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

July 28, 2010 | RISA-3D, Plates, New

How Do Plates Connect to Members?

In order to understand the interaction between plates and members it is important that you know the basics of Physical Members and Plate Connectivity. Remember that plates only connect to other elements at their corners, and you will instantly recognize why the model below will not work as intended. In this model there are two plates, defined between members M2, M5, and M9. The moment in all three beams is zero though, because the surface load on the plates goes to their corners, which fall only on the girders and the columns. In order to get a more realistic one-way load distribution to the beams the plates actually need to connect to them (since plates do not connect to anything along their edges). Above is a submeshed model. The moment diagram for member M9 is plotted on the right. As you can see, the plates are only connected to that member at its midspan, so it is being loaded with a single point load right in the middle. This is not realistic for how a slab would load this beam though, and is therefore an inadequate mesh. In order to get that realistic loading we need enough connection points between…

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…

April 22, 2014 | RISAFoundation, New

How can I check a Slab on Grade for Post Loads?

Need to first know how to model a slab on grade? See our Slabs on Grade article to learn how to model slabs on grade: One of the most critical aspects of slab on grade design is the consideration of punching shear due to posts/columns/racks which bear directly on the slab. With RISAFoundation it is easy to model these conditions and get complete punching shear checks. In the example below an L-shaped slab has four concentrated loads from an equipment rack which is sitting atop the slab. The slab will automatically be designed for flexure and one-way shear, but in this case it is the punching shear which will most likely control the design. In order to check for punching shear you must model the profile of the baseplate at the bottom of each post. Click the button highlighted in the image above in order to draw a post. Choose to draw a Post/Column, as opposed to a concrete pedestal, and define the dimensions of the baseplate: Posts are always centered on a node, and any load which the post is carrying should also be applied directly to that node: After solving you can review the punching shear results, which…