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Document Type: FAQ Product(s): LEAP CONSPAN Version(s): All Original Author: Bentley Bridge Technical Support Group
Document Type: FAQ
Product(s): LEAP CONSPAN
Version(s): All
Original Author: Bentley Bridge Technical Support Group
ANSWER:
Please note that the tenth points in the output under 'Service Load moments and Shears' are referenced with respect to the centerline of the left bearing. Let us follow a sample calculation using Tutorial 1 in the manual. Precast Length is 116.0 ft. Now 0.4L = 0.4 x 116 ft = 46.4 ft. However under Shear and Moment envelopes program shows a value of 45.90 ft. This difference of 0.5 ft comes from the overhang over bearing CL. Since the program always assumes that the beam is placed symmetrically over the bearing, this overhang is calculated as (Precast Length (116.0 ft) - Design length (115.0 ft)/2 = 1.0/2 = 0.5 ft.
AASHTO LRFD (Article 5.9.4) and AASHTO Standard Specifications for Highway Bridges (Article 9.15.2) specify stress limits for concrete – specifically, pre-tensioned and post-tensioned concrete components. Tensile and compressive stress limits are specified for fully prestressed components for two stages of the Service Limit State – before and after prestress losses.
Tensile stress limit keeps the bridge component from cracking in service, thus minimizing the increased stresses and fatigue of the prestressing strands at the cracks. Tensile limit (6√(f'c )) is lower than modulus of rupture (7.5√(f'c )) to safeguard against fatigue in tension. Fatigue of strands is not a problem in uncracked concrete.
Traditional compressive stress limits (0.45f'c and 0.60f'c) protect concrete from excessive creep deformations and fatigue (0.40f_c^' limit).
Although the above stress limits are only specified for fully prestressed components and not for reinforced concrete components of composite members, the practice varies across the United States.
LEAP CONSPAN, by default, only checks the stress limits for the top and bottom fibers of precast, prestressed concrete girders. Optionally, the user may elect to also check the stresses at the top fiber of the cast-in-place, reinforced concrete deck.
Uniform temperature is used to calculate horizontal displacement. The program calculates and reports the displacement in X direction and this feature was added in the version V12.01.00.57 and the same uniform temperature is not used for stress calculation .
To figure out the stresses user needs to use Gradient and Multi linear Temperature option.
To calculate the distance between the supports ,Conspan considers the station numbers from the center of the abutment to center of abutment and does not consider the station based on the alignment. User needs to calculate the stationing based on center of the bridge and later on in Layout option input the deck offset.
This is the moment capacity by Strain Compatibility. Choice for SC or AASHTO equations is under CONBOX Settings/Design parameters.
Yes , According to AASHTO interim (2008-2009) release. Please check Section 4 (4.6.2.6) of Aashto code.
Optional Live Load Deflection (LRFD Design Code Only)
According to the LRFD design code, if the user invokes the optional live load deflection criteria specified in LRFD Article 2.5.2.6.2, the deflection should be taken as the larger of (a) the resulting deflection from the design truck alone, or (b) the resulting deflection from 25% of the design truck taken together with the design lane load. If you select the “Include LL Deflection” option, the above two conditions are checked and the deflection under the governing load is printed in the camber and deflections section of the printout.
Detensioning is the process of releasing (by cutting) the strands, typically two at a time.
Release stresses are the stresses after all the strands have been cut.
Pedestrian load is applied as pounds per linear foot load and it should be Total load and Conspan distributes this total load to each beam by tributary width factor. The load is analyzed in the same manner as a lane load, loading only those parts of the bridge that would cause maximum moment or shear at each section.
Please see AASHTO article 3.6.1.6 for pedestrian loading info.
Yes the user can separate the live load reaction into truck and lane loads. Go to Analysis TAB --> in the load type select LOAD CASE --> Select the span and the beam and in the cases option select the lane or truck and in the Fy column then the user can see the reaction.
No, the program does not apply distribution factor to account for centrifugal force. Centrifugal force effect is used in the design of substructure ,Please RCPier design examples for more information.
Right now if the user is using Standalone conspan then it is not possible to input the 6% Super elevation information but you can still input the Radius info.
If the user is using Leap bridge Suite then in ABC wizard you can input both Radius as well as cross slope (%) information which will then get transferred to conspan.
The other option is inputting the super elevation (6%) information in Geomath.
User can model the AREA E-80 Truck under Design Truck in the LRFD library.
Then user can simply use this truck. User has to make sure a dummy tandem and dummy lane is defined also.
1) In conspan once the analysis is done please go to the file menu and click EXPORT TO RCPIER.
2) In the new window please save the file as a TEXT File .
3) Open RCpier and select the Dead Load-load case and click EDIT button -->Click generate --> Select option “ Input composite Dead Load reaction from conspan”.
4) Click IMPORT button --> select the conspan reaction file and click IMPORT button à finally click Generate button and you should be able to see the reactions generated on the bearing.
5) Please follow the same procedure for Live load – Load case too.
This is just a limitation of Conspan and not based on AASHTO code.
These are the few suggestions I would recommend the user to do.
1 )Please download the latest version from the select server.
2)Try to see if the file is saved in the server or the local machine (Highly recommended to install and run the software from the local machine).
3) Check if the license is expired.
4) If the user is working on Conspan please close all the other program because sometimes Conspan may not be compatible with other programs.
5) Check if the user has minimum system requirements which are as follows :
• Microsoft® Windows® 2000, XP or Vista,• 512 MB RAM
• Mouse or other pointing device,• 100 MB available uncompressed hard disk space
• Internet access (recommended)
• 800 x 600 dpi with 256 colors minimum (1024 x 768 dpi with thousands or millions of colors
recommended)
6) If the user has big file and less memory then there are chances that you get Run time error.
Generally the Live load reaction shown in the conspan output is for the whole support and whatever LL reaction the user gets for the whole support is not multiplied with DF and IM factor. To get the unfactored LL reaction for each bearing location just multiply it by appropriate DFs based on exterior and interior beams .
Its left to the user to decide whether he/she wants to multiply with the DF and IM factor . Some cases the user wants the reaction with DF only and in some cases both IM and DF.
Original Author: Bentley Technical Support Group
For non-continuous model Conspan does not have an option to make interior supports continuous for composite loads.
Currently expansion joint option is not available in Conspan.
Rotation values are not available in the output.
In reality the program is simply using statics to find the deflection and then the program applies long term factors per PCI. User can see the Tutorial hand calculation on how the program is calculating the deflection.
Mcr reported in the ultimate moment table is the cracking moment which includes both live load and dead load So the values are high.
Mcr reported in the vertical shear is Moment causing flexural cracking of section due to externally applied loads (beyond Dead Load) so this value is used for Rating and the formula.
Vmu and Vu are 2 different entities where
Vmu = Factored shear occurring simultaneously with Mu .Basically shear is based on where the max moment is occurring, generally the shear value is less than Vu.
Example - Truck load in the mid span of the beam
Vu =Total factored shear force at section . Shear is based on where the max shear occurs .
Example - Truck Load is placed at the starting or end of the span.
User needs to go to BEAM TAB -->RESULTS --> In the new PRINT window check the Properties option under Design .
In the new Enhanced report Viewer window under Span 1 , Beam 1 option --> Straight Option, the user should be able to get the Beam Shortening (PL/AE) information in "inches" .
For time dependent losses Conspan does not provide the length information , user has to calculate it manually.
Please see the attached screenshot for more information.
To change the Fy value to 36 ksi then the user needs to go to rebar library and add a new rebar or modify the existing Fy value to 36 ksi . Please see the attached screenshot for more information.
When you use the FDOT provision, program uses the theta angle, draw a line, to the support and calculate the area of shear reinf. under this. It may be more conservative in some cases.
If the user does not select the FDOT alternative option then the program just considers the area of shear reinforcement. at the section under consideration. Let’s say if the user is looking at a section 4' from the end. Program will simply look at the area of the shear stirrups provided at the distance.
The default Strand cutting sequence in CONSPAN, is to start with one row of strands at the bottom, move all the way to the top and move down from there. User can see this information in the first column under Group/Height Yend and Ymid.
The table is part of the output primarily for precasters/fabricators. Not really for the designers.
This is the answer…..
1.) Using the value of concrete strength that you specify, the Youngs modulus is computed using the formula
2.) Using the Youngs modulus the loss in strength is calculated using the formula
3.) This loss in strength is substracted from total strength to give residual strength , which is then divided by the total area to give the tensile stress.
If the user has moved from LFD code to LRFD then according to LRFD there is no provision to add a concentrated load to the lane load as it is in LFD. Live Loads can be input in accordance with AASHTO Standard (LFD) Art. 3.7.1 by selecting either the H/HS lane loads or H/HS truck loads. CONSPAN analyzes the H/HS lane and truck loads separately. Therefore, the user must select both the H/HS lane and truck loads for analysis.
Moment and shear envelopes for live load are developed in two phases:
The truck load comprises the axle loads and preceding/trailing uniform loads acting together. When performing live load analysis, the program first considers the lane load (including associated concentrated loads) acting alone. Then, it considers the truck load acting alone. At each section, the values of the live load envelopes are obtained by comparing these two envelopes and taking the critical value. This process is done for each of the trucks selected on the Loads screen; the final envelope covers each of these envelopes.
As per article 3.6.1.2 AASHTO LRFD 2007 code nothing has changed with respect to HL93 truck loading in the latest AASHTO 2010 code.
In Conspan if the LRFD option is selected in the loads tab then the user can see the Design lane and Design truck is preselected and are applied at the same time.
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Calculation of K value as per the LFD code which is used under rating is shown under the hand calculations for Tutorial 1 (page HC 16).
According to AASHTO , DESIGN TANDEM is not used to calculate the deflection . If the user checks the Conspan output then the deflection value will be 0.00 for Design Tandem.
The Detensioning stresses are compared against the RELEASE stresses which can be either compression or tension stresses.
The application of 90% of the negative moment as per AASHTO code is hard coded and can’t be changed but one work around solution to this problem is to do 2 runs one by creating custom vehicle by increasing the present axle loads by 10% (that will be 110%) and then the program will apply 90% of it and then the user should be getting 100% of negative moment between dead load contraflexure points .And also the user needs to run one more file with default axle loads just to compare the results.
The permit vehicle loading is not used in the calculation of moments in the negative region. But in Analysis tab --> Load case option --> Permit vehicle, the user is able to see the moments and shear values.
Right now it is not possible to differentiate if the final reactions is for double truck or tandem as the output results does not mention what type of vehicle it is . The users have to do some testing to figure out the vehicle which is generating the maximum reactions.
Conspan runs the trucks from left to right and right to left. But nothing like one facing one direction and the other simultaneously in the other direction.Conspan still use 2D continuous beam models for our bridges not 3D models.
CONSPAN does not calculate cracked section properties. It will only flag the overstresses.
As per the code it is required to have double truck as the default truck and it is optional to have double tandem since no requirement is suggested by the code, as the commentary suggest as “PROBABLE CONSIDERATION “. Please see the attached screenshot for more information. And lot of states in US does not use double tandem for calculation of negative moment.
Yes, the user does NOT have to calculate the skew correction factor for shear by hand as the program will model true skew supports and distributes the loading.
CONSPAN allows the user to enter a starting skew angle as well as an ending skew angle for each span. In LRFD mode, when using the AASHTO formulae for computing the Moment Distribution skew correction factors, the smaller of these two angles is used. When computing the Shear Distribution skew correction factors, the larger of the two skew angles specified for the spans is used. When the refined method of analysis is used in either specification mode, the program sets up the grillage model considering the true bridge geometry created by these two skew angles. The input fields are grayed out in the Standard Specifications (LFD) mode, unless the option to use refined methods of analysis has been selected in the Analysis Factors dialog box.
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