Design and Rating of Precast Culverts for LRFD

Design and Rating of Precast Culverts for LRFD
ETCulvert Design and Rating of Precast Culverts for LRFD ACPA PipeSchool March 5, 2015 Brian Barngrover, P.E.

Brian Barngrover, PE Brian Barngrover, P.E., is Vice-President of Eriksson Software, Inc. Brian is in charge of software development at Eriksson Software. He has over 30 years experience in software development for the engineering community, along with experience in engineering design for the precast/prestressed concrete market. Brian began his career at a major precast/prestressed concrete fabricator gaining firsthand experience in all aspects of the design, detailing, and fabrication of precast concrete.

Agenda 10:30 am Introduction 1. ETCulvert Design Process
2. Common Design Questions 3. Getting to 1.0 4. Advanced Topics 5. New Features 6. Questions Corey has given you a good summary of what goes in each input dialog box, but you also need to know what goes on ‘behind the screens’. Why? What if a reviewer asks where this or that number comes from? Also, how do you handle typical design situations that come up every day. We have both a software development firm and an engineering consultancy. Here are some of the common design questions that our engineers face regularly, along with questions we get from reviewers.

Common Design Questions
Skewed Culverts How are # of Lanes Determined Pinned/Roller Supports Single Layer of Reinforcement The LRFD specification states that you must take the skew of the culvert into account if it is greater than 15 degrees, but they do not tell you how to do this.

Skewed Culverts: Case 1 Only the Exterior Culvert is Skewed
Skew angle is < 15 degrees Skew angle is > 15 degrees If only the exterior culvert is skewed and the skew angle is less than 15 degrees, use the dimension shown as the clear span, and keep the lanes parallel.

Skewed Culverts: Case 1 Only the Exterior Culvert is Skewed
Skew angle is < 15 degrees Skew angle is > 15 degrees If only the exterior culvert is skewed and the skew angle is greater than 15 degrees, use the dimension shown as the clear span, but now place the lanes perpendicular.

Skewed Culverts: Case 1a
Straight culverts, skewed roadway Skew angle is < 15 degrees Skew angle is > 15 degrees A subset of this type of culvert, the culverts are straight, but the roadway is skewed less than 15 degrees, use the distance shown as the clear span, and keep the lanes parallel.

Skewed Culverts: Case 1a
Straight culverts, skewed roadway Skew angle is < 15 degrees Skew angle is > 15 degrees If the culverts are straight, and the roadway is skewed greater than 15 degrees, use the distance shown as the clear span, but place the lanes perpendicular.

Skewed Culverts: Case 2 All Culverts are Skewed
Magnitude of Skew is not Important Use the Skew dimension as the Clear Span If all of the culverts are skewed, then the magnitude of the skew is not important. Use the skew distance as the clear span, and keep the lanes parallel.

Skewed Culverts How are # of Lanes Determined Pinned/Roller Supports Single Layer of Reinforcement

How are # of Lanes Determined?
Traffic is Parallel to Main Reinforcement Less than 2 foot of fill depth Greater than 2 foot of fill depth 5 cases, with a maximum of 4 lanes 2 lane case is checked twice If less than 2 foot of fill depth, then the code states to use 1 lane.

Traffic is Parallel to Main Reinforcement Less than 2 foot of fill depth Greater than 2 foot of fill depth 5 cases, with a maximum of 4 lanes 2 lane case is checked twice

Traffic is Perpendicular Code gives no guidance here 10 foot lanes are used, rounded up May be overly conservative Can limit maximum number of lanes In both the parallel and perpendicular cases we assume 10 foot wide lanes (it makes the lane positioning algorithm way too complicated to try to place a 10 foot truck inside a 12 foot lane at varying positions). So the program might be overly conservative in the case of long clear spans and perpendicular traffic direction. I will show you later how to handle this if you feel that we are placing too many lanes on the culvert in the perpendicular case.

Pinned/Roller Supports
3-Sided Culverts Default Support Conditions Discuss long span to depth ratios observed in the NE.

Pinned/Roller Supports
3-Sided Culverts Alternate Support Conditions If Span/Depth Ratio > 4 This support condition allows the top span to behave as a simple span, with no negative moments in the corners.

Single Layer of Reinforcement
Cover Dimensions Main Reinforcement Temperature and Shrinkage Steel for Walls Distribution Steel + T & S Steel for Top and Bottom Slabs Check that Main Reinforcement satisfies T & S steel

Cover Dimensions Set Cover Dimensions such that Layers line up
Coordinate with Bar Diameter

Main Reinforcement A1, A2, A100, A200 and B2 Bars only
Program assumes singly reinforced Main reinforcement is in the direction of the clear span.

Temperature and Shrinkage Steel for Walls
Can use the C1 bars in bar schedule May be better to use calculated required

Distribution Steel + C1 Bars for Top and Bottom Slabs
Need greater of Distribution Steel (C100 and C200) + C1 Bars or 2 * C1 Bars May be better to check required column

Final Check Check that Main Reinforcement satisfies Temperature and Shrinkage requirements Main reinforcement is parallel to the clear span.

Global Stability Sometimes precasters are asked to check global stability In our design practice, sometimes we are asked to check the global stability of the exterior culvert. We check sliding and overturning.

2. Common Design Questions 3. Getting to 1.0 4. Advanced Topics 5. New Features 6. Questions

Getting to 1.0 Designs must now include ratings
MBE and LRFD are diverging, mainly through different load factors Ratings can be found in the Critical Section Tables In the first edition of the MBE, if your design was OK by LRFD, then you should get a rating factor of 1 or greater. However, as the MBE matures, it is starting to diverge from LRFD. In our opinion, they should make the ratings just another chapter in LRFD. So, how can you get to 1.0? Here are some legal methods of increasing your rating without spending any extra money.

Reduce # of Lanes Program may be too conservative
Only allowed for perpendicular lanes To get the # of lanes placed on the culvert when the lanes are perpendicular, the clear span is divided by 10 (not 12). This may place too many lanes on the culvert. In earlier version of ETCulvert, we allowed users to specify the # of lanes in both directions, however, it does not make sense to do this in the parallel direction. This is the way we currently do this, in new version we will do it slightly differently, as you will see later in this presentation.

Use Maximum Moment with Maximum Shear
For use with those equations for Vc that use an interaction between V and M Forces the program to use the moment at the flexural critical section Forces the program to use the moment at the critical section for moment for the interaction equation used in the calculation of Vc, if unchecked will use the corresponding moment at the critical section for shear.

Use Axial Loads to Your Benefit
Program uses a P-M diagram Usually increases flexural capacity Members of box culvert are typically either in the tension controlled zone, or the transition zone, so you will get an increase in the flexural capacity under axial compression. Note that some states do not allow this.

Use Pipe Equation Use pipe equation from LRFD
Multiplier available for Nu You also have the option to use the pipe equation to calculate the flexural resistance. This will also allow an increase in the flexural capacity if the member is in compression. Note that you can set the Nu multiplier to zero in those states where this is not allowed.

Increase the Steel Yield
2013 Interims allow higher yields Mesh typically has higher yields as well

Consider Haunches in Location of Critical Section
Used to locate the critical sections for flexure and shear 3 possible locations for flexure 2 possible locations for shear Three possible options for locating the critical section for flexure and two possible locations for shear.

Illustration of dv vs d

Haunches not considered:
If you select to NOT consider haunches in the location of the critical section, probably the most conservative option.

Haunches considered: If you select to consider the haunches in the location of the critical section, default option, does not change the location of the critical section for shear, only that for moment.

Extended beyond haunches:
Can also select to extend the location of the critical section for both flexure and shear, probably the least conservative option.

Haunches in Structural Model
Re-distributes slab moment from positive region to negative region

Always Distribute Wheel Loads
When fill depth is < 2’, will still treat wheel loads as distributed, not point LRFD allows wheel loads to be distributed when the fill depth is less than 2 foot, this option accomplishes that. Later on I will show you exactly what the program does with the LRFD equations here (Advanced Topics).

Always Check Iterative Beta
Forces program to always check general case (Appendix) for shear resistance Will use largest value of Vc found If unchecked will only use calculate one Vc value (using an internal decision table that is based on several factors). If checked will calculate 2 values for Vc and use the higher value. Note that sometimes when checked will still just calculate one value for Vc, if the internal decision table leads to the selection of the iterative beta method for Vc.

2. Common Design Questions 3. Getting to 1.0 4. Advanced Topics 5. New Features 6. Questions Here we will get down into the weeds a little bit, because some of these questions may come in in the review process.

Live Load Distribution
Traffic is Parallel to Main Reinforcement Less than 2 foot of fill depth Perpendicular Width = S Parallel Width = Tread Length + LLDF * Fill Depth (2nd term is optional) One lane only ‘Always Distribute Wheel Load’ option adds or removes the 2nd term Live load distribution is broken down into 4 sub cases, divided by fill depth (less than or greater than 2 foot) and traffic direction (parallel or perpendicular to main reinforcement). Also, remember that the parallel direction is always the direction parallel to main reinforcement. Perpendicular is always the direction perpendicular to the main reinforcement. The truck is then moved back and forth in the parallel direction to build the envelopes. The ‘Analysis Option’ to always distribute the wheel load adds or removes the 2nd (optional) term.

Traffic is Perpendicular to Main Reinforcement Less than 2 foot of fill depth Perpendicular Width (Table ) Parallel Width = Tread width + LLDF * H Perpendicular distribution width is calculated based on the referenced table. Then the program moves the truck around to find the combination of axles that fit within this width to give us the highest load. This load is then used to construct a new ‘truck’ that is also distributed (or not) in the parallel direction. This new ‘truck’ is then moved back and forth in the parallel direction to build the envelopes (just like before).

Traffic is Parallel to Main Reinforcement Greater than 2 foot of fill depth Distribution widths calculated same for both directions Basically only the number of lane cases is different.

Traffic is Perpendicular Greater than 2 foot of fill depth Similar to Parallel Case for Distribution Width calculations Again uses 10 foot lanes, rounded up

How are axles combined when fill depth is greater than 2 foot?

Combining Axles 6th Ed. LRFD language demands that axles are always combined We believe that this may be un-conservative for fill depths just over 2 foot Sketch up this case on something while you talk about it.

Combining Axles New code equations in 2013 appears to require that loads be combined only when prisms overlap This seems more appropriate Sketch up new concepts while you are talking about it.

Lateral Soil Loads (Max/Min)
For LRFD 5th Edition use load factors For LRFD 6th Edition use pressure ratio The program checks both maximum and minimum lateral soil loads. To achieve this max/min effect, the program uses the max and min load factor for the 5th Edition of LRFD. This mirrors the approach in the 1st Edition of the MBE, which was published at about the same time as the 5th Edition.

Lateral Soil Loads (Max/Min)
For LRFD 5th Edition use load factors For LRFD 6th Edition use pressure ratio For the 6th Edition of LRFD, the program uses the ratio of the maximum and minimum equivalent soil pressures. This mirrors what is being done in the 2nd Edition of the MBE.

Load and Resistance Factors
Factors for LRFD P-M Diagram Phi for moment is not used unless axial load is ignored Back on the load and resistance factors screen you have two phi factors related to the internal P-M diagram. The phi for moment is not used unless the axial load is ignored on the Analysis Options page.

Upper Limit on As For use with Design Mode
Iterative beta method can produce very large member thicknesses We found that in the Design Mode algorithm that we got crazy member thicknesses at times, so by limiting this option you can circumvent that and break out of the design mode algorithm looping.

Always Include Distribution
Both Design and Analysis Modes Fill depths greater than 2’ Right now, we use distribution steel in both top and bottom slabs. However, this may be too conservative for the bottom slab. Will allow you to eliminate the bottom slab distribution steel in next release.

Longitudinal Steel For non-distribution longitudinal steel
Follow specification recommendations Only use % of cross section area Only use temperature/shrinkage If the follow specifications option is used, then the program uses the larger of the two. Otherwise, you can set it to use either.

Multiple Types of Vehicle Loads
Design Load Typically HL-93 ‘Notional’ truck Legal Loads Represents real trucks (unlike HL-93) Typically not required for Design Permit Loads Varies by state Also represents real trucks Ratings are required at design time by many states. Also, states have more than one truck that needs to be designed for. You should always check the Design truck (HL-93), you will typically have other types of vehicles that you have to check during the design. Legal loads are typically reserved for existing bridges that do not rate out by Design loads. Used for posting of load limits. Can be used for design if required by a specific state.

Legal Loads New load factors in MBE
Treat as a Design Load in ETCulvert (use Strength I)

Permit Loads New Load Factors in MBE Remember to treat as Strength II

Permit Loads New Load Factors in MBE Remember to treat as Strength II
Remember to switch to Strength II if you are analyzing or designing for permit loads. (we have a new way of doing that now)

Multiple Vehicle Support
Removed trucks from loads screen New menu option for Moving Loads

Removed trucks from loads screen New menu option for Moving Loads ETCulvert now has support for multiple vehicles.

Unlimited # of Vehicles Supports all 3 types of vehicles Can also turn trucks in the list on and off, without removing them from the list

Unlimited # of Vehicles Supports all 3 types of vehicles Can set rating and design parameters for three vehicle types.

Unlimited # of Vehicles Supports all 3 types of vehicles

Load factors input here are for ratings only Remember that these are MBE load factors for ratings only, still have separate load factors for design.

Still have separate load factors for design Remember that these are MBE load factors for ratings only, still have separate load factors for design.

Adding new trucks to the list Edit any truck properties

Can also save and retrieve moving load configurations Will save each configuration in a separate library file Current moving load configuration will be saved with the ETCulvert data file. Do not have to re-load a configuration when you re-load the data file.

Can also save and retrieve moving load configurations Will save each configuration in a separate library file

Additional options available at the bottom of the dialog box Note that ability to set # of lanes is now only available for perpendicular lanes These options will be used for all moving loads

Additional options available at the bottom of the dialog box Note that ability to set # of lanes is now only available for perpendicular lanes

Finding the Controlling Case(s)
Program now finds controlling case Note that different members may be controlled by different vehicles However, this may result in very long text reports, especially with multiple fill depths included In our engineering practice, we have had up to 1000 page culvert design reports, probably upwards of 95% of which were text reports from ETCulvert.

Text Report Navigation
Reports will be getting longer, as you add multiple trucks and multiple fill depths For the foreseeable future, printed reports will be limited in width to 8-1/2” Therefore, we have “white” space to the right that we need to utilize

Text reports now have a navigation panel on the right side of the report

Highlights show you where you are Clicking on navigation tree will take you to that place in the report

Can remove portions of report Report will be updated in real time Report layout will be saved in data file This allows you to craft your own custom report, as the specific report layout will be saved in the data file.

Text report now includes summaries of both design and ratings

Wing Wall Design Add in module to ETCulvert
Basically a retaining wall design Supports both US Customary and Metric units Implements AASHTO, IBC and CHBDC specifications Output is automatically updated as the input changes

Wing Wall Design Window is divided into three panels

Wing Wall Design Left most panel contain all of the input divided into tabs

Wing Wall Design Center panel contains the output tabs

Wing Wall Design Visual model is updated in real time as the user changes the properties

Wing Wall Design Center panel also contains the report

Wing Wall Design Right most panel contain the report navigation and design status

Wing Wall Design Navigation portion of the summary panel similar to that of ETCulvert

Wing Wall Design Design status provides the user visual cues, and can be used for navigation as well

Foundation Design Only for 3-sided culverts Spread footing only

CHBDC Support ETCulvert will support CSA-S6-06, including all supplements

CHBDC Support Program also supports Metric units in both the input and the output

Support for Metric Reinforcement
Soft Conversions for Rebar

Canadian Rebar Standard

Also support metric WWR sizes (soft) If there is a ‘hard’ form of WWR sizes for metric we have not found them.

Size/Spacing Pairs Library
ETCulvert ships with an internal size/spacing list for use in Design Mode Now users can create their own lists These lists can be built from any of the US Customary or Metric rebar and WWR sizes that we support

Size/Spacing Pairs Library
Supports either rebar or mesh Create multiple lists and share them

Enhanced Mesh Support 4, 6, and 8 sheet layouts
Can simply combine 2 and 4 sheet layouts for both inner and outer layers We started with the 4, 6, and 8 sheet layouts, and realized that it was simply combinations of 2 and 4 sheet layouts for both the inner and out layers. Not sure if you can have laps in the same place (draw sketch on easel).

Enhanced Mesh Support Will also support ‘nesting’
Plus you will still be able to have 2 layers in various locations Show sketch of nesting on the easel.

Complete Graphics Overhaul
Will probably lead to user defined section cuts and views on reports Will eventually allow you to select the specific section cut or external view that you want for the report. Instead of our choices for what to display, you will be able to select them. Will also allow for better mixed text and graphic reports.

New Structural Analysis Core
Why is this important? Re-Built from the ground up Have added many new features More new features are planned Will allow use to easily add extensions to our core routines. Will allow the use of non-symmetrical haunches, for example. We will also be able to add hinges (joints) in the walls (talk about U-shaped section with lid here). We will be able to add support for dynamic analysis (currently not required by code). We will also eventually add support for non-linear springs, which leads to soil-structure interaction, which will lead to arched culverts.

Future Plans Stripping and Handling Hinges in walls Mesh sheet layouts
CAD ready drawings I have alluded to some of the changes we are thinking about in the program during this presentation. Here is a list of the major changes.

Design and Rating of Precast Culverts for LRFD

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