STAAD.Pro Loading and ISM
Tuesday, 20 August 2013
The information in this document has been prepared for STAAD.pro SELECTseries 5 which has not been publically released yet.
Introduction
The STAAD.Pro analysis model can integrate with an ISM physical model using the StrucLink ISM module. During development of the analysis model, STAAD.Pro will include loading data on the analytical elements. This means that any update on the STAAD.Pro model using the menu option ‘File > Update from ISM Repository’ which includes changes in the physical model, can result in significant changes in the analytical model and the loading which was defined on one member may need to be redefined or moved to another location or member. This document outlines the current scope of the changes that are managed automatically and highlights loading that will require user intervention.
When a STAAD.Pro model is updated from an ISM repository, changes from the repository will result in the following key changes in the STAAD.Pro model:-
1. New physical members added into the ISM repository.
These will be imported into STAAD.Pro and will create a one or more new analytical members to represent the new physical member. Additionally, if they are found to lie on exiting analytical members, these members will be split and any loading defined on the original member will be evaluated and may be redefined or re-allocated to maintain the equivalent distribution.
2. Existing physical members deleted.
The analytical members that represented the deleted physical members will be removed with any loading that was assigned to them. If a deleted member was spanning between two supporting beams, then the supporting beam would have been divided into a number of analytical parts to ensure correct analytical connectivity. If the removal of the member does not require the supporting beams to remain split, they will be merged and the loading re-defined to maintain the equivalent distribution.
3. Existing members moved.
Moving of physical members can result in both the above effects taking place.
The following outlines the effects on the loading defined on the analytical model and describes what loading is handled automatically and what needs to be managed by the engineer after running an update.
Loading Types
Member Loads
There are 10 types of member load that can be defined on a STAAD.Pro model. The STAAD.Pro ISM StructLink module currently automatically manages the following member load definitions during an update:
- UNI, uniform force
- UMOM, uniform moment
- CON, concentrated moment
- CMOM, concentrated moment
- LIN, linearly varying
- TRAP, trapezoidal
Loading of these types on members that are split or merged due to changes in the geometry of the physical model are managed and may be changed to provide an equivalent loading pattern.
Therefore, if a member is split due to the addition of a member or due to the move of a member so that it now interacts with an existing member line, any of the above loads will be positioned on the new set of members.
Action Required:- None
Example
Consider the following model which has loading on members which are affected by a change in the ISM model:
Updated model with cross member deleted, support beams combined:
Add a member that divides existing members:
Analysis model updated:
These other member loads are currently not managed during an update and should be managed by the engineer are less commonly used:
- TEMPERATURE
- PRE/POST STRESS
- FIX END
- STRAIN
Action Required:- Members that have been subject to these 4 load types should be reviewed and if modified by the update, should have the loading reassigned for the updated geometry.
Self weight
Self-weight can be used in STAAD.Pro either without a specific member list.
a) If the selfweight command is defined without a specific named list, then no action is needed.
b) If the selfweweight command is defined with a specific named list, then:
- If new members are created during an update, by the addition of a new member or by splitting an existing member, these are not automatically managed and will need to be manually added into the selfweight command.
- If members are removed, say by merging a member, then they are removed from the selfweight list.
Action Required:- None
Node Loads
Nodes are always retained, thus node loads are also retained. However, it may be that during the update, removal of members can leave nodes isolated from the main structure and will need to be manually re-located back on the structure.
Recommendation, it may be better to use member concentrated loading instead of node loads so that it is clear what the intension of a load at a point which is a junction of several members so that if a member is moved, the load can move with the appropriate member.
Action Required:- Check for isolated nodes using the menu item ‘Tools>Check Orphan Nodes’ and convert to member loads
Floor and Area Loading
STAAD.Pro supports two methods for creating loads on members using a definition of a zone which is used to calculate loads on members.
- FLOOR, this can create either 1 or 2 way or loading by defining either:-
- A group of members which are then used to determine closed loops within which the loading is assigned and then a calculation performed to establish the loading to apply to these members.
- A zone in 3D space and let STAAD.Pro find the members in that space, and use all these members to find closed loops of members in a given plane, then apply loading in that area, establish the loading that then gets assigned to the members of the loops.
- AREA, this is a legacy command and not a recommended method due to a number of significant limitations in the algorithm.
The update process manages members that are in load groups such that
- If a member is removed from the model as it has been deleted, thus is removed from the group definition.
- If members are merged into a new member, the resulting member is maintained in the group.
- If members are split due to the introduction of a new member, then all the new parts are included in the group. However, note that the analysis members formed from the new physical members are not considered part of the group. If this is required, then it should be managed by the engineer.
Action Required:- Where a member is inserted into an area that has been defined as a member group for floor loading and intended as a load bearing member, add into the floor group definition. Where the volumetric form of the floor load command is used and a whole structure move has been implemented, the definition of the volume should be updated by the engineer to suit the new location of the model.
Reference Loads
Reference load cases are similar to primary load cases, but not analysed. These cases include similar load items to the primary load cases and should behave in the same way. The same limitations exist for the equivalent load items that exist in Primary load cases.
Action Required:- Follow the same guidance as for the equivalent loading as in primary load cases.
Finite Element (Plate and Solid) and Surface Loading
STAAD.Pro does not support integration of finite elements (i.e. plates and solids) or the STAAD.Pro surface with ISM, thus loading on these entities is not an issue.
Action Required:- None
Seismic Loads
A seismic definition can include loading to determine the mass matrix. These take the same form as the loading found in a regular load case and are managed with the same limitations.
Note that alternatively, the mass can be taken from a Reference Load case where the type is defined as MASS (see reference load cases above).
Action Required:- Follow the same guidance as for the equivalent loading as in primary load cases.
Dynamic Loads
The solution of a dynamic analysis is dependent on the geometry and a mass matrix. The mass matrix is defined once and can be:-
- A Reference Load Case classed as MASS (see Reference Load Cases above)
- Loads included in the first Time History load case
- Loads included in the first Response Spectrum load case
Therefore any loads that have been included in a Time History or Response Spectrum load case to determine the mass matrix will be affected in the same way as when used in the regular static loading condition.
Time History. The definition of a time history forcing function is not affected by any changes by the ISM updates. However, the forcing functions are assigned to nodes and any modification on nodes, such as deletion would impact this. As nodes of deleted members are not deleted, the applied loads remain, but it may be that merging the member leaves nodes unconnected to the model which means they would need to be manually reintroduced into the model.
Response Spectrum. No part of a Response Spectrum definition or application is dependent on the changes that take place during an ISM update.
Action Required:- Check any loaded nodes that may have been disconnected due to deletion or movement of members these may need to be reintroduced into the analysis model
Moving Load
Moving Loads are specified in 2 parts, the definition of the vehicle and the inclusion in a load case.
The definition is an independent load definition, not affected by any modification by the ISM updates. However, this definition is given an absolute starting point and direction, hence any global offset of the model would need to be replicated in the starting position.
Action Required:- None
Wind Load
Wind loads are defined in 2 parts, the definition of the wind, typically in terms of intensity over height, and then the inclusion of this in a load case. The actual resolution of the loading onto the members is done at analysis time; hence changes in the model due to ISM update do not affect the result.
There is one area where a modification by ISM can impact on a wind analysis. Part of the definition can be an EXPOSURE parameter; this is intended to allow local reductions (on increase) in forces due to the wind at specific nodes on the model. If the nodes are modified/removed, these definitions will need to be reviewed. This has to be a manual procedure.
Additionally, the extent of the structure subject to wind loading can be constrained by a number of dimensions in the global axes. If the model has been modified with a global move, then this will need to be modified.
Action Required:- Check nodes that have been removed or added to ensure if they need to be included/removed from the exposure list.
Snow Loading
Snow loading is defined in 2 parts, a definition which is generic and not affected by the structure and an assignment in a load case in which the zone in which it is applied is defined by a FLOOR GROUP. Hence the issue of floor loading defined above (i.e. groups that have members split with the introduction of new members not getting added to the GROUP) will impact on the snow loading.
Action Required:- Where a member is inserted into an area that has been defined as a member group for snow loading and intended as a load bearing member, add it into the floor group definition.
Summary
The main loading types are automatically updated, but it is still prudent to review all the loading after an update to ensure that the loading on the updated geometry adequately reflects the specific engineering requirements for the analysis that is to be performed.