Multiframe News October 1999

We will soon be releasing our next update to Multiframe for Windows - Version 6.1. This update will contain a number of improved modelling features, including the ability to apply rigid offsets at the ends of members.

When modelling most structures it is generally assumed that the size of connections is small relative to the length of the members and that members are aligned between nodes. It is also usual to assume in the model that members are connected through the shear centre of the section. However there are many situations where this assumption is not valid and more detailed modelling techniques are required. For example, for sections which are not doubly symmetric, the connection to the member may be significantly offset from the shear centre such as a connection to the leg of an angle.

In cases where the above assumptions do not apply, the model of the structure can be improved by using a rigid link to connect the end of a member to a node. In Multiframe version 6.1, these rigid links can be modelled using Member Offset connections. These are rigid connections between a node and the end of the flexible portion of a member.

Member Offsets can be used to improve the modelling of rigid connections. When members meet at a rigid joint, the over-lapping portion of the members is relatively stiff and may be considered to be a rigid body for the purpose of analysis. This rigid zone can be modelled using member offsets as shown in the diagrams on this page.

When members are not aligned between nodes, actions are introduced into the members due to the eccentricity of the connection. The eccentric connection can be represented as a rigid link between the node and the end of the member and hence can be modelled using a member offset connection.

Another use for member offsets is in the modeling of shear cores. The shear core may be represented as a large section with the appropriate section properties and shear area. The member offset can be used to take into account the substantial distance from the centreline of the shear wall to the actual end of the flexural member.

Although most users of Multiframe use the suite of programs for preliminary and detailed structural design, there are also a large number of construction companies using Multiframe for design and investigation of construction procedures and temporary site works. Peter Hawkins of Hawkins Engineering is a consultant specialising in construction methods using the principles of "Value Engineering" to help his clients deliver maximum customer benefit for least cost.

Over the past 4 years, Peter has used Multiframe on a wide range of construction projects including investigations into launching techniques for incrementally launched bridges, complex temporary works for large scale concrete projects, and numerous other projects. Peter's projects have taken him throughout the South East Asian region including Laos, Vietnam and throughout Australia. His laptop and Multiframe travel with him at all times and form a core part of his mobile office. Email transfer of Multiframe and other files to the various design and construction offices in the project, ensures he can quickly react to the short deadlines typical of the construction industry.

Typical of Hawkins work was the formwork for the construction of four huge offshore towers constructed in Western Australia for the Wandoo project. These towers formed the basis of the Concrete Gravity Substructure (CGS) and were built in a casting basin 12 metres below sea level to facilitate flotation of the CGS when construction was complete. A total of 28 000m3 of concrete, 8 500t of reinforcing steel and 550t of prestressing strand was used in constructing the CGS. Multiframe was used to investigate a range of supporting structures including the "spiders" circling each tower and supporting the formwork as the pour progressed.

Peter finds that most of his temporary structures tend to be deflection rather than strength limited, so Multiframe is an ideal tool to do quick member sizing, deflection checks and investigate bracing and support alternatives. The graphical tools for rapid manual iteration help him in this process. Once an appropriate solution has been developed, the graphical printouts from Multiframe are quickly marked up and included in design reports. As an interesting note, like many of our users, Hawkins Engineering have now moved away from traditional large format drawings to smaller A3 (11"x17") size drawings for construction purposes.

Hawkins Engineering's most recent project is a part of the duplication of the Narrows Bridge in Perth, Western Australia. The key issues investigated on this project are the likely loads on the trolleys and other supporting structures used to incrementally launch the bridge across the Swan River. With reaction loads as high as 3000 kN (200 kips), correct assessment of likely behaviour is critical to ensure a safe and trouble free jacking process. One of the Multiframe models used is pictured on the previous page - this structure models multiple phases of the launch process in a single design to determine reactions and deflections during various stages of the launch.

The next release of Multiframe for Windows will include a capability for exporting Multiframe models in VRML (Virtual Reality Modelling Language) format. VRML is a file format developed to allow the transfer of 3D models across the Internet and their display in standard browsers like Netscape Navigator and Internet Explorer. VRML has found application in a wide variety of areas including data visualisation and 3D product modelling. As far as we know, Multiframe is the first structural engineering software to support this format to assist engineers in creating web sites which allow them to share their models with other members of their design and construction teams.

VRML models can either be viewed in a separate window in your browser or they can be embedded in a web page along with other standard HTML data. This means it is quite simple to set up web pages that include a Multiframe VRML model which can be viewed along with any other required tabular, graphical or text data. To view VRML models and to use the 3D controls for zooming, panning and navigating through your scene, you will need a browser plugin. Cosmo Software (formally a part of Silicon Graphics), Intervista and Blaxxun all produce VRML browsers.

The best reference point for information on VRML and places to download a browser is the web site for the Web3D consortium that manages the VRML standard. You can find them at http://www.web3d.org. VRML has been approved as an ISO standard and so is firmly established as standardised data of viewing 3D data. The Web3D consortium is currently investigating extensions to VRML (currently at version 2), to extend the usage of 3D browsing on the web. We will continue to monitor these developments and support them as new 3D options become available. We expect that more and more Multiframe users will use web sites for project management and promotional purposes and we see 3D models forming an important part of the data which will be communicated via these web sites.

Normally Multiframe does not allow you to analyse more than one structure at a time and it also requires that all parts of the model are connected together in some way. It is possible however to create a Multiframe model which contains multiple frames allowing you to model more than one structure at a time or to create multiple versions of the same structure to investigate design alternatives.

The trick to this is that all the structures can be drawn in the usual way however they must be connected together in some way. This can be achieved, without affecting analysis results, by simply drawing at least one connecting member between the restraints so that each structure is connected to at least one other structure. You may have to assign section properties to these dummy members and release all the moments at the ends so that their stiffness is ignored.

You can use the Masking option to mask out these dummy members so that they don't appear in your results. An example of this can be seem in the screen shot on page 2 of this newsletter where multiple versions of the bridge structure have been modelled in one Multiframe file.