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Load Cases & Combinations

Load Case and Combination Types

Case Type Description
Service Load Case A container for holding physical loads, usually grouped by load source. May also include self-weight of the model. These cases may be analyzed or not, and may be 'patterned' for live loads (advanced). For wind loads there are settings specific to ASCE7 that you may use in conjunction with area loads. These are not used directly for design checks.
Equation Combination A way of combining service cases, with load factors based on load sources. These are used for Building Code combinations, and you can create your own custom combinations as well. These can be used for design checks.
Factored Combination A way of combining service cases with arbitrary load factors. These are good for patterned loading and other custom load combinations where you are picking & choosing the cases and factors. These can be used for design checks.
Dynamic Response Case A way of specifying a seismic event along with direction factors. Dynamic response case results are computed by combining results from mode shapes. No design checks directly -- use a superposition combination.

Requires: Advanced Level

Advanced Cases Description
Time History Case A load case that applies a 'forcing function' load that varies with time. No design checks directly -- use a superposition combination.
Moving Load Case A load case for applying simple moving loads (a truck and/or patterned uniform) along a single beam-line. Moving loads produce envelope results and are handled independently of other load cases during analysis.  No design checks directly -- use a superposition combination.
Result Superposition Combination This is a post-analysis load combination. These are used to combine time-history results, dynamic response results, and moving load results with normal static results. Note that these load combinations are really just an "envelope" with extreme+ and extreme- values. Superposition combinations may not be accurate in the presence of nonlinear features! Design checks may be performed on superposition combinations if you mark them for design (ASD or LRFD or Deflection)

 

Service Load Cases

Service cases are used to organize the physical loads on a model, with an eye toward load combinations used for design checks. VisualAnalysis supports many different load types to represent physical loads on your model such as concentrated nodal loads, distributed member loads, etc. These physical or actual loads are grouped inside a container called a Service Load Case.

Each Service Load Case has a load source associated with it. The source defines the physical origin of the loading. Common load sources include dead load, live load, and wind load. Load sources are used to make load combinations easier. They are described in more detail below.

Most design codes require that you check various combinations of loads. The LRFD specification for steel, the ACI specification for concrete, and the IBC specification for buildings define combinations using equations like 1.2D + 1.6L. In these combinations, loads are grouped according to their sources and given factors to account for the relative uncertainties. In many cases it is important, as a designer, to look at other loading configurations that may also take place. One such technique is called patterned loading. VisualAnalysis supports these operations through load cases.

What are Load Sources?

VisualAnalysis provides convenient ways to both organize and combine your loads. Each Service Load Case can be defined as coming from a particular load source. These load sources are derived from the most common design specifications in the USA such as those produced by organizations such as ASCE, IBC, and AASHTO. There is a preference setting under Edit | Preferences, Project to specify which service load cases are set up when you start a new project. The default is to generate load cases for ASCE 7 sources only. You may manually create service load cases with any names you desire, without much regard to the "load source" definition.

The existence of a load source type in the software does not mean that a corresponding physical load is supported. For example, there is no "creep" load source available in VisualAnalysis that you use for loads on a concrete frame. However, you can still create member loads and place them in a load case to represent the effects of creep on your structure.

Load sources are used in VisualAnalysis to create Equation Combination load combinations and (automated) Building Code combinations. These load combinations allow you to combine groups of service loads together, while automatically applying the appropriate multiplication factors based on the source type. VisualAnalysis can also generate the set of factored combinations needed for working with LRFD, UBC, IBC, or ACI codes. The support is not entirely foolproof however as careful attention is needed for patterned loading situations and other possible complications. For more information refer to the Loading section.

Load sources are not customizable in VisualAnalysis. However, you may create any number of service load cases with arbitrary names and use them for whatever you like and create custom factored load combinations. The existence of a load source in VisualAnalysis does not imply that there are specific loads or calculations done in the software--these are just ways of organizing and combining your loads.

 

Simple Load Sources

Load Source Abbreviation
Dead D
Live L
Seismic (directional) E+X, E-X, ...
Wind loads(directional) W+X, W-X, ...

 

ASCE 7 Load Sources 

Load Source IBC / ASCE 7 Explanation
Dead D Structure self-weight or permanent fixtures on or in the structure.
Ice Weight Di Weight of ice.
Live L Loads due to moveable equipment or occupancy. Live loads may be "patterned" in the advanced level, and if so are denoted with L1, L2, L3, etc. with a pattern ID. Caution: Live loads are reduced by 50% in some IBC load combations, use Lpa source for loads > 100 psf.
Live
(Public Assembly)
Lpa(>100psf) Garage loads, Public Assembly areas, or loads greater than 100psf. They have also been called "Exception" loads, because they appear in a separate clause in some building codes.
Roof Live Lr Live loads on a roof.
Rain or Ice R Initial rainwater or ice exclusive of ponding contributions.
Snow S Snow loads. Notes: There is no separation in VisualAnalysis for non-shedding loads. There is no load 'source' for unbalanced snow loading, which should be handled like 'patterned loading' using multiple snow load cases and load combinations.
Seismic
(directional)
E+X, E-X,
E+Y,E-Y,
E+Z, E-Z
Earthquake or Seismic loads. EX is for seismic loads in the positive global X direction, E-X is in for the negative global X direction, and similarly for the Y and Z directions.
Seismic E Earthquake or seismic loads to include with each of the directional seismic loads. (You should rarely need to use this, it is provided primarily for backward compatibility and ultimate flexibility!)
Earth Pressure H

Loads due to differential settlement, backfill on a wall, etc.

Caution: Combinations do not distinguish the difference between whether H adds or resists the primary load variable in an equation. It is up to you to handle the load combinations for special circumstances with earth loading.

Fluids F Water in a tank.
Flood Fa The river flowing over your bridge or through your building.
Temperature T Thermal loads.  IBC calls these self-straining loads.
Wind loads
(directional)
W+X, W-X, W+Y, W-Y,
W+Z, W-Z,and more!

Wind pressure loads. As in the seismic loads above, using just W will apply the same load in all directions. WX is for wind loads in the positive X direction, W-X is for wind loads in the negative X direction, and similarly for the Y and Z directions.

Optional "Skewed" wind load directions are available (+X+Y, -X+Y, +X-Y, etc.), but these are disabled by default and they do not participate in Building Code Combinations. You may enable these using Edit | Preferences, Criteria and use them for your own custom load cases and combinations, but they are primarily here for backward compatibility.

Wind on Ice WiX, Wi-X,... Wind on Ice. Ice will increase the surface-area of members and therefore the wind forces.
Other loads U User-defined source available for special loads that you want to factor independently of other sources.  These are not used in Building Code Combinations.

 

New in version 10.0AASHTO Load Sources

The following AASHTO load sources may optionally be used for service load cases. There are no automatic AASHTO load combinations generated, but you may create or import your own load combinations.

Load Source AASHTO
Creep CR
Downdrag DD

Dead, Components

DC
Dead, Wearing Surface DW

Horizontal Earth

EH
Misc. Locked-In Forces EL
Earth Surcharge ES
Vertical Earth Fill EV
Post-tensioning Secondary PS
Shrinkage SH
Vehicle Braking BR
Vehicle Centrifugal CE
Vehicle Collision CT
Vessel Collision CV
Earthquake EQ
Friction FR
Ice Pressure IC
Vehicle Dynamic Impact IM
Vehicle Live LL
Live Load Surcharge LS
Pedestrian Live PL
Settlement SE
Temperature Gradient TG
Temperature Uniform TU
Water and Stream WA
Wind on Live WL
Wind on Structure WS

 

Load Case Manager

The load case manager is accessed by selecting Load | Load Case Manager. Essentially the load case manager is a convenient place to view and manage all load cases and load combinations. The only exception to this is the advanced-level Project Envelope, which acts similar to a load case but is really just a 'result case'.

Load Case Manager is where you go to setup 'design load cases'. Load cases for design are either load combinations or superposition results. You may specify load cases to be checked for strength (ASD or LRFD) or deflections. See more about design checks.

Service Load Cases

Service load cases are containers for real loads. Typical service load cases are automatically created with a new project, but more can always be added by going to Load | Load Case Manager and clicking on the Service Cases tab. The weight of a piece of machinery, the pressure of the wind, and the settlement of a support are all real, static loads that may be included in a service load case. All static loads must reside in a service load case. Generally it is best to separate loads into easily managed groups based on load sources or some other preferred organization.

Self Weight

Self-weight is automatically included in the Dead Load service case, in the vertical direction. The self-weight of the model itself is automatically calculated based on member, plate, and cable element properties, but can, if necessary be scaled or factored using the load case settings. Seismic load cases may also include the effect of self-weight, factored into a horizontal direction.

Patterned Live Load Cases

Requires: Advanced Level

Live load service cases may be given a pattern ID (a positive integer) to help you easily model various loading patterns, such as odd/even span loads. Each patterned service load case is combined independently in building code load combinations from other patterned load cases. For example, you can place odd-span loads in a "Pattern 1" service case and even-span loads in a "Pattern 2" case and the building code combination will generate something like:

1.2D + 1.6L1 + 1.6L
1.2D + 1.6L2 + 1.6L

Note how any loads in an unpatterned live load case are included in both combinations.

If you have the advanced level of VisualAnalysis three patterned service cases are automatically generated and available for your immediate use: L1, L2, L3, but you may manually create as many different patterned service load cases as you need.

There are other ways to create load patterns in VisualAnalysis, for example: using custom factored load combinations with various load factors--which would not require the advanced level of the software!

ASCE 7 Wind Load Settings

Service load cases that have a wind load source may be used to help generate area loads based on ASCE 7 criteria. There are three places for defining wind load settings: Project Settings (for project-wide settings), the service load case (for settings that are affected by load direction), and in the area load itself. The settings are defined in ASCE 7, and the What's This? help in the Service Case dialog box can be used to display reference information.

ASCE 7 Seismic Settings

The Project Settings allow you to define seismic parameters that are currently ONLY used in the generation of IBC or ASCE 7 load combinations in the Building Code Combination system. These settings determine the Ev +/- Eh portions of seismic loads and automatically include the vertical component as a function of your 'self weight' service loads "D". The system will use the rho factors and then also generate any required Overstrength load combinations. You will see the effects of this in the "Effective Equation" shown in the Load Case Manager, or load combination reports.

Building Code Combinations

Building code combinations from a variety of building codes are built-in to VisualAnalysis. When selected these combinations are automatically maintained as loads are added or removed to service load cases. The desired building code or codes to use are selected at the bottom of the Load Combinations tab of the Load Case Manager. This system was designed for use with IBC or ASCE 7 load sources. If you are using the AASHTO load sources, you will need to manually generate (or import) your load combinations.

The building code combinations implemented in VisualAnalysis do not necessarily represent all possible load combinations or variations present in a particular building code. For example, in the implementation of ASCE 7-10 load combinations, the major equations are implemented, but exceptions clauses of section 2.4 dealing with H, F, Fa are not implemented directly. Similarly VisualAnalysis does not deal directly with T (self-straining) conditions at all--so you may need to manually create custom load combinations to deal with special circumstances.

How it Works

When one of the building code combinations is selected, VisualAnalysis will use the current service load cases and generate the necessary combinations prescribed for that code. Note that any custom equation or factored combinations that are created manually will remain unaffected. When VisualAnalysis generates building code combinations, it will generate combinations including the effects of wind and seismic in various directions, including (+/-X, +/-Y, +/-Z). Only load cases that actually contain loads or self-weight are included in the combinations. The Load Case Manager will then display the 'effective' combination of the equation which may be different than the equation in the building code.

Built-In Code Combinations

The following are hard-coded into VisualAnalysis 10.0, you generally only need to select one of these to get the load combinations you need. You might use both ASD and LRFD if you are doing some strength-level design and some service-level design.

 

Taking Charge, Controlling Performance

The automatic system is fast, convenient, and comprehensive--but not very intelligent. You can end up with redundant load combinations, depending on the actual loads you have applied. One of the biggest drains on VisualAnalysis performance is having extra load combinations to analyze, search, and report. 

You can manually delete redundant load combinations--but they may get regenerated if you add or remove loads or service load cases. You can convert all the combinations to manual combinations and take full control. Reducing the number of load combinations to analyze and report may dramatically improve VisualAnalysis performance.

A Customizable System!

The building code combination system is fully customizable through a data file CodeCombo10.txt. You may manually add, remove, or disable sets of code equations, or add, edit, or remove individual equations from the sets. There is a hyperlink on the Load Combinations tab of Load Case Manager that allows customization. You should share your customized CodeCombo10.txt file with everyone who uses VisualAnalysis in your office, ideally everyone would have the same settings. You should also manually back-up this file, because IES may update the default load combinations and overwrite your file. You can manually edit this text file in a simple editor like Notepad, be careful though as subtle differences between tabs and spaces or extra lines or characters can cause problems.

Import Custom Load Combinations

Requires: Advanced Level

You can import load combinations from the Windows clipboard, into the Load Combinations tab of Load Case Manager. You can create your definitions in a word processor or spreadsheet (table) and copy to the clipboard. The data format is flexible, basically a set of load factors and service-case names, you can optionally name the combination.

[Name:] f1*Case1 + f2*Case2 +…

Sample text import format:

Combo1: 1.0*D + 2.0*L + 3.0*S
# Comment lines ignored
1.0*D + -2.0Lr + 1.4E+X

Tabbed-delimited columns of data from a spreadsheet with a format like this:

#Header Line (Ignored) Factor Case Name Factor Case Name
Combo Name1 1.1 D 2.4 L
Combo Name2 2 D -4 S

Limitations / Rules

Equation Load Combinations

Design codes require a check of the structure against the combined effects of loads. The typical combination looks like this: 1.2D + 1.6L. In VisualAnalysis an Equation Load Combination will be used to accomplish this. Use the Create new Load Combination feature located in the Load Case Manager to create a single factored combination of service load cases.

Before creating an equation load combination, it is important to have service cases defined with appropriate load sources. The load factors are implicitly determined by the chosen equation and the correlating load sources. Use the Load Case Manager to create a custom equation load combination. If you preselect an existing equation load combination, you will get a "clone" of this when you click the button to create a new one.

Factored Load Combinations

When an equation load combination does not work, a Factored Load Combination should be used which lists all desired load cases with a specified factor. One common use for this type of load combination is patterned loading. In this case, multiple load cases might have the same source, yet require different factors for each. Use the Create a Factored Load Combination button located in the Load Case Manager to create a new combination. If you preselect an existing factored load combination, you will get a "clone" to start with.

Sometimes you want to combine loads with different factors on cases with the same source. For example, two live load cases may need to be included in the same combination, but a load factor of 1 on the first and 1.5 on the second is desired. Factored load combinations are more flexible than the equation load combinations because factors can be explicitly set to each load case regardless of source.

Response Load Cases

To perform a dynamic response analysis you need to have at least one Response Load Case defined. A response case contains direction multipliers and a design spectrum. Use Load | Load Case Manager and click on the Dynamic tab to create a Response Load Case.

The direction multipliers define the direction of the seismic event and also scale the magnitude. You might think of them as direction cosines. If you want to simulate an earthquake in the X direction use X=1, Y=Z=0. Similarly to indicate the direction in the XZ plane, 45 degrees off the X-axis, use X=Z=0.707 and Y-0. If the square root of the sum of the squares of the direction cosines adds up to a number other than 1.0, then this scale factor is also applied.

The design spectrum is a data set representing a seismic event. Building codes often provide design spectra to use, or you may have more accurate local information from previous events. The data represents a time history of displacements or accelerations. You can include your own custom data in the file (the default is 'Spectrum.txt' located in the Customizable Data folder).