L-shaped buildings can present unique design challenges because of their asymmetric geometry. One such challenge is the distribution of lateral loads especially when utilizing a flexible diaphragm.
For background on flexible diaphragm load distribution, refer to the related article below for some details on the load distribution based on beam layout. L-shaped buildings require additional assumptions and clarifications from the general beam layout guidance described there.
The program uses different distribution methods for wind and seismic loads based on the application of the load type. Wind load is generated using the full building face dimensions, while seismic load is generated using mass tributary area. In both cases, the generated loads are assigned to the lateral framing members that define each diaphragm region.
The following sections compare three conditions: the theoretical distribution based on engineering fundamentals, the distribution generated by an initial integrated RISAFloor and RISA-3D model, and the distribution after modifying the member designation assumptions. Each condition uses the same floor plan, with equal-sized bays and lateral frame lines shown in red.
To understand and verify how RISA distributes lateral loads, first establish the theoretical distribution for an L-shaped building with a flexible diaphragm. These theoretical values provide a baseline for comparison with the program generated results.
Wind load applied to the building face is assigned to the lateral framing members based on tributary area. For the wind load on an L-shaped building, the total tributary area will be based on the surface area of the building face. Although the exterior walls do not align on the same gridline, the total surface area will be the same as in a rectangle building. Based on the tributary areas, the theoretical wind load distribution is approximately:
1/4 of the total load to the west gridline, shown in green
1/2 of the total load to the center gridline, shown in purple
1/4 of the total load to the east gridline, shown in orange
Seismic load distribution will be based on the floor mass rather than building face area. Unlike the wind load case, the total tributary area for the seismic load of the L-shaped building differs from that of a rectangular building. Therefore, the seismic distribution also differs from the corresponding rectangular building. Based on the floor tributary area assigned to each lateral frame line, the theoretical seismic load distribution is approximately:
1/3 of the total load to the west gridline
1/2 of the total load to the center gridline
1/6 of the total load to the east gridline
In the initial model, the interior beams are designated as Gravity members because they are not included in the lateral frames noted above.
With an integrated RISAFloor and RISA-3D model, RISA-3D performs the lateral analysis, and only members designated as Lateral in RISAFloor will transfer to RISA-3D. Because the interior beams are designated as Gravity members, they are ignored in the lateral model and do not have any lateral load attributed to them.
For wind load generation, the program idealizes the building face as a rectangle formed by infilling the L-shaped floor plan. This approach reflects the assumption that the wind exposed area corresponds to the full building width.
Because no lateral member is present in the interior bay, the load associated with that infilled area is redistributed to the remaining lateral framing members. This produces a larger load share at the east gridline than the theoretical distribution would predict. Correspondingly, the center gridline receives less load than predicted by the theoretical distribution.
The resulting wind load distribution is:
3/8 of the total load to the west gridline
1/8 of the total load to the center gridline
1/2 of the total load to the east gridline
The seismic load is distributed strictly based on the tributary area to the lateral framing members. Since the interior beams are excluded from the lateral model, the generated distribution also differ from the theoretical distribution.
The resulting seismic load distribution is:
1/2 of the total load to the west gridline
1/6 of the total load to the center gridline
1/3 of the total load to the east gridline
Changing the lateral designation for the members will modify the generated load distribution. By switching the interior beams to be part of the lateral system, they will be included in RISA-3D analysis and will participate in the load distribution. For this example, the two highlighted interior beams are changed from Gravity to Lateral in RISAFloor.
Although these beams may not be primary elements of the lateral-force-resisting system, they may still contribute to the lateral load path at that level. They likely act as drag struts or collectors that transfer lateral forces to the braced frame. The appropriate designation should reflect the actual structural load path and the engineer’s design intent.
The methodology remains unchanged, but the tributary area assignment changes by including the interior beam in the lateral system. The load from the infilled floor will now be distributed between the center gridline and the eastern gridline.
The resulting distribution aligns with the theoretical distribution:
1/4 of the total load to the west gridline
1/2 of the total load to the center gridline
1/4 of the total load to the east gridline
Similar to wind loading, the tributary area assigned to each gridline for the seismic load distribution is adjusted by incorporating the interior beam in the lateral system. Again, the resulting distribution aligns with the theoretical distribution:
1/3 of the total load to the west gridline
1/2 of the total load to the center gridline
1/6 of the total load to the east gridline
Theoretical Loading |
Initial RISA Results |
Modified RISA Results |
||||
|
Wind |
Seismic |
Wind |
Seismic |
Wind |
Seismic |
|
|
West (Green) |
1/4 |
1/3 |
3/8 |
1/2 |
1/4 |
1/3 |
|
Center (Purple) |
1/2 |
1/2 |
1/8 |
1/6 |
1/2 |
1/2 |
|
East (Orange) |
1/4 |
1/6 |
1/2 |
1/3 |
1/4 |
1/6 |
These results demonstrate that flexible diaphragm load distribution in an L-shaped building depends on the building geometry, the members included in the lateral analysis, and the loading type. By understanding the underlying assumptions and adjusting the modelling techniques, users can better control the lateral load attribution in their model.
Where a project requires a lateral load distribution that differs from the automated result, RISA provides the option to skip the automated lateral load generators and simply input the lateral loads in the model. This provides complete control over the applied load magnitudes, locations, and distributions.
To skip the lateral load generator, select “None” for the code in the load generator window prompt and click “Generate”.
This clears the generated loads and nodes. The same action can be performed during the RISAFloor to RISA-3D integration or from the Advanced tab in RISA-3D. After the generated loads are cleared, manually enter the required lateral loads.