Limitations of wind decomposition to diaphragms

Tekla Structural Designer
2021
Tekla Structural Designer

Limitations of wind decomposition to diaphragms

Certain building shapes need extra consideration if simple wind loads have been applied, or if wind loads have been applied to wall panels set to decompose to diaphragms.

As illustrated by the following examples, buildings containing discrete towers (and thus containing disconnected rigid diaphragms) are a particular concern.

Wind load perpendicular to disconnected diaphragms

In this example wind load is to be applied in the Global Y direction, perpendicular to the disconnected diaphragms that exist in the highlighted slabs at the second floor level shown below.
An issue arises when the wind load is applied as an area load that has to be decomposed to both diaphragms. This could happen when the load is applied either via a Simple Wind load, or a wind wall panel:
Irrespective of the method used to apply it, the area load within the building profile is shared between levels prior to decomposition.

The load is then decomposed to the diaphragms at each level in proportion to the width of each diaphragm.

In this case because both diaphragms the second floor level are of equal width, the load is shared equally between them. This is not satisfactory as more of the load should have been applied to the left hand diaphragm in this case.
The workaround varies according to the method of loading, but basically involves replacing the original load with separate loads in each bay:

In both the above cases, the load is then decomposed as originally intended.

Wind load parallel to disconnected diaphragms

In this example although the two blocks are now the same height, another issue arises when the wind load is applied in the Global X direction, i.e. parallel to the disconnected diaphragms at the second floor level:
The issue arises because one diaphragm is ‘hidden’ from the applied load by the other diaphragm. The issue occurs irrespective of whether the load is applied via a Simple Wind load, or a wind wall panel:

The area load within the building profile is first shared between levels prior to decomposition.
At the second floor level, instead of all the load being decomposed to the diaphragm facing the wind; because it is decomposed in proportion to the width of each diaphragm at that level, it ends up being shared equally to both.
To avoid the load being shared equally:
  • if using wind wall panels - you would need to decompose to members or nodes instead of to diaphragms
  • if using Simple Wind loads - there is no workaround, you would have to manually input the loads as diaphragm loads instead.

Diaphragms less than 5% of simple wind load width ignored

In Tekla Structural Designer, if the projected width of a rigid diaphragm in the direction that the load is acting is less than 5% of the simple wind load width it is assumed to be ineffective.

Consider the following example which consists of 4 levels and has a simple wind load applied over the full height of the building.

The first 3 levels are identical. The width of the diaphragm perpendicular to the load exceeds the width of the load, so the diaphragm at each of these levels is effective.
This changes at the roof (level 4). The only structure at this level is the lift core as shown below.
At this level the width of the diaphragm is now significantly smaller than the width of the load. In this case, because it is less than 5% of the load width, the diaphragm is ignored. As it is the only diaphragm at the top level a "Wind load profile applied above top diaphragm" warning message is displayed against that level in the Simple Wind Loading dialog when the load is created.

As it is only a warning it does not prevent the analysis from proceeding, however the diaphragm load applied to level 3 is significantly larger than it would otherwise have been.

This is because the full width is used for the loaded area above and below level 3 when calculating the diaphragm load to be applied to it.

To have the load calculated more accurately, the workaround would be to reduce the top level of the original simple wind load to align with level 3, then apply a second wind load from levels 3 to level 4 with a reduced width equal to that of the lift core.
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