V Slab Technical Help Section

V Slab Web App follows code requirements of the American Association of State Highway Transportation Officials (AASHTO) LRFD Bridge Design Specifications, Customary U.S. Units, 10th Edition. Design requirements and details vary by state and can also vary based on the preferences of the design engineer. V Slab Web App allows the designer to make the decisions. This software must be used under the direct supervision of an experienced design professional to ensure compliance with Federal, State and local requirements.

The sign convention used in this program assumes positive moments create tension on bottom surface and likewise negative moments create tension on top surfaces.

For the purposes of this help menu, items included in brackets [  ] are references to the applicable section of the AASHTO LRFD Specification. For references to the AASHTO Manual for Bridge Evaluation, 3rd Edition 2018 with Interim Revisions through 2019, items are included in brackets preceded by an R as [R-  ].

Once you have registered as a user and your email is verified, you will be directed to the Main Menu Page. This is the center for user controls, company setup and V Slab project files.

Initially, a new user will be set up in Demonstration Mode until a license is purchased and the user is assigned to a company. In Demonstration Mode, the user can create a new project for testing purposes, but the project cannot be saved. The user can recreate a new project as often as desired but there are limitations on what will be displayed on screen and in the report.

The user first purchasing a license becomes the Company Admin. The Company Admin can add user emails to the company based on the number of users purchased for the company. When a user, matching one of those emails, logs into V slab, they will be automatically added to the company. All users under the company will be able to open and use the project files stored for that company.

The Company Admin has access to renew and change the company subscription license. The Company Admin will be the main point of contact for licensing.

The Company Admin also has access to the Company Admin Page which provides Company and User Maintenance options. There can only be one Admin per Company but the Admin can be changed to a different User on this page. However, once the Company Admin is changed to another user, the original Company Admin will no longer have access to this page to make changes.

This page is also where the Company Admin can invite users to the company by adding the users' email to the company.

Every user has access to their own set of Default Values which will populate a new project.

There are two types of Default Values: Basic Bridge Settings and Loading Settings. The Bridge Default Values Page is presented below:

The second type, the Loading Default Values Page is presented below:

This page is where any Special Vehicles are defined. Special Vehicles cannot be defined inside of a project. Only stored Special Vehicles can be added inside of a project. A Special Vehicle in a project can be added to the user's Default Loading Values from that project but it can only be edited on this page.

After entering a job description, you can begin entering span lengths.  Up to nine spans can be entered.  The span lengths must be between 10 and 90 feet.  If more than one span is entered, adjacent spans are continuous.  If you want adjacent simple spans, you must run them as separate projects and complete the substructure design by other means.

The total Bridge Section Width must be between 12 and 120 feet.  The roadway width cannot exceed the total section width.  The difference between the bridge width and roadway width is the railing width.  The roadway width also serves as the loaded area for the future wearing surface (see load specification section).

The number of design lanes is left to the designer, but the value in [3.6.1.1.1] is presented and checked when the user leaves the page.  Overriding this value affects the design of both the superstructure and substructure.  Overriding the calculated value should be completed only after careful consideration of the affects.

The bridge skew must be between 0 and 45 degrees.  The upper and lower clear cover must be between 0.5 and 5.0 inches.  The tire wear depth must be between 0 inches and the upper clear cover depth.

The exposure factor (γe) is used in equation [Eq. 5.6.7-1] for crack control maximum reinforcing spacing. This value is described as 0.75 for Class 2 Exposure and 1.0 for Class 1 Exposure.  Since the crack width limits are subject to individual owner requirements, values from 0.5 to 2.0 are allowed in the program.  Different values can be provided for top and bottom steel layers to account for potentially different exposure conditions.

When the bridge data page is complete, click the next button to enter the slab data page. The bridge can be designed as a flat slab or one with a variable depth. Variable depth slabs have haunches at interior supports for continuous spans. Three variable depth slab options are available: Constant Depth Haunch, Slope-Tapered Haunch and Parabolic Haunch.

Variable depth slabs pull positive moments away from the span to the negative moment region over the interior supports where they can be more efficiently carried by the deeper slab thickness. The interior slab can therefore be thinner than the minimum recommended depth by AASHTO as the depth at the interior support is typically thicker than the minimum. This is not only more structurally efficient, but the dead load moments are also reduced due to the distribution of concrete thickness.

There are 14 design points for each span. For a flat slab these locations are equal to the span length divided by 14. For a variable depth slab, a point will be placed at the end of the haunch and at the offset location. The distribution of the other points depends on the relative length of the span and haunch length.

The structure stiffness matrix and fixed end forces are numerically integrated using Gauss Quadrature for variable depth members. Since the slab dead load is not constant for a variable depth slab, there is an option for printing a unit load table if the designer needs it.

Depending on the haunch layout, the required maximum negative moment steel may not occur over the interior support. The designer should review the calculated required steel at each design point to ensure proper haunch configuration.

For parabolic haunches, the depths at 1/5 points can be printed to ensure proper dimensioning during slab detailing. The equation used to determine the slab depth is determined by calculating a constant “K” which is equal to ( transition length² ) / ( slab depth @ support – slab depth @ span ) . The slab depth at a point “X” measured from the end of the transition is equal to the slab depth in the span plus ( X² ) / K.

For example, to find the depth at 6 feet from the end of a 10ft. haunch with a 20 inch slab and 30 inch deep haunch at the support, the following is calculated:

Note that the calculation is always from the span side of the transition to the offset location.

When the slab data page is complete, click the next button to enter the material data page. Enter the reinforcing steel yield strength and whether the top and bottom reinforcing bars are epoxy coated. This information is used for the cut bar development and splice length calculations.

The unit weight of reinforced concrete is entered. Only normal weight concrete, as defined by the code (135pcf minimum), is used for calculation purposes. After the concrete compressive strength is entered, the calculated modulus of elasticity is displayed. This value can be overridden for live or dead load deflection calculations.

Live load deflections are optional and can be owner specific. For this program, the live load deflection calculations are the larger of a single truck or 25% of the truck with full lane loading [3.6.1.3.2]. Deflections are based on service loads (load factor of 1.0) with the dynamic load allowance of 33% applied to the truck portion only. The live load is adjusted for the number of loaded lanes, and gross cross sectional properties of the bridge are used.

The live load deflection can be adjusted to a pseudo-cracked condition by modifying the modulus to a value lower than calculated, see [5.6.3.5.2] for guidance. To account for lower initial concrete strength when the forms are removed, the dead load modulus can also be adjusted. The modulus of elasticity is prescribed in [5.4.2.4]. This equation has correction factors for lightweight concrete. Since V Slab assumes normal weight concrete, the modulus is calculated using equation [C5.4.2.4-3].

When the material data page is complete, click the next button to enter the reinforcing development and splices page. This page allows the user to select the development and splice data for the project. The values can be calculated or user defined values can be entered. For calculated values, the assumptions related to development factors are displayed.

Splice types must be selected for top and bottom bars if the calculated values are used. The development and splice values in the table will be used for the project when the next button is selected. These values can be stored as default value, or retrieved, if previously saved.

A special feature is included to address the cut bar development length location calculation. The code requires the cut bar to be extended a certain distance past the point where it is no longer needed. This is called the cut bar extension. The required extension is calculated for both ends of the bar.

The extension is physically a part of the bar, but prior to the 10th Edition, the code was silent on whether the extension contributed to the development length of the bar. The Commentary now provides Figure C5.10.8.1.2a-1 which allows the extension as part of the development length. However, it is conservative to ignore it. As such, the designer has the option of ignoring the extension portion from the development length, or including it. See Section Checks for a discussion on the area of steel calculations related to these options.

Once the Development/Splice data is complete, the applied loading must be specified. The railing load is entered in pounds per foot (plf) and is assumed to be applied on both sides of the bridge. In other words, the total load is twice the plf load entered. This rail load is applied by adjusting the track bar. The specified percentage is distributed over the entire bridge width, and the remaining percentage is applied directly to the exterior Strip. If the bridge span is long and the width is narrow, the rail loading will tend to distribute more evenly over the width. Therefore, the distribution percentage will be higher. If the bridge span is relatively short compared to the width, the rail load will likely be more concentrated along the edge beam.

It is up to the designer to specify the appropriate percentages. Some transportation departments may require the railing load to be entirely distributed over the bridge width, while others may choose to apply the rail load entirely to the edge beam. The code does not specify a provision regarding this distribution. However, it is the author’s opinion the load will be shared based on the three dimensional effect of the span to width ratio.

The future wearing surface is provided in pounds per square foot (psf). It is applied directly to the interior strip width. For the edge beam, it is applied to the roadway portion only. (i.e. if the roadway width is less than the bridge width, the edge beam future wearing surface will be a ratio that reduces the load to account for the railing location)

Three additional loading options are provided for unique cases. A distributed line load can be applied to the interior and exterior strip. These are unit width loads and must be calculated by the user accordingly. If a live load is being modeled, the pattern loading toggle can be selected to produce the maximum force effects. Otherwise, the entire surface can be loaded using the full load toggle. A load factor between 1.0 and 1.75 at 0.05 increments can be selected. This same load factor will be used for design and load rating if the loading is applied. An additional dead load (applied with a DC load factor) can be applied to the exterior strip. This was intended to account for additional sidewalk dead load on the exterior strip.

To reduce the amount of output, you can check the “Suppress Detailed HL-93 Printout (Summary Only)” box, and/or the “Suppress Detailed Special Vehicle Printout (Summary Only)” box. If the designer requires more detailed information about the HL-93 envelope, the “Print HL-93 Lane Load Only Table” can be selected to help determine the component parts of the envelope.

If the designer wants to see shear loading values for the superstructure, select the “Print Shear Summary Table” checkbox. A table is provided in the report with factored values for the slab only, and for the design unit width for interior and exterior strips. The strips include dead and live loading.

In addition to HL-93 loading, the program will analyze special vehicles and design for the maximum resulting load cases. Special vehicles can only be created on the Default Loading Values Page. Stored Special Vehicles can be added to the project on this page. Also, Special Vehicles in a project can be stored in the users Default Values.

Special vehicles can be analyzed with or without the standard 640plf lane load included. In addition, the user can set the strength load factor to be used. The 33% dynamic load allowance is applied to the vehicle portion, but not the lane load if included.

After defining the loads, the next step is to determine how they are applied.  Two types of vehicular loading can be applied to the structure: HL-93 and Special Vehicles. HL-93 loading [3.6.1.3] consists of a tandem, with lane loading or one variable spaced axle design truck, with lane load. In addition, for negative moments and interior support reactions, the provisions of [3.6.1.3.1] for 90% of two trucks spaced at 50 ft. apart with lane loading is checked. Lane loading is pattern applied at tenth points to achieve maximum affects. A dynamic load allowance [3.6.2] of 33% is applied to the truck and tandem (not the lane loads).

Fatigue loads are calculated in accordance with [3.6.1.4]. A dynamic load allowance [3.6.2] of 15% is applied to the fatigue truck with a load factor of 1.75 for fatigue type I and 0.8 for fatigue type II. The distribution width for a single lane is used in accordance with [4.6.2.3-1]. This width is multiplied by 1.2 in accordance with [3.6.1.1.2], specifically the commentary regarding fatigue. The fatigue load on the exterior strip is equal to the interior strip.

Special vehicles are analyzed similar to the HL-93 loading. A dynamic load allowance [3.6.2] of 33% is applied to the vehicle (not the lane load if applicable).

Two load application definitions are available. The first defines the design envelope case for creating the required reinforcing layout. The second is intended for the post-constructed and potentially deteriorated load rating case.

The code allows an adjustment (reduction) of the live loading to account for bridge skew. Since bridge skew creates a shorter span distance diagonally between the supports, the live load longitudinal superstructure moments are reduced as the skew increases. Select the checkbox, for each definition, if you want the program to reduce the live load moments according to [Eq. 4.6.2.3-3]. It is always conservative to leave this unchecked.

DESIGN ENVELOPE

LOAD RATING

The two terms ∅c and ∅s are the condition and system factors. These are described in [R-6A.4.2.3] and [R-6A.4.2.4]. The system factor for a slab bridge [R-Table 6A.4.2.4-1] is typically 1.0 and the condition factor depends on the deterioration of the bridge. The condition factor ranges from 0.85 to 1.0 depending on the SI&A Inspection Rating (See Commentary). For the purposes of the rating equation, these terms should be entered as a combined rating factor consisting of the product of the two selected values. In the event a system factor other than 1.0 is desired, multiple the two factors together and enter the resulting combined factor.

A detailed fatigue rating can be completed if desired. This is provided to check the deteriorated condition for fatigue loading and to provide reinforcing stress ranges. The effective reinforcing steel percentages are used, but the system and condition factors are not applicable, because this is a service condition check.

When the analysis is complete, the results will be displayed graphically for designer interpretation as described in this section.  The strip width is calculated in accordance with [4.6.2.3]. This can be changed to a user defined value. The maximum negative and positive area of steel are calculated using the effective depth for bar sizes #4 through #11. In accordance with [5.5.4.2 & 5.6.2.1, the reinforcing strain is checked. If the maximum strain is greater than the tension control strain, the section is tension controlled and a Phi Factor of 0.9 is used. For strains less than the compression control strain, a Phi Factor of 0.75 is used. Between these two limits, a transition Phi Factor is used.

The required area is compared to the minimum area of steel, which is equal to 1.2 times the cracking moment [5.6.3.3]. This utilizes variability factors that include reinforcing steel types. Since yield strength is the only reinforcing parameter entered, the more conservative of each factor is utilized. In addition, the modulus of rupture [5.4.2.6] includes a concrete density modification factor. V Slab assumes the use of normal weight concrete only and therefore uses a factor of 1.0 in the equation. The lesser of the minimum area or 1.33 times the calculated area of steel is provided. The fatigue stress range of the resulting area of steel is checked as specified in [5.5.3.2]. If the stress range is exceeded, the area of steel is increased until it is satisfied.

Temperature and Shrinkage steel is also calculated and provided as a minimum value. Minimum T&S steel is calculated using equation [5.10.6-1]. The slab depth at midspan is used for the least thickness, and the least width is the smaller dimension of the bridge width or the bridge length (in inches). This minimum steel is required in each face and in each direction. Additional reinforcing requirements may be necessary at haunch conditions. The design engineer is responsible for determining these specific details.

After calculating the required area of steel at every design point, a bar spacing with two appropriate bar sizes are selected to satisfy the required area of steel and spacing limitations. The program begins with a trial spacing of 6 inches which reduces it if a suitable solution cannot be found. This process is completed for the Interior and Exterior Strips. The results are presented graphically on the Reinforcing Layout page.

Use zoom and pan to navigate to an area for editing. The mouse wheel can be used to zoom in place of the toolbar buttons. Use either the scroll bar or the left mouse button to pan left and right. After zooming to an area, move the mouse cursor over the bar you want to edit and click the left button.

A dialog box will open for the bar selected. The light blue bar is the cut bar, and the dark blue bar is the continuous bar. The cutoff points can be changed for the cut bar.

The continuous bar can be modified to add up to two splice bars right and left of the main continuous bar. The reinforcing rating condition can be modified for both the cut bar and continuous bar. Enter the percent of the bar area that remains effective for use in determining the load rating moment calculations.

The toolbar buttons provide the following functions:

If the designer wants a different spacing than originally presented, modify the spacing and select Reset Top/Bottom Bar to have the program create a new bar layout. If the project was previously saved after a reinforcing layout was presented, the new layout will remain unless the user does one of the following: changes the number of spans, or selects the “Reset Top/Bottom Bars” button.

To determine if the reinforcing bar layout is adequate, the program creates section checks at appropriate locations for the top and bottom bars. Section check points are placed at the ends of spliced bars and at the extension points of cut bars. In addition, if the maximum area of steel occurs away from these points another section is added.

If the area of steel provided is greater than required, and if the spacing provided is less than the maximum allowed, then the section check box displayed on the layout window will be green. If these conditions are exceeded, the box will be displayed as red. To display the detailed information at the section, place the mouse cursor over the box and a textbox will appear.

DETERMINING REINFORCING STEEL AT A SECTION

For the cut bar, the program calculates the development length of the bar without reduction factors. The top bars use the top bar modification factor and the development length of both bars are adjusted for epoxy coating. The bar spacing and cover conditions are used in the calculation. The designer must select whether the development length is measured from the end of the bar or from the extension location. For the cut bar, the following detail describes how the program determines the useable reinforcing steel at a section:

The provided area of steel is the sum of the continuous and cut bar reinforcing steel. The required area of steel is determined using the effective depth calculated using the largest bar at a check section point.

LOAD RATING SECTIONS

For load rating purposes, a section is provided at each design point as well as the ends of splices and cut bar extensions. To determine the reinforcing steel provided, the effective reinforcing rating percentage is applied to the bar size before the ratio is calculated as described above. However, the development length of the cut bar is calculated assuming the full bar capacity.

Each support can be designed as either a cap type or wall type substructure. The cap type support is flexible while the wall type is considered to be rigid. This affects the way the pile loads are calculated as well as the reinforced concrete design.

If a wall type is selected, only the cap drop (which becomes the wall depth), the bottom width, and if it is the first or last support, the pavement dead load will be enabled. The other entry fields are needed for the cap type support.

PILE DESIGN

CAP DESIGN

WALL DESIGN

When the design is complete, the program output is presented in the form of a report. In demonstration mode, not all of the output will be included in the report. If the software is licensed, the report will provide complete Information.

The report is provided in PDF format and can be viewed in a browser window or downloaded directly. Some of the data can be suppressed using the appropriate checkboxes on the loading pages. This can significantly reduce the number of pages in the report.