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Static Loads

Static loads represent true physical loads on model objects to represent weights or forces acting on the structure. In VisualAnalysis the direction of the load may be changed  by changing the sign on the magnitude (by 180 degrees). For example, a +1.0K FY load acts in the +Y direction, and a -1.0K FY load acts in the -Y direction. Some loads are placed in terms of global coordinates {X, Y, Z}, while others are in element-local coordinates {x, y, z}, we always use the upper-case for global and the lower-case for local coordinates. See also: Working with Loads.

Self-weight

Self-weight of the model (weight of members and plates actually modeled) is automatically included, by default, through the "Dead Loads". Otherwise any Service Load Case with a load source of Dead loads or Seismic loads may incorporate self-weight. Self-weight may be applied in any direction and scaled to accommodate different needs. If self-weight is included in more than one load case, careful attention to load combinations needs to be taken to ensure duplicate self-weight is not being included. Self-weight is automatically factored in load combinations. Caution: VisualAnalysis cannot include any self-weight for items that are NOT part of your model!

Nodal Loads

Nodal Force or Moment Loads

Forces and moments in some structure types may be applied directly to nodes in the model that are not supported in the same direction. Nodal loads are always applied parallel to a global coordinate {X, Y, or Z}, using the sign on the magnitude to place the load in the opposite direction. If skewed loads are to be applied, two component loadings should be used to achieve the effect.

Support Settlements

A settlement is a specified displacement or rotation at a supported node. You can "apply" settlement loads just like you would apply any other load. Select the node first and define the "load" specifying the type as a settlement rather than a force or a moment. The node must be supported in the direction of the settlement.

Member Loads

Member Load Types

The following types of member forces or moment loads are possible, depending on the structure type:

Loads may be applied in either a local direction or a global direction. Distributed global loads may be applied to the projected length of a member, as is typical for applying snow loads to a sloped roof. The sign of the magnitude can be used to reverse the action of the load, e.g., from "up" to "down". If your local-direction loads are going in the wrong direction on some members, you can use the Model | Reverse Local Axes command to insure your members are all aligned in the same direction.

Generating Multiple Member Point Loads

VisualAnalysis has a feature for rapidly generating multiple point loads across members.

Select either Fractional Span Points or Evenly Spaced Points to generate multiple concentrated loads on each member. Fractional loads may be placed randomly; they also work well for placing loads at say 1/3 points of members with different span lengths. Use the evenly spaced option to create loads a fixed distance apart.

Although the multiple point loads are generated in a single step, they immediately become independent concentrated loads for editing or deleting.

Eccentric Member Loads

There is no built-in facility for defining an eccentric load on a member in VisualAnalysis. All member loads pass through the centroid (or shear center) and cause no secondary effects. For eccentric loads that cause an additional moment, then modifying the model or applying both the load and the associated moment manually can account for the eccentricity.

Wind + Ice Loads on Members

New in version 10.0 For open truss and frames you may wish to apply wind pressure loads directly to member elements. VisualAnalysis can calculate the wind pressures per ASCE 7 requirements using the settings in the project Project Settings and the settings in a Service Load Case used for wind loads. You may also apply pressure values you calculate yourself--you are not required to use ASCE 7 methods or requirements. The surface area used for wind pressures is the length of the member multiplied by the member shape 'size', this area is reduced as the orientation of the member becomes closer to parallel to the wind direction. The 'size' of the member is conservatively assumed to be the diameter of the smallest circle that would contain the shape cross-section, regardless of the direction of the wind.

The ASCE 7 approach to wind pressure is this: Pressure = q(z)*G*Cf, where q(z) is from Equation 29.3 in ASCE 7-10. Where q(z) = 0.00256*Kz*Kzt*Kd*V^2, and we use the average height (z) of the member element for the uniform load pressure. G is the gust factor you supply as input data in Project Settings. We use a Cf from the ASCE Publication 113 Substation Structure Design Guide, p. 40-41 and varies between 0.54 and 2.0 depending on the shape and length of the member. You may also override Cf to use values from ASCE 7-05 or 7-10 (which are MUCH more complicated) for various types of structures. A conservative value is 2.0.

VisualAnalysis Values for Cf
ShapeType Aspect Ratio

Cf

Rectangular or Square Any 2.0
Round or Pipe Any 0.6
Other Shapes L/d > 40.0 (long members) 1.6
L/d > 8.0 1.28
L/d > 4.0 1.12
L/d < 4.0 (short members) 0.96

Because ice build-up on members can dramatically affect the surface area for wind loads on members, you may define ice thickness for the wind pressure calculation. If you use ASCE 7 wind loads, the ice thickness is also adjusted by your Project Settings and Service Load Case settings and the input thickness is treated as a nominal thickness that is adjusted over the height of the model. Ice loads are always applied in the same direction as self-weight, using the vertical direction specified in the Project Settings. The ice value entered for a Wind load is used ONLY to increase the surface area--you should apply the ice weight in a separate load case to account for the weight of the ice.

Both ice and wind loads in this version of VisualAnalysis are applied as constant uniform loads over the length of the member. The 'average height' of the member will be used for ASCE 7 wind calculations. For more fine-grained control of member wind pressures you could split member elements into multiple pieces.

Plate Loads

Area loads may be more convenient than plate loads because they can be modified independently of the plates themselves and won't get deleted if you remove the plate elements from your model to re-configure the layout.

Plate Load Types

Plate loads may be applied individually to plates or across a plate mesh. All plate loads act perpendicular to the plate element to cause bending, except for 'Change in Temperature' loads which are in-plane. Positive magnitude loads act in the +z local direction while negative loads act in the -z direction. If loads are being applied in the wrong direction on some plates, the Model | Reverse Local Axes command can be used to ensure your plate elements are all aligned in the same direction.

Plate element loads are applied as "equivalent nodal loads", which means that you cannot get any plate bending out of a single plate element! You MUST use a plate mesh to get accurate plate behavior.

 

Using Mesh Loads

When applying linear or hydrostatic loads across a plate mesh the definition of the load depends upon global coordinates and directions. The linear variation must be parallel to one of the three global axes. Be careful!  Defined loads that do not vary across your plates or loads that end up being zero on the plates can be created. Use Analyze | Check Model for Errors or the Find tool to help locate any problems. 

Once loads are created for the "mesh," the loads act independently on each plate. They may be modified or deleted separately, but their definitions still depend upon the global coordinate locations specified. This means that if plate elements are moved in the model, the magnitude of the loads will change accordingly and could become zero or change direction! All the loads in the group may be selected to edit them; however there is no automatic-selection for just this set of loads.

Hydrostatic loads are a special case of the linear mesh loads, and are unique in that above the fluid datum the load is always zero. With a linear load on the other hand, if it crosses the 'zero' mark, the load changes direction and increases in magnitude as the distance from this line increases.

Apply Concentrated Loads on Plates?

VisualAnalysis does not directly support concentrated plate element loads. Generally a plate will be modeled as a plate mesh and concentrated loads can be applied to the nearest node. The nodes can even be moved (distorting the neighboring plate elements) to position the load in the correct location.

Apply Edge Loads on Plates

VisualAnalysis does not directly support loads on plate edges, either in-plane or out-of-plane. Equivalent nodal loads will need to approximate for this type of loading. In some situations, a member element may be used along the edge the loads are applied to the member. Just remember that members and plates are only connected at common nodes.

Area Loads

Areas distribute pressure loads to either members or plate elements that lie within their plane. Loads are applied in either a global direction or perpendicular to the plane of the area. To reverse the load direction, change the magnitude to a negative value. The load magnitude may be uniform, linear, or determined based on ASCE 7 Wind load criteria. Holes in the area are automatically taken care of for generating loads. Area corridors can receive separate load magnitudes that are specified as uniform or linear pressures.

Area Side Loads 

  A uniform loading can be applied to area sides (in any direction, including in the plane of the area) by entering a resultant magnitude of the load.  The loading is distributed to the nodes along that edge (both manually created nodes and nodes from auto-meshing).

Area Loads on Plates

When area loads are applied to plates, the pressures are transmitted directly to plate elements within the plane and confines of the area. If there are no plate elements (holes in the plate mesh) then this "area" will not be loaded--that is the gap in plate elements is interpreted just like a hole defined in the area. If your area load extends beyond the plate element mesh, or is completely within a single plate element (but not completely over that plate), then you may not get the loads you expect!

Area Loads on Members

A one-way span distribution can be specified (as would be used for a metal deck) or a two-way distribution may be used for members. The loads are generated using a numerical integration procedure, distributing the load at each "point" over the area to the nearest members, based on the span distribution directions. You may have a model with primary members (beams & columns) and also auxiliary members such as braces or other types of framing that should not receive area loads (direct loads from floors, snow, wind, etc.) and you can handle this situation by making sure that each member's "Framing" type is set appropriately: bracing and dummy members will not receive area loads, beams and columns will.

If you have problems with area loads not working properly, check these items:
a) Nodal coordinates line up with the area vertex coordinates (this can be an issue if nodes move, for example).
b) Area "span direction" would direct load toward one or more members.
c) Area load is defined as "loading members" (rather than loading plates).
d) Members are marked as "Framing = Bracing", but the Area is not set up to "Load Braces".

ASCE 7 Snow Loads

IES has not (yet) specifically implemented snow load support, but by using area loads you can accomplish what you need to do. Each area can only accept a single uniform or linear varying load in each load case, so there is no direct or built-in way to accomplish "drift zones" or unbalanced loading. You can accomplish this in at least two different ways:

  1. Use multiple areas on a roof system so that you can apply different kinds of loads to different areas.
  2. Use a combination of area loads and loads applied directly to members or nodes.

ASCE 7 Seismic Loads?

IES has not implemented any direct way to handle seismic loads using area loads. Generally you will want to use direct application of member, plate, or nodal loads, or you may use Rigid Diaphragm Loads to accomplish static-equivalent loading for seismic analysis.

Area Load Tips & Cautions

 Because the actual loads are generated behind the scenes and applied to your model during analysis, special precautions for verifying that your loads are correct should be taken.  You should use the Analyze | Check Model for Errors report to help validate area loads before you analyze them. 

VisualAnalysis offers an Area Loads: Generated Loads report item that can help you validate the actual loads that were applied to members or plates for analysis.  You should compare this with your total applied area load.  They may be 'off' due to numerical integration or round-off, but if they are significantly different you may have a problem.

ASCE 7 Wind Loads on Areas

VisualAnalysis implements a wind-load generator that can help with ASCE 7 load application, following the Directional Procedure (Wind Loads on Building- MWFRS) in chapter 27 of the specification, for Enclosed or Partially Enclosed buildings. Open structures are not supported for Area Loads, you may be able to use member loads for open structures. The system is designed to give you the power to apply wind loads, but not to automate anything beyond the actual pressure calculation.  About 20 input parameters are required, along with a good understanding of ASCE 7 wind provisions.  

Steps to Create Wind Loads:

  1. Define project-wide data for wind loads in the Project Settings Tab of Project Manager.
  2. Create Areas to represent the places where you would like to apply a wind load pressure. Each area can accept one wind load pressure, in each load case. Additional loads may be placed directly on nodes, members, or plates, if the area loads are insufficient for your needs.  It is a good idea to define all your areas so that their "normal direction" (perpendicular) is consistently outward to the building!  Knowing the "outward" and "inward" directions for areas is critical to getting wind loads applied properly.
  3. Set up case-specific data in each wind load service case that you need to use. (Note: empty service cases are hidden by default in Load Case Manager, you can display them with a check-box at the bottom of the list.)
  4. Apply wind loads to areas, in each directional wind-load service case; each load offers a number of possible choices for the type of pressure to use.

The real “trick” to our system is the creation of Areas. This is a manual process and you can use single areas over an entire face of a structure, or you can create multiple areas to represent the different types of wind loads that ASCE 7 requires. You can place holes in your areas (by drawing an area within an area). Areas do NOT need to be created with nodes to represent their shape, you can use an Edit Grid to place Area vertices anywhere, and they will have the effect of loading any members (or plates) that lie within their plane.

We have deliberately not automated the whole process because we don’t think it is rational or even possible for us to do it correctly. We are trying to provide a tool that you can use in the way you see fit. If you find it easier to determine the wind pressures outside of our tool, you can still do so (but we would like to know why, so we can improve it!)

Rigid Diaphragm Loads

Requires: Advanced Level

Point loads and moments may be applied to a rigid diaphragm for a convenient way to model wind or seismic effects on a building frame. The load you apply to the diaphragm is distributed accordingly to nodes in the diaphragm in order to achieve appropriate behavior and to maintain the centroid. This is not as accurate as modeling individual loads applied directly to nodes or elements, but can be immensely convenient for certain jobs!