May 11, 2023

RISA | Inputting the Wood Adjustment Factor, le, in RISA-3D

Do you need to include effective length, le, per NDS Table 3.3.3 for your wood design?

Yes? While RISA-3D does not automatically follow this le procedure per NDS Table 3.3.3, you as the engineer can input these values once you have them calculated. Inputting the effective length value in RISA-3D allows the program to use the correct value for the Rb calculations.

NDS provisions use an effective length approach to account for different load and support conditions. How does NDS do this? By calculating the slenderness ratio, Rb using the effective length, le, instead of the unbraced length, lu.

Follow the steps below to calculate and input the effective length into RISA-3D.

STEP 1: Determine your span type and loading condition.

STEP 2: Calculate and determine the effective length values from Table 3.3.3 based on the unbraced length and loading.

lu = 10ft

d = 9.25in = 0.77ft

lu/d = 12.97 > 7, therefore

le = 1.63lu + 3d = 18.61ft

STEP 3: Enter the effective length value calculated into your unbraced length properties section in RISA-3D.

Additional information regarding this topic can be found in our Help file in the Unbraced Lengths section.

Tags: RISA-3D, Wood

March 25, 2015 | RISA-3D, RISAFloor, New, Unbraced Lengths

How are Unbraced Lengths coordinated between RISAFloor and RISA-3D?

In a RISAFloor model the columns and beams can be assigned unbraced lengths, or the program may calculate them automatically. This unbraced length information can then be used in RISA-3D. A great example is the frame below: The beams along Grid Lines 1 and 2 have fixed ends. Therefore their bottom flanges are in compression, so they need to be braced. The 12” deep gravity beams framing into them should provide sufficient bottom flange restraint for the lateral beams, so the Segment command was manually entered for the lateral beams: The program now considers the gravity beams (spaced at 7.5 feet o.c.) to brace the lateral beams: When the model is transferred into RISA-3D, the gravity beams are no longer present though. Not even a node exists at their end locations: How can we ensure that the RISA-3D model knows about the gravity beams providing bracing? In the RISA-3D Members spreadsheet the unbraced lengths all report “Floor”. This means that RISA-3D is using the unbraced length information from RISAFloor: When solving in RISA-3D the unbraced lengths from RISAFloor are then used for those members:…

December 3, 2014 | RISA-3D, RISAFloor, New, Unbraced Lengths

What do the Unbraced Length commands Segment and Lbyy mean?

Aside from leaving an unbraced length blank or inputting a fixed distance, you can harness the program’s ability to use a limited intelligence for determining unbraced lengths. In order to do this you can simply type in the name of the Unbraced Length Commands into the unbraced length field. Below are the two most common commands: Segment When this command is used, all Joints which fall along the length of the physical member are assumed to provide bracing. If Joints are not evenly spaced along the member then an unbraced length is calculated for each “segment”. A segment is defined as the distance between adjacent Joints. In the example below, the member is defined as one physical member spanning from N1 to N2. The intermediate node N3 is considered as a brace point when Segment is specified for this member. Therefore the forces which occur in Segment 1 will be compared against a capacity calculated using Lb1, and the forces occuring in Segment 2 will be compared against a capacity calculated using Lb2. When using the Segment command the program will assume that each joint along the length of the member can act as a brace location regardless of whether…

October 19, 2016 | RISA-3D, RISAFloor, New, Instability

Instabilities in RISA-3D with Flexible Diaphragms

In a model that contains both RISAFloor and RISA-3D data it is possible to define your diaphragms as either flexible or rigid for lateral design. You can do this from the Diaphragms spreadsheet in either program: Rigid diaphragms will create a rigid link mesh in RISA-3D that makes the diaphragm act as a rigid body. There is a physical effect on your model that affects the analysis by locking all nodes at the diaphragm level together. Flexible diaphragms act solely as a load attribution mechanism. The program calculates the tributary area to the lateral force resisting elements and applies the load to the collectors. However, there is no physical effect on the analysis. The flexible diaphragm behavior can cause lateral stability problems with your model. Here is an example where this could be a problem: In this scenario our braced frames in our flexible diaphragm model have no out-of-plane stability provided, as the diaphragm has no physical effect on the model. When we solve this model we will receive instabilities in the solution. To handle this scenario we will use very small spring boundary conditions in the out-of-plane directions on the frames. These are dummy boundary conditions of course. Because…