chapter3 input data

8/10/2019 Chapter3 Input Data

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Cracked Section Properties

CTBridge allows input of a variety of cross section types including box girder shapes,rectangular and circular shapes. Sometimes the user desires to model sections that havebeen cracked and thus have smaller properties than those for the full section.

CTBridge allows the user to apply cracked factors to the following properties:

Area

X Axis Moment of Inertia

Y Axis Moment of Interia

Torsion

The default cracked factor is 1.0 for all sections. The cracked sections properties areused in all subsequent calculations from the finite element model generation to thespecification checks.

The use can choose to show the gross section properties or cracked section propertieswhen working in the cross section input window. In addition, a note appears in theproperties part of the input window that indicates a cracked section.

Loading in CTBridge

The current CTBridge loadings are

Dead Load

Added Dead Load

Live Load

Prestress Load

Temperature Load

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Creep and Shrinkage Load*

*Creep and Shrinkage Load is not included in the superstructure load combination.

Trucks Lanes Vehicles

CTBridge is set up so that the user is required to enter a minimum of live load data, solong as default information obtained from the design specification is valid. However,the user is allowed to depart from the default information and create used definedtrucks, lanes and vehicles. CTBridge does not place restrictions on what the user candefine, but there is a process to making it all work that the user should be aware of.

The first and perhaps most important concept to understand is the difference between liveload information derived from the design specification and that defined by the user. The

specification derived live loads include a suite of trucks, lanes and vehicles. The usercannot change this information, and the GUI enforces this by disabling all edit fieldsassociated with specification data.

The user can create new trucks, lanes and vehicles. The user can also copy specificationdata into user data, and then modify it at will. The GUI distinguishes betweenspecification and user defined data by allowing the user to select the appropriatebutton.

The second important concept to understand is the relationship between trucks, lanes and

vehicles. Truck and lane definitions are fairly self-explanatory. Vehicles on the otherhand are a collection of trucks and/or lanes that are analyzed and enveloped. WhenCTBridge analyzes a point in the structure, the results from each truck and lanecombination within a vehicle are determined, but only the controlling truck and/or laneresults for that vehicle are included in the load combination for that point. In somecases (permit and fatigue), the vehicle will only include one truck. In other cases(design), a number of truck and lane combinations are included in the vehicle andenveloped accordingly.

With this concept in mind, the user can define a truck or lane, but this information willnot be analyzed unless the truck or lane is included within a vehicle.

The following figure shows the definition for an LRFD HL-93 specification truck.




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There are two or three default vehicles based on the design specification: Design, Permitand Fatigue. Since they are specification based, they cannot be changed. However, thesevehicles can be used as a starting point for a user defined vehicle by choosing the copyoption. Any number of specification or user trucks and lanes can be included in the uservehicle.

The following figure shows the definition of the LRFD Design Vehicle. Note that it ismade up of a series of trucks and lanes. These are enveloped at each analysis points.




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This leads into discussion of the third concept. Once a user defined vehicle has beencreated, appropriate load factors must be set to include the vehicle in the analysis. Bydefault, all load factors for a user defined vehicle are initially set to 0. Thisconcept also holds true for user defined point and concentrated loads that are not partof the dead load or additional dead load.

The last concept is that of ctive loads. For the user defined vehicle to be includedin the analysis, it must be identified as an active load. By the same token, the usercan deactivate any vehicle (or load for that matter) thereby excluding it from analysis.

An example might help illustrate these concepts. Suppose a user wishes to create a userdefined permit vehicle. These are the steps the user might take:

1. Create the user defined permit truck.

2. Copy the default specification permit vehicle to a new user defined vehicle.

3. In the new vehicle, replace the existing truck with the user defined truck.




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4. Add the appropriate load factors for the new permit vehicle they might match thefactors for the existing specification permit vehicle.

5. Go to the load control dialog. Make sure that the new user defined vehicle isactive, and deactivate the specification permit vehicle.

Number Of Lanes

The live load distribution (number of live load lanes) may be program determined or userspecified. The user can specify one or more 'lane layouts' to override programcalculated values. If there are no lane layouts, the program will calculate the numberof lanes based on geometry and specification equations.

It should be noted that the multiple presence factor is not used in any live loadfactoring.

Program Calculated:

There are two main divisions of live load distribution: for superstructure members andfor substructure members. When applying the LRFD specification for superstructuremembers, the program uses whole width design equations for box girder sections. Theequations are found in Chapter 4 of the specification and depend on the force effectunder consideration. If a box girder section is not found at the point underconsideration, the number of lanes is set to 1.0.

When applying the LFD specification for superstructure members, the overall width isdetermine from the section in question and the number of lanes is computed by dividingthe width by 14.0, conforming to whole width design for box girders.

For substructure members, the integer portion of 12 ft wide lanes that can fit within theoverall width of the section in question is used. There are other specificationprovisions for decks less than 24 ft wide that come into play.




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User Specified:

The user can specify a layout of lane information, but the application is slightlydifferent for superstructure and substructure members. The user can also choose to applydifferent lane distribution criteria for different vehicle types, and for superstructuremembers can further refine the layout depending on force effect.

For superstructure members, there is always a beginning number of lanes. The user canspecify the number of lanes at any other point in the superstructure. For analysisbetween the described points, the program will linearly interpolate the number of liveload lanes. For points beyond the last description, the program will use the value in

the last description.

For substructure members, there is also a beginning number of lanes. The user can thendescribe the number of lanes at any other substructure. For substructures not describedby the user, the number of lanes from the previous substructure is used. So the usercould only describe the number of lanes at the beginning and that number would be used atevery other substructure location.

Uniform Temperature Load

Uniform temperature load was added in CTBridge version 1.4.8. California Amendments toAASHTO LRFD Bridge Design Specification amend to us Procedure A temperature range todesign thermal movement associated with a uniform temperature change. LRFD 3.12.2.

CTBridge adapt the CA amendment recommendation to use the base construction temperatureas 60 F and

T, 35 F, both rise or fall of less as default values. Users can overwrite the defaultvalues as shown in the dialogue below.




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Creep And Shrinkage

Creep and shrinkage force was added in CTBridge version 1.5.3. Internally, CTBridge usesan equivalent uniform temperature load to calculate creep and shrinkage response. Thedefault value for Creep & Shrinkage is 0.63" per 100 feet span. This value isrecommended in Memo to Designer 7-10 for post tension reinforced concrete bridge.

Currently, creep and shrinkage is for substructure design only. It is not included inthe load combination for superstructure design.

Support Settlement Analysis Using CTBridge

Bridge settlement was added in version 1.5.1 under user defined load. Settlement




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analysis allows the user to compute the demand on bridge components due to supportsettlement. The implementation is pretty general allowing specification of settlement inthe form of displacements as well as rotations in any direction, utilizing the existingUser Load graphic interface in the program.

The following is a brief guide intended to assist the user with using this new option inCTBridge:

The User may specify settlement at abutments and/or at bent column supports.

The specified settlements may be in the form of displacement or rotation.

The settlements may be specified in local or global coordinates system as defined inCTBridge online Help document. The use of Global Coordinates system is recommended.

The settlement force effects are input to CTBridge through the existing User Loads

interface.

The participation of Settlement force effects in LRFD or LFD Load combinations ishandled like any other user specified load cases. The user will populate theCTBridge generated cells of the Load Factor Table with proper load factors for allthe relevant limit state load combinations.

Imposed displacements and rotations may only be applied to a support that is fixedin the direction of the settlement. Unpredictable and erroneous results will beproduced otherwise. It should be noted that a support fixed in global coordinatesmay not be fixed in the local coordinates.

The imposed displacements at the abutments are applied to the superstructure spansat location of the support. The followings are examples of abutment settlement:

For settlement of 1st abutment, specify displacement (or rotation) at thebeginning of the 1st span.

For settlement of last abutment, specify displacement (rotation) at the end ofthe last span.

For settlement at bent column supports, specify displacement (or rotation) at thebeginning of the column where it is attached to the support in question.




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Effective Dimensions

Effective dimensions in a span are meant to describe the boundary between the end spandiaphragm and the more flexible interior part of the span. The practical effect ofentering an effective dimension is to move the reporting of forces away from thecenterline of support. This typically reduces the fictitiously high forces found at thecenterline of support.

An effective dimension can be modeled with or without a rigid link. A rigid link is anelement with very stiff properties and no mass. If a rigid link is applied, the usershould computed the weight of the entire diaphragm and enter it as a point load at thespan end.

If a rigid link is not applied, the span properties are used all the way to theconnection at centerline of support. In this case, the effective dimension just shiftsthe analysis point away from the support centerline. When a rigid link is not used andassuming a box girder span section, the weight of the diaphragm concrete within the cellsshould be applied as a point load at the span end. CTBridge reports the cell areas toallow easier calculation of this weight.




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Support Width

See span effective dimensions.

Default Supports

During the creation of a bridge model, the user will encounter the concept of defaultsupports. Basically, when the user creates a span, the program doesn't know what tosupport the span with so it places a default support at one of both of the span ends. Adefault support gets applied whenever the true support condition is unknown.

When bents are created, they can replace existing default supports. If a bent is removedfrom the model, a default support is placed in its stead.

The user can also replace defaults supports with a user-defined support. The user-defined support is similar to a default support except that it will apply to only onelocation in the bridge. When a user-defined support is removed, it is replaced by adefault support.

The user is able to modify the default support, but the modifications will apply wherevera default support is found. For example, if the user changes the default support to aroller, all default supports will be rollers.

The default support cannot be deleted.

The following figures show a progression of model building where default support comeinto play.




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The first span is supported by default supports at either end.

A second span is added and is supported by another default span at the end.




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The user has replaced the default support the end of span 2 with a user created rollersupport. In addition, a bent is added to the structure and is placed between the spans.The bent replaces the default support that previously existed.

Note that a bent can also be placed at the abutments.

Units

CTBridge allows input data and output results to be in either English (US) units orMetric (SI) units. These units are commonly referred to as "user units".

CTBridge also allows the user to choose between the LRFD specification (in US and SIunits), or the LFD specification (in US units only).

The specification units are completely separate from the user units. In other words, itis possible for a user to describe a bridge in US units, and analyze the bridge accordingto the LRFD SI specification.

The following figure is a depiction of the typical toolbar set up in CTBridge. In thisfigure, both the user units and specification units can be seen:




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User units:

Specification units:

In addition to user and specification units, CTBridge also provides unit conversion withmost input data. This conversion is not permanent and amounts to a local "unitcalculator".

For example, a user could choose to view unit conversions of a data item, change the itemto a different set of units and exit the window. When the user re-enters the window, thedata item will be shown in the user units of the program, not the converted units.




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An example of the unit calculator is shown below:

Specification

CTBridge applies the provisions of the following specifications:

AASHTO LRFD, 4th Edition, 2007 Interim Revisions and Incorporating CaliforniaAmendments (Blue Sheets) Dec. 2008, US or SI version

Caltrans Bridge Design Specifications, LFD 16th Edition, 1998 (Caltrans 2000)

The default specification is LRFD - US, but this may be changed at any time. However, itis recommended to set the specification prior to entering model data. CTBridge appliesdefault values where possible, and these defaults sometimes depend on the specification.This is particularly true with the live load trucks, lanes, and vehicles, and the load

factors.

If the user has not modified load factors or created user defined live load information,there should be no problem switching between specifications. If this type of data hasbeen created, some of it might be lost during the specification change.

Sign Convention

There are several sign conventions at work in CTBridge. These conventions affect the




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layout of the model, the orientation and loading of members within the model, and themeaning of results.

The global sign convention is used when laying out the nodes. Loads can also be placedin the global direction. An example of a global load is gravity, which is oriented in

the negative global Y direction. The global directions can be seen by double clickingthe compass at the lower left of the model view. The global X direction coincides with N90 E, global Y is directed upward vertically, and global Z is due south following theright hand rule. These directions are shown in the figure below.

Member local sign conventions are important for orienting the member and its sectionproperties. In addition, members can be loaded in the local directions. Prestress forcecoefficients are resolved and applied in the local span directions. The reporting ofmember forces is also done in the local directions of the members. The local signconvention for each type of member is illustrated the figure above.
