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Area & Wind Loads

Area loads are used primarily in conjuncture with members, which is useful when applied correctly. They take pressures and generate equivalent distributed loads (DL’s) that are applied to members. These equivalent distributed loads are what are used for the member analysis. Area loads are useful when not wanting to model the plate, which may introduce unwanted stiffness into the model. Like with other loads, you can assign a load group and subsequent load case to area loads.

Types of Area Loads

  • One-Way:
    • Loads are distributed using one-way action
    • Requires 3 or 4 nodes to identify extents of area load
    • Span direction of internal members must be identified
  • Two-Way:
    • Loads are distributed using two-way action
    • Requires 3 or 4 nodes to identify extents of area load
    • Support orthogonal orientation of internal members
    • Loads do NOT auto-update
  • Column Wind Loads:
    • Requires 3 or 4 nodes to identify extents of area load
    • Can step pressure values along length of column/member
    • Multiple elevation and pressure changes supported
    • Loads members that follow a single column direction
  • Open Structure:
    • Requires 3 or 4 nodes to identify extents of area load
    • Distributes loads to members based on tributary area of member itself
    • Apply to members in global X,Y,Z planes, or all members

The following examples will go through each type of area load and their differences.

Area Loads vs. Equivalent Distributed Loads

When area loads are created and applied to an area, they will originally show up as a cube shape, indicating a constant pressure. What is actually used in analysis is the equivalent distributed loads that are applied to members. Depending on the type of area load, these distributed loads will depend on various factors but will mostly depend on tributary width/spacing of the members located within the area load. These equivalent distributed loads can be toggled on and off in the viewing space.

To toggle on and off the equivalent distributed loads, go to the Visibility Settings on the right hand side (looks like an eye) and click on Equivalent Area Loads:

area loads 2

The results for a simple square frame can be shown below:

area loads 1

Note: It is always good practice to check the equivalent distributed loads to verify the area loads you are applying are distributing and loading the members correctly.
Note: Normal Distributed Loads and equivalent distributed loads are distinguished by their red and green colors, respectively.

Example: One-Way Area Load

One-way area loads are the most common loading type when it comes to loading for floor framing or other orthogonal framing instances. One-Way load action means that a load is supported by two sides, so the load path will only travel to either of the support, or One-Way.

The input fields required to apply a One-Way Area Load are:

Corner Node IDs: Identifies he extents of the area load – must be 3 or 4 nodes
Pressure Magnitude: Load magnitude that is going to be applied
Load Direction: Can be Global X, Y, Z or the planes’ Local Axis
Span Direction: Direction of interior members; span direction of members that will experience the load
Load Group: Load group as identified in the Load Group and Load Cases documentation

For this example we’ll be looking at how one way area loads can be applied a one-story warehouse-type structure with beams framing into girders. The beams are spaced at 5 feet on-center. Instead of modeling a slab with a plate and affecting the stiffness of our structure, we will use one-way area loads. Lets assume that the floor load is a Live Load, and it 100 psf. One-way area loads are meant to be used when framing is orthogonal to each other, as shown in our example structure:

area loads 3

To apply a One-Way Area Load, click the Area Loads, button to open the menu. Select “‘One-Way” from the type dropdown. Put in the corner nodes of the floor to identify the extents of the area load. The pressure is 100 psf, so input -0.100 ksf. We input a negative pressure because it will be acting along the Global Y-axis, but in the downward direction. Change the span direction to go from Node 16 to Node 18, the direction of our internal members (beams). Type in “Live Load” for the load group, as you would for other loads.

Note: The order of the corner nodes must follow the clockwise or counterclockwise direction.

area loads 4

By default, the area load will be represented by a pressure load and magnitude as shown below.

area loads 5

As aforementioned, previously, toggle on and off the equivalent distributed loads to see how the loads are being distributed.

area loads 6

Note the values presented here. The pressure is uniform, however the subsequent equivalent distributed loads are not. This can be expected since the spacing between the members will affect the result. As you can see, the edge members see half of the load as the other interior members because they have half the tributary width affecting the resulting load magnitude. To change the value of the area load you simply go back to the menu and edit the value.

 

Example: Two Way Area Load

Two-Way loads require a much larger number of computations to be completed by the solver compared to One-Way loads, which is why we highly recommend using One-Way Loads whenever possible.

The other common loading method is using Two-Way loads. Two-Way action assumes that the load is supported on four sides, so at any location, the load path will travel to the closest support, which in this case, can be a beam OR girder. Because of this, span direction does not need to identified.

Note: Two-Way loading will only support orthogonal framing within the extents of the area load. Angled framing will not work respond properly to Two-Way Area Loads.
Note: Two-Way Area Loads will only distribute correctly if all interior members are split, meaning: unlike in the One-Way case, “girders” contained in your area load must be split at each point where another member frames into it. This can be accomplished very easily by selecting all of those members at once and using the “Split Members” function.

Because of the impact on the calculation time, you will need to confirm in the Settings > Other that that you would like Two-Way loads to distribute to interior members as shown:

Screen Shot 2019-10-10 at 2.03.39 PM

The input fields required to apply a Two-Way Area Load are:

Corner Node IDs: Identifies he extents of the area load – must be 3 or 4 nodes
Pressure Magnitude: Load magnitude that is going to be applied
Load Direction: Can be Global X, Y, Z or the planes’ Local Axis
Load Group: Load group as identified in the Load Group and Load Cases documentation

We will use the same structure as in the One-Way example for consistency. We will assume that the Two-Way area load is a “Dead Load” with the same extents, direction and magnitude as in the previous case:

area loads 7

The resultant area load will look the same as the previous case when looking at the pressure.

area loads 5

Make sure that all members inside the area load are split and do not have nodes along their length. When first applying a Two-Way load, you will be greeted with this message from Structural 3D:

Screen Shot 2019-10-10 at 2.11.40 PM

Toggle on the equivalent distributed loads to see the differences between One-Way and Two-Way Loads:

area loads 18

 

Example: Column Wind Load

 

Column Wind Loads are unique to the Area Load function because they can vary, and do not need to exhibit a single pressure value. They are very useful when applying Wind loads to a closed structure that has lateral pressures that vary step-wise as the elevation of the structure changes. You are able to define a bounded plane without having that plane being bounded completely by members. As a general rule of thumb, you should have (1) less pressure value for the number of elevation points that you define.

The inputs required for application of Column Wind Loads are:

Corner Nodes: Identifies he extents of the area load – must be 3 or 4 nodes
Elevations: Comma separated Elevations (Global Y Axis) for wind loads. These are the elevation ranges that match up to the Pressure Magnitudes below.
Pressure Magnitudes: Comma separated Pressure Magnitudes for wind loads. These will be applied based on the above Elevations.
Span Direction: Similar to One-Way Loads: the span direction of the columns. Direction is indicated by the vector between two nodes

We will use the same structure as in the One-Way example for consistency. The height of the one-story structure is 10 ft. We will assume our structure is closed, and apply Column Wind Loads to the “front” side of our structure; or the Global minus-X direction. We will assume from wind load calculations – You can get your wind load calculations directly from the SkyCiv Wind Load Software – that we have a lateral wind pressure of 20 psf from 0-3.33 ft, 40 psf from 3.33 to 6.66 ft, and 60 ft from 6.66 ft to the top of the structure. We also need to make sure that the column direction is correct, so we will the direction from Node 3 to Node 6 (Global Y-axis). If we input all of this information correctly, we should see this input window:

Note: The order of nodes in the ‘Span Direction’ field will indicate the “zero elevation” and “end elevation”

area loads 8

Then subsequently, with the equivalent distributed loads off, it will display a constant pressure as the average of all pressure on the structure:

area loads 9

Turn on the equivalent distributed loads to see the expected steps in loading and distribution among the columns:

area loads 10

A note about Column Wind Loads:

There are other similar functions you can use the Column Wind Load function for:

  1. You can offset the load from the column ends. The first number in the “Elevations” field must be greater than the Y-coordinate of the bottom node of the columns, and vice versa for the top.
    area loads 11
  2. You can also apply a Column Wind Load to a slanted plane (shown using the Local axis direction). In this case, the “Elevation” corresponds to the location along the length of the sloped roof member. Using the same structure, but with the middle ridge raised 4 feet:area loads 12
  3. Lastly, you can use a triangular shape as the bounded extents of a Column Wind Load. Using the same structure, but with bottom chords connecting the triangle (rest of structure hidden for clarity)area loads 13

Example: Open Structure


The last method of applying Area Loads is the Open Structure Area Load. Like it sounds Open Structure Area Loads are meant to applied to open structures, or those which have their members subject directly to outside elements. with this method, Structural 3D will calculate the equivalent distributed loads based on the tributary width of the section, which depends on its orientation to the load. As the exposed area of the section gets wider, the equivalent distributed load increases for the same constant Open Structure Area Load. Similar to the Column Wind Load method, this will most commonly be used in conjuncture with wind loads. You can get your wind load calculations directly from the SkyCiv Wind Design Software

The input fields required to apply an Open Structure Area Load are:

Corner Node IDs: Identifies he extents of the area load – must be 3 or 4 nodes
Pressure Magnitude: Load magnitude that is going to be applied
Load Direction: Can be Global X, Y, Z or the planes’ Local Axis
Load Group: Load group as identified in the Load Group and Load Cases documentation
Loaded Members Axis: Apply the loads to just members in the X,Y,Z axis, or all members

Similar to before, we will use the same structure as shown in the previous modules except this time, we will assume there are two bays of “X-braces” on the front face of the structure. The main difference between the Open Structure and Column Wind Loads is that the latter can vary the pressure, while Open Structure loads cannot. Lets assume there is a constant lateral wind pressure towards the “X-braces” of 50 psf. For Loaded Members Axis, we want to load ALL members, including the braces, which are not in any orthogonal axis. If we input all the relevant values, we should see this input window:

area loads 14

By default, the area load will be represented by a pressure load and magnitude as shown below:

area loads 15

Toggle on the equivalent distributed loads to see how the loads are being distributed:

area loads 16

As noted previously, we selected the option “All Members” for the Loaded Members Axis selection. If you wanted to load just members following the orthogonal axis, you would select “X, Y, Z Members” and see this result:

area loads 17

Note: Open Structure Area Loads can be applied in any Global Axis direction, or in the Local Axis of the plane bounded by the Corner Node IDs

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