridge:

_RS_X_SDReq1, _RS_Y_SDReq1 and _RS_XY_SDReq1. The first two response

spectrum load cases apply the dynamic loads along the U1 and U2 directions.

The U1 direction is defined as the longitudinal loading direction that is chosen to

be from the start abutment to the end abutment, both points located on the refer-ence line of the bridge object. If the user wants to apply a response spectrum load

along a different axis, a directional overwrite is available in the Substructure

Seismic Design Request Parameters form (see Chapter 2).


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4 - 2 Response Spectrum Load Cases

Figure 4-1 U1 Direction Response Spectrum Load Case form

The third response spectrum load case uses a Directional Combination option of

ABS, with an ABS scale factor of 0.3. This response spectrum load case will

satisfy the AASHTO Seismic Guide Specification, Section 4.4, which requires

the response spectrum loads to be combined using the 100/30 percent rule in each

of the major directions. The single response spectrum load case,

_RS_XY_SDReq1, envelopes the maximum response spectrum results for each of

the combinations 100/30 and 30/100. The Load Case Data form for the response

spectrum load case _RS_XY_SDReq1 is shown in Figure 4-2.

The modal damping coefficient is set to 5 percent, but this value can be modified

as necessary by the user in the Substructure Seismic Design Request Parameters

form (Chapter 2).


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Step 4 - Response Spectrum and Demand Displacements

Response Spectrum Load Cases 4 - 3

Figure 4-2 ABS Response Spectrum Load Case form

To illustrate the ABS directional combination feature, the following BENT1 dis-

placements are summarized for example model MO_1C:

Figure 4-3 BENT1 Displacements for the three

Auto-Defined Response Spectrum Load cases


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4 - 4 Response Spectrum Results

Figure 4-4 Modal Load Case Definition

4.3 Response Spectrum ResultsUpon completion of the response spectra analysis, the displacements are tabu-

lated for each bent. The displacements are calculated using Generalized Dis-

placements to account for the average cap beam displacements and the relative

displacement between the cap beam and foundation. The displacements for the

ABS response spectrum load case also are tabulated for each of the bearing ac-

tive degrees of freedom. These can be viewed using the Home > Display > Show

Tables command to display the Choose Tables for Display form. Select the De-

sign Results for Bridge Seismic, Support Bearing Demands-Deformations item.

These displacements also can be displayed and animated on screen or read from

the quick report created using the Design/Rating > Seismic Design > Report

command.


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Overview 5 - 1

Step 5Determine Plastic Hinge Properties and Assignments

5.1 OverviewFor bridge structures having a Seismic Design Category (SDC) D the

AASHTO Seismic Guide Specification requires that the displacement capacity

be determined using a nonlinear pushover analysis. This requires that the col-

umn plastic hinge lengths and plastic hinge properties be determined for each

column that participates as part of the Earthquake Resisting System (ERS).

In this step, the methodologies used to calculate the plastic hinge lengths and

properties will be explained. After the hinge properties have been determined,

the plastic hinges are assigned to the ERS columns. The automation of the plas-

tic hinge assignments will also be explained in this step.

5.2 Plastic Hinge LengthsThe plastic hinge lengths used in the Seismic Design Request is determined for

the AASHTO Seismic Guide Specification, Section 4.11.6, as follows:

Plastic Hinge Length, 0 08 0 15P ye blL . L . f d ,

where


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5 - 2 Plastic Hinge Lengths

yef = the effective yield strength of the longitudinal reinforcing, and

bld = the diameter of the longitudinal reinforcing.

The hinge length is compared to the value for the maximum hinge length value

described as, 0 3P ye blL . f d , and the controlling value is used. After the hinge

lengths and properties have been determined, the hinges are placed on the bent

columns at each end of the column at distances from each end equal to 1/2 the

hinge length, as shown below in Figure 5-1.

Figure 5-1 Hinge Locations


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Step 5 - Determine Plastic Hinge Properties and Assignments

Nonlinear Hinge Properties 5 - 3

Figure 5-2 Hinge Locations

5.3 Nonlinear Hinge PropertiesCurrently, the CSiBridge Automated Seismic Design Request uses a hinge

property that is consistent with the AASHTO/CALTRANS idealized bilinear

moment-curvature diagram, as shown in Figure 5-3 (click the Display menu >

Show Moment Curvature Cure command on the Section Designer form).

From the moment curvature shown, the yield and plastic moments along with

the I,cracked

properties can be observed for a specific axial load, P. Note that this

form is made available to allow users to better understand the effects of axialloads and fiber mesh layouts on the frame member properties. The axial load

values input on this form are not used in the analysis and design of a model.


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5 - 4 Nonlinear Hinge Properties

Figure 5-3 Moment Curvature Diagram

Typically, the axial loads in the bent columns change as the bent is pushed over

due to the overturning effects. Therefore, the yield and plastic moments will

change depending on the amount of axial load present in a particular column at

a particular pushover step. These effects are captured in the nonlinear hinge re-

sponses whenever P-M or P-M-M hinges are specified. For this reason, the

Automated Seismic Design procedure assigns coupled P-M-M hinges to the

bent columns. The default settings are shown in Figure 5-4 (select the frame(s)

to be assigned a hinge, clickAdvanced > Assign > Frames > Hinges, select

Auto, click the Modify/Show Auto Hinge Assignments Data button). Thelength of the plastic hinge also is calculated by CSiBridge when using the

Automated Seismic Design procedure.


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Nonlinear Hinge Properties 5 - 5

Figure 5-4 Auto Hinge Assignment Data

Figure 5-5 Sample Hinge Data form


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5 - 6 Nonlinear Material Property Definitions

Upon completion of the Pushover Analysis, the Hinge Results can be traced.

This feature is explained in detail in Step 6.

5.4 Nonlinear Material Property DefinitionsThe ductile behavior of a plastic hinge is significantly affected by the nonlinear

material property used to define the frame member receiving the hinges. The

material nonlinear properties must be defined using the Advanced Nonlinear

Material Data forms.

5.4.1 Nonlinear Material Property Definitions for ConcreteFor concrete, the nonlinear material property data form appears as shown in

Figure 5-6 (Components > Type > Material Properties > Expand arrow >

check the Show Advanced Properties check box > Add New Material > set

Material Type to Concrete > Modify/Show Material Properties button >

Nonlinear Material Data button):

Figure 5-6 Nonlinear Material Data form for Concrete


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Nonlinear Material Property Definitions 5 - 7

Figure 5-7 Nonlinear Stress-Strain curves for Confined and Unconfined Concrete

Figure 5-8 Concrete Model - Mander Confined


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5 - 8 Nonlinear Material Property Definitions

5.4.2 Nonlinear Material Property Definitions for SteelSimilarly, for steel, the nonlinear material data form appears as show in Figure

5-9. The user can specify the parametric strain data, which includes the values

for the strain at the onset of hardening, ultimate strain capacity, and the final

slope of the stress-strain diagram.

Figure 5-9 Nonlinear Material Data form for steel


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Plastic Hinge Options 5 - 9

Figure 5-10 Nonlinear Stress-Strain Plot for steel

5.5 Plastic Hinge OptionsConcrete column section properties can be defined for use in two ways such

that hinges properties can be assigned to them during the Automated Seismic

Design procedure. One method is to use the Section Designer and the other isto define a rectangle or circle using the Components > Type > Frame Prop-

erties > New command and define a rectangular or circular shape. Internally,

CSiBridge will convert the rectangular or circular shapes into Section Designer

sections for the purposes of determining the hinge and cracked section proper-

ties. The advantage of using the Section Designer feature is that the user can

choose to have the hinge defined using fibers. This option is applied when the

user activates the Design menu > Fiber Layout command from within Section

Designer and sets the Fiber Application to Calculate Moment Curvature Using

Fibers, as shown in the following form.


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5 - 10 Plastic Hinge Options

Figure 5-11 Plastic Hinge Fiber option

The fiber mesh also can be specified in this form. The mesh can be rectangular

or cylindrical depending on the shape of the column. Another advantage of us-

ing the Section Designer feature is that complex sections, similar to the one be-

low, can be handled.

Figure 5-12 Section Designer options


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Displacement Capacities for SDC B and C 6 - 1

Step 6Capacity Displacement Analysis

This step describes the automated procedure that CSiBridge uses to determine

the bridge seismic capacity displacements. The method used varies depending

on the Seismic Design Category (SDC) of a particular bridge. A flowchart that

describes when an implicit or pushover analysis is used to determine the capac-

ity displacements is shown in Figure 6-1:

Figure 6-1 Rectangular Beam Design


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6 - 2 Displacement Capacities for SDC B and C

Figure 6-2 Design Requirements for SDCA B C D

Identification ERS Recommended Required Required

Demand Analysis Required Required Required

Implicit Capacity Required Required Required

Push Over Capacity May be required

Support Width Required Required Required Required

Detailing Ductility SDC B SDB C SDB D

Capacity Protection Recommended Required Required

Liquefaction Recommended Required Required

The user can overwrite the program determined SDC to enforce that a push-

over analysis is used to determine the displacement capacity. The differences

between the implicit and pushover approaches are described in the following

sections.

6.1 Displacement Capacities for SDC B and CFor structures having reinforced concrete columns, the displacement capacities

for SDC B and C are found using the following equations. The AASHTO

Seismic Guide Specification equations are also noted.

For SDC B:

0.12 1.27 ln( ) 0.32 0.12LC o oH x H (4.8.1-1)

For SDC C:

0.12 2.32 ln( ) 1.22 0.12LC o oH x H (4.8.1-2)

in which

o

o

Bx

H

(4.8.1-3)

where,


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Step 6 - Capacity Displacement Analysis

Displacement Capacities for SDC D 6 - 3

Ho

= Clear height of the column (ft)

B0

= Column diameter or width parallel to the direction of displace-

ment under consideration (ft)

= Factor for the column end restraint conditions

6.2 Displacement Capacities for SDC DWhen the Seismic Design Category for a bridge structure is determined to be

SDC D or the user overwrites the SDC as D, CSiBridge uses a pushover analy-

sis in accordance with the AASHTO Seismic Guide Specification, Section

4.8.2 to determine the displacement capacities. This requires that CSiBridgeactually perform several pushover analyses, depending on the number of bents

that are part of the Earthquake Resisting System (ERS). Each bent is analyzed

in a transverse and longitudinal direction local to the specific bent. For the ex-

ample bridge used in this manual, there are three spans with two interior bents.

Bents can be used as abutment supports so it is possible to have additional

bents participating as part of the ERS. But, for the example bridge, there are

two interior bents. This means that a total of four pushover analyses is needed

to determine the displacements capacities for each bent in each of the trans-

verse and longitudinal directions.

To perform multiple pushover analyses on a single bridge model, CSiBridgeuses several nonlinear single-staged construction load cases.

For the example bridge, the four separate pushover load cases are named as fol-

lows:

_PO_TR_BT1_SDReq1

_PO_LG_BT1_SDReq1

_PO_TR_BT2_SDReq1

_PO_LG_BT2_SDReq1

The SDReq1 is the name provided by the user to identify a particular seismic

design request.


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6 - 4 Displacement Capacities for SDC D

TR denotes Transverse and LG denotes Longitudinal.

The _ symbol is added to the beginning of each auto load case name to dis-

tinguish the load cases that are automatically provided by CSiBridge from user

defined load cases.

Figure 6-3 shows the nonlinear single-staged construction load case for the

BENT1 transverse direction.

Figure 6-3 BENT1 Transverse Pushover Load CaseThe user can not modify this load case because it is defined automatically. The

_PO_TR_BT1_SDReq1 load case starts from the end of the initial nonlinear

load case named, _ bGRAV_SDReq1.

The _ bGRAV_SDReq1 load case is shown in Figure 6-4 and is needed to iso-

late the bents from the rest of the bridge model and to apply the cracked section

property modifiers as well as apply the dead load.


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Displacement Capacities for SDC D 6 - 5

Figure 6-4 BENT1 Application of Property Modifiersand Dead Loads to BENT1

The load pattern used to apply the lateral pushover loads or displacements to

BENT1 is named, _PO_TR1_SDReq1. A 3D view of the _PO_TR1_SDReq

loads is shown in Figure 6-5. The magnitudes of these loads are based on the

reactions from the superstructure.


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6 - 6 Pushover Results

Figure 6-5 BENT1 Pushover Load Pattern for the Transverse Direction

6.3 Pushover ResultsAfter the pushover analyses have run, the capacity displacements are automati-

cally identified as the maximum displacement of the pushover curve just before

strength loss (negative slope on the pushover curve) for each of the pushover

runs.


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Pushover Results 6 - 7

The pushover results can be viewed using the Home > Display > More >

Show Static Pushover Curve command. An example output is shown in Fig-ure 6-6 for the BENT1 transverse and longitudinal pushover load cases.

Figure 6-6 Display of BENT1 Pushover Curves


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Demand/Capacity Ratios 7 - 1

Step 7Demand/Capacity Ratios

After the demand displacement (Step 4) and displacement capacity (Step 6)

analyses have been completed, CSiBridge computes the ratio of the De-

mand/Capacity displacements and reports these values in the Seismic Design

Report. The table of D/C ratios can be viewed using the Home > Display >

Show Tables command, and then selecting Design Data > Bridge > Seismic

Design data > Table: Bridge Seismic 01-Bent D-C-AASHTO LRFD 2007.

The subject table will appear similar to the table shown in Figure 7-1:

Figure 7-1 D/C Displacment Ratios

In the table shown, all four D/C ratios are reported, namely, the transverse andlongitudinal direction for each bent (the example model has two bents). Note

that the Generalized Displacement name also is reported. Generalized dis-

placements are used to average the top of bent displacements and to determine


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7 - 2 Demand/Capacity Ratios

the relative displacements between the bent cap beam and the foundation. The

generalized displacement definition is automatically defined by CSiBridge andcan be viewed using the Advanced > Define > Generalized Displacements

command.


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Design 01 D-C Ratios 8 - 1

Step 8Review Output and Create Report

This step describes the two methods of viewing the seismic design results. The

first way to review the results is to use the Home > Display > Show Tables

command. The second way is to create a report using the Orb > Report > Cre-

ate Report command.

The entire list of output tables for the Bridge Seismic Design includes the follow-

ing:

The seven Bridge Seismic Design tables are described in the sections that follow.


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8 - 2 Design 01 D-C Ratios

8.1 Design 01 D-C RatiosThe Demand/Capacity ratios are summarized for each bent in each direction.

Values less than 1.0 indicate that an adequate capacity exists for a given bent and

direction for the ground motion hazard used in the seismic design request. Values

greater than 1.0 indicate an overstress condition.

8.2 Design 02 Bent Column Force DemandA summary of the bent column seismic demand forces are tabulated.

8.3 Design 03 Bent Column Idealized Moment CapacityThe idealized column plastic moments are calculated and tabulated. The axial

load P represents the demand axial load. The idealized plastics moments are de-

termined using the associated axial load value, P.


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Step 8 - Review Output and Create Report

Design 04 Bent Column Cracked Section Properties 8 - 3

8.4 Design 04 Bent Column Cracked Section PropertiesA summary of the cracked property modifiers that get applied to each of the bent

columns is tabulated.

8.5 Design 05 Support Bearing Demand ForcesThe forces in the bearing due to the seismic loads are presented in the following

table. All bearings at the abutments and bents that are found to resist seismic

forces are included in the subject table.


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8 - 4 Design 06 Support Bearing Demand Displacements

8.6 Design 06 Support Bearing Demand DisplacementsThe displacements for all bearings at the abutments and bents that resist seismic

loads are tabulated and reported.


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Design 07 Support Length Demands 8 - 5

8.7 Design 07 Support Length DemandsThe support lengths are calculated from the bearing displacements and represent

the amount of displacement normal to a specific bent or abutment.

8.8 Create ReportA single command can be used to create a report using the Design menu >

Bridge Design > Create Seismic Design Report command. Several representa-

tive pages of the report that can be created using the previously noted report re-

quest are included in the following pages. Theses have been excerpted from a 30page summary report that CSiBridge writes as a Microsoft Word document.


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8 - 6 Create Report

CSiBridge V15CSiBridge V15


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Create Report 8 - 7



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8 - 8 Create Report



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References

ACI, 2008. Building Code Requirements for Structural Concrete (ACI 318-08)

and Commentary (ACI 318R-08), American Concrete Institute, P.O.

Box 9094, Farmington Hills, Michigan.

AASHTO, 2009. AASHTO Guide Specifications for LRFD Seismic Bridge

Design. American Association of Highway and Transportation Offi-

cials, 444 North Capital Street, NW Suite 249, Washington, DC 2001

bridge seismic design. automated seismic design of bridges aashto lrfd seismic design - csibrigde

Documents

csibridge seismic designii1

example bridge

bridge model1

bridge deck section

bridge object definition

bridge layout line

bent geometry

bent property assignments