September 6, 2013

Why Are The Steel Beam Deflections So Large?

When solving a RISA-3D model with hot rolled steel members under the AISC 360-05 (13th Edition), AISC 360-10 (14th Edition) or AISC 360-16 (15th Edition) codes you may find that your member deflections are roughly 20% larger than you would expect. To understand why see the AISC 360-10 Specification, Section C2.3.1:

“A factor of 0.80 shall be applied to all stiffnesses that are considered to contribute to the stability of the structure”

By applying this 0.80 factor to the axial and flexural stiffnesses of the members (EA and EI respectively) the deflections for those members increase by approximately 20%. This code provision is only intended to be applied to members that contribute to the stability of the structure though, such as moment frames and braced frames.

Since RISA-3D does not inherently know which members in a model are contributing to stability (i.e. moment frames) versus which members are simply present to carry gravity loads (i.e. floor joists) the program conservatively defaults to applying this stiffness adjustment to all hot rolled steel members when using the AISC 360-05 (13th Edition), AISC 360-10 (14th Edition) or AISC 360-16 (15th Edition) codes. This occurs because those members are assigned the Lateral Function when drawn. To disable this adjustment for certain members in a model you can set their Function to Gravity under the Hot Rolled tab of the Members spreadsheet.

For models where this stiffness adjustment is unnecessary altogether, such as models of gravity systems where lateral stability considerations are being ignored, this adjustment may be turned off altogether in the Global Parameters under the Codes tab.

Tags: RISA-3D, New, Analysis, Deflection

March 26, 2010 | RISA-3D, RISA-2D, New, Physical Member

What Are Physical Members?

Members (beams, columns, braces, etc.) are defined in RISA by an I-Node and a J-Node. While you and I see a beam occupying physical space between two columns, most programs see a line between Point I and Point J. This is known as a non-physical member. See the image below: In this model the red side of the beam is the I-End (Node N1) and the blue side of the beam is the J-End (Node N2). There is clearly a midspan support, but this member is only defined with two joints, and it is not defined as “physical”, so the program is unaware that Node N3 is connected to the member. As a result, when the model is solved we get this deflection: This has always been an issue for Finite Element Analysis programs which don’t support Physical Members. These programs recommend that you model the beam shown above as two beams: While splitting the beam into two will give you correct analytical results, it also means that you have twice as many results to sift through for one continuous member. Instead, RISA offers the ability to define the member as Physical, which means that it will recognize and automatically…

October 9, 2013 | RISA-3D, New, Analysis

What is the Tau b Factor and how does it affect my analysis?

When solving a RISA-3D model with hot rolled steel members under the AISC 360-05 (13th Edition) or AISC 360-10 (14th Edition) codes you may find references to a factor called Tau_b. To understand what this factor is you should first be familiar with the Stiffness Adjustment in RISA-3D, then see the AISC 360-10 Specification, Section C2.3.2: “An additional factor, Tau_b, shall be applied to the flexural stiffnesses of all members whose flexural stiffnesses are considered to contribute to the stability of the structure” Link: Stiffness Adjustment in RISA-3D By applying this factor in addition to the 0.80 factor to flexural stiffness (EI) of the members the deflections for those members increase significantly. This code provision is only intended to be applied to members whose flexural stiffness contributes to the stability of the structure though, such as beams and columns within moment frames. The calculation of Tau_b is a function of the ratio of axial force in the member to its axial yield strength. For members where this ratio is less than 0.5 (members which are less than 50% stressed axially) the value of Tau_b is 1.0, which means that is has no impact on the member stiffness. For members which…

May 19, 2020 | RISA-3D, New, Reporting, Detailed Reports

Why is my force demand higher than what is reported in my force…

A question that we are asked from time to time in technical support is “why is my required force higher than what is reported in my force diagrams”. For instance, in the detail report below we can see that the maximum moment about the member’s weak axis is 28.9 k-ft. However, the required weak axis moment is reported as 31.8 k-ft. The reason there is a discrepancy between these two values is that the force diagram is solely reporting the moment about the weak axis whereas the required force is also including contribution from the torsional warping stresses. The required shear force shows a similar difference in that torsion is also included. If you are using RISA-3D v18 and newer, you are able to access the new detailed reports which include the full limit state calculation that was used to determine the required forces. In this example, click on the “Applied Loading - Bending/Axial” or “Applied Loading - Shear” section for a breakdown of these calculations. After expanding the “Applied Loading - Bending/Axial” section it becomes clear that required flexural strength is derived from both the weak axis bending stress and the local warping bending stress. For more information on…