Guide To: Flexible Diaphragm Load Distribution in RISA

It is important to understand how RISA is modeling your load distributions when applying flexible diaphragms. The flexible diaphragm distributes loads based on tributary area, but RISA breaks up the areas based on the nearest support elements on either side. When RISA sees support elements on both sides, the load is split (or distributed) to both element sides. However if one side is missing a support element, then all that load will be attributed to the one side.

Load Distribution Examples

Baseline Assumptions: 

Let’s see this theoretical distribution in a simple example shown below. Consider a rectangular building with lateral load applied in the North-South direction (plan up/down), and resisting shear walls along grids “B” and “F”. However, the shear wall along grid “B” does not extend the full building length and is located between gridlines “3” and “5” only. In this case, only the tributary load area highlighted in green is assigned to the left shear wall. The rest of the load area highlighted in purple and orange is assigned to the right shear wall.

Alternative Methods:

If the program’s load distribution method does not align with the design intent, alternative modeling techniques can be used to modify the load distribution. Below are two options to adjust the loading.  These will need to be reviewed based on the project specifications to determine if they satisfy the design intent.

• To redistribution the load based on a revised tributary area, consider incorporating a drag strut or collector element.  These added lateral elements will redistribute the lateral loading based on this revised tributary area.  In the above example, designating a beam from grid B1 to B3 as lateral member would split the orange tributary area evenly between grids B and F.

• To have full manual control of the load distribution, run the load generator with the code set to "none" in order to clear the generated loads.  The lateral loads can then be added manually.  

Multi-Wall System:

To see how the program recognizes flexible diaphragm tributary areas in a more complex lateral system, let’s consider another example shown below. Considering how a North-South direction lateral load (plan up/down) will be distributed to the lateral system shown in the image below. Lateral shear walls are shown in red with a few gravity beams shown in blue. Again for each area section, the program is looking for the nearest support element. Tributary areas that belong to each shear wall are color coded with arrows indicating which wall that area belongs to. Also notice how the diagonal wall is handled. Users may need to consider these effects if they have diagonal walls or walls that aren't aligned with the global X or Z axis.

Gaps in Lateral Elements:

Now let's look at the case where there is no lateral system at a specific cross section of the floorplan.  Consider the floorplan below for this example where there is no lateral elements in either direction at the midpoint of the floor.  In this case, the program will incorporate the load from this tributary area and attributed it to the nearest available lateral element as a short, concentrated line load at the end of the adjacent wall.  This load distribution is consistent with the gravity member behaving as a drag strut to deliver that load to the nearest lateral element.  

To keep learning about load distribution with flexible diaphragms, see the article below focused on L-shaped buildings.