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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON CIVIL AND STRUCTURAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Three-Dimensional Structural Modelling of Multi-Storey Buildings for Obtaining Moment Envelopes
T.M. Nahhas and M.H. Imam
Department of Civil Engineering, Umm Al-Qura University, Makkah, Saudi Arabia
T.M. Nahhas, M.H. Imam, "Three-Dimensional Structural Modelling of Multi-Storey Buildings for Obtaining Moment Envelopes", in B.H.V. Topping, (Editor), "Proceedings of the Eighth International Conference on Civil and Structural Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 16, 2001. doi:10.4203/ccp.73.16
Keywords: moment envelopes, buildings, structures, modelling, software packages.
Structural designers commonly use shear and moment envelopes to determine the maximum positive and negative moments and shear forces in the structural members. To obtain these envelopes, for multi-storey building structures, there are two distinctly different approaches in practice. One of them, referred to as "Traditional Moment Envelope" or the TME approach in the paper, is based on the textbook method of considering a single continuous beam at a time. Thus, it involves only one-dimensional analysis using simple closed form expressions for moments and shear forces. Despite the advanced state of the art in structural analysis software, the TME approach is still in common use by the practicing structural designers. In contrast to this, the "Current State of the Art" or the CSA approach involves the three-dimensional structural modelling of the building using a general- purpose structural analysis software package. Both approaches have certain advantages over each other but for a real-world problem both fail to offer the automation in modelling as may be expected with the current state of the art in structural analysis software and CAD. The TME approach is inaccurate and limited and no enhancement is possible to enable it to handle the three-dimensional structural model. The CSA approach is accurate due to the three-dimensional structural modelling and advanced structural analysis capabilities of the available software packages but it suffers from being laborious, time-consuming, and error- prone because the input data does not relate to the floor plan and apriori visualization of the wire-frame model is required for using the joints and elements generation options.
In this paper, a conceptually different new approach is presented that does not require apriori visualization of the structural model and generates the three- dimensional structural model using the information available mainly from the floor plan of the building. The live loads for obtaining the shear and moment envelopes can therefore be easily applied and any live load patterns for generating the moment envelopes can be generated. The key to the generation process is a primitive building block called "Building Unit Structure" or BUS, which is composed of four columns, four beams and a slab. The beam and column elements of a BUS represent the structural properties and the slab element is merely to apply the live and dead loads. The slab may be specified as rigid to represent the high in-plane stiffness. Each BUS may be identified on the floor plan of a multi-storey building by its position and the dimensions. Any number of storeys can be assigned to a BUS. The topology of a BUS is controlled with a Boolean string called "topology string" containing nine bits. The total number of possible topological configurations for a BUS equals 29 or 1024. Four different union operations of two BUS's while maintaining the attributes of each individual BUS allow fast and easy modelling of multi-storey building structures with any number of storeys and any required topological configurations.
The concept has been implemented in the software package called FEMBUS . FEMBUS generates the data representing the structural model in the form of ready- to-go input data files for general-purpose structural analysis software packages [2,3,4,5,6,7]. Once the model has been generated, any type of analysis can be performed using a software package of user's choice. Extremely large number of possible live-load patterns that occur due to the consideration of the three-dimensional structural model can be easily applied.
The presented approach drastically cuts down the time and effort required for the structural modelling of multi-storey buildings for obtaining the moment envelopes as well as for other types of analyses. For a small academic example problem the moment envelopes are produced within 50 minutes as compared to 900 minutes using CSA approach and 1300 minutes using TME approach. For real-world problems, the time required to generate the envelopes using FEMBUS approach is only in hours as compared to weeks using CSA and TME approaches.
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