October 1, 2020
Designing Pultruded FRP Structures Using RISA-3D and PulCalc
FRP or Fiber Reinforced Polymer composite materials are becoming more and more popular in the construction world of today. There are many reasons for using composite materials, which go beyond the scope of this article but suffice to say that composite/FRP materials are now used in almost every facet of the construction/engineering industry. PulCalc was developed to allow a designer to use RISA-3D data in a model in order to evaluate FRP member capacities.
Pultruded FRP profiles are manufactured worldwide and are very similar to the standard AISC profile geometry found in the “Hot Rolled” Materials section of RISA-3D. The designer can therefore use these standard AISC profiles as they closely match the pultruded profiles of a FRP manufacturer (ex. Creative Pultrusions, Bedford Plastics, etc). When using these members, it’s very important that the user disregard the AISC Code data for FRP members when a model is run due to the fact that the material properties, behavior under load and design constraints are very different than steel! This is where Pulcalc comes in to assist the designer using RISA-3D.
PulCalc has several worksheets that look at FRP member capacities as well as connection data. The worksheets available to the user are as follows:
- Member Worksheets using ASD or LRFD (Imperial or Metric units)
- General FRP Members
- Angle FRP Members
- Connection Worksheets using ASD or LRFD (Imperial or Metric units)
It is important that the user selects the worksheet that matches the load combinations and design code established in RISA-3D.
PulCalc follows the ASCE 2010 Pultruded FRP Pre-Standard. The material characteristics and allowable (or factored) data are based on the formulas within the standard and not specific to any manufacturer. As most projects that utilize FRP go through bidding, the characteristic data is generic and not specific to a manufacturer or resin type (polyester or vinyl ester). In this way a contractor or fabricator can use any known manufacturer.
Setting Up a RISA-3D Model
There are a couple different ways that an engineer can set up FRP members in RISA-3D, however the easiest method is to create a New Material through the Data Entry Tables and then set up a New Member under the Hot Rolled Section Sets tab. FRP material properties can be found under the Properties Worksheet in PulCalc.
The properties listed in red are typically used and can be input in RISA-3D. If a specific manufacturer is going to be used for a project the user can match the Young’s Modulus (E) and/or Shear Modulus (G) to the material being used within the RISA Material tab.
Typically, every FRP member type should be set up individually and labeled with a Type (such as beams, columns, braces, etc). Setting up RISA-3D in all other respects is the same as any other created model. These inputs include boundary conditions, joint coordinates, basic load cases and load combinations.
Run the RISA-3D Model
After completing the setup of the model, make sure to run a batch solution. The required data can be found by cycling through the individual member detail reports. When evaluating each member, we are interested in the Axial Forces (A) and Bending Stress (Fc) for each FRP member. This data can then be input manually into PulCalc.
With the RISA-3D model results available, open PulCalc and enter your password if required. Next, choose the desired members worksheet that applies to your analysis and selected design code. An example of a member worksheet that corresponds to the information shown the previous detailed report from RISA-3D is shown below.
In this example, we entered both the axial (A) and bending (Fc) data from RISA-3D into the member line (these value are marked in purple). The user can then input the selected profile type and member length. With the values from RISA-3D input, the spreadsheet automatically calculates the factored capacities as well as the unity checks for axial, bending and combined forces. If any of the unity check values exceeds 1.0, the value is shown in red, indicating there is an issue with the model or member. Each of the other member worksheets (angle and general) function in a similar fashion and allow users further control over the member properties, allowing for input of either custom or manufacturer specific shapes.
Note: Engineers can enter up to 40 members on a single worksheet.
In the world of FRP structures, connections are perhaps the most important aspect of the design. Determining the proper connections is at times a real challenge. PulCalc evaluates typical connections including bolted, PIN bearing, shear and FRP clip capacities. For bolted connections, the user can select input options such as bolt type, number, size, etc. With the input added (including the loading found in the RISA-3D detailed report), the sheet will list the capacity check of the bolt(s) as well as the FRP material. These input values are then compared to the shear and tensile strength values for FRP bolts.
A final note, there is a new ASCE FRP Standard in the works with an intended release date of early 2021. This will result in an updated version of PulCalc, as some of the formulas will change. For more information about PulCalc, click on the link below:
Link: Fibr Design Website