
November 19, 2014
What do the Unbraced Lengths represent?
Understanding unbraced lengths and how RISA software accounts for them is essential in the design of any project. In RISA there are three main types of unbraced lengths:
In structural engineering, few design challenges are as rewarding—or as unforgiving—as the tall building. While gravity systems and code checks form the backbone of any structural project, once a structure rises beyond ten or fifteen stories, a shift occurs. Wind and seismic forces begin to dominate. Story drift and torsional irregularities become non-negligible. Load paths grow increasingly indirect. And design decisions, if not carefully made early on, can have exponential consequences higher in the structure. Tall buildings are not simply “bigger” versions of short ones. They behave differently. And understanding those differences is essential for any engineer working in an urban environment where building vertically is often the only viable path forward. Modeling for Reality, Not Just Code The foundation of any successful tall building design lies in the model—its assumptions, resolution, and degree of abstraction. Many engineers begin with simplified representations: rigid diaphragms, idealized connections, and linear material properties. This is practical and often sufficient for early design phases. But as the building increases in height and complexity, those assumptions may start to mask critical behaviors. Semi-rigid diaphragm modeling, for instance, allows engineers to capture in-plane flexibility of floor systems—especially important in buildings with irregular cores, open floor plans,…
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Understanding unbraced lengths and how RISA software accounts for them is essential in the design of any project. In RISA there are three main types of unbraced lengths:
The axial compression and flexure strength of beams and columns is dependent on the spacing of elements which provide bracing along the length of those members. You may specify unbraced lengths as a fixed distance or by using RISA’s Unbraced Length Commands. These unbraced lengths are used for...
Code requirements for consideration of panel zone shear deformation can be confusing. This blog post discusses some of the basis for these code requirements and when one can consider them to be met and when one cannot.
Concrete buildings often have large cantilevers. In RISAFloor ES, the Support lines can be drawn to support points but also they can be drawn as cantilevers (shown below).
In RISAFloor ES, you draw a support line from support point to support point to define the Design Strips. The program will automatically create Design Strips based on the tributary width. When you have walls in the model, it is often a question of how you draw in the support lines.
By default RISA-3D draws all members as line elements located at the centroid of the cross-section. However, connections between members are not always aligned with a member’s centroid. There may be horizontal or vertical offsets in the connection. To account for these you can add rigid links to...
A rigid link is a member element in RISA-3D that can be used for many advanced modeling procedures. It is so useful that it is included as one of the default member Section Sets, as you’ll see below.
The deflected shape is really helpful to understanding your model’s behavior.
RISA-3D and RISA-2D will allow you to add a moving load pattern to your model and include it in your envelope solution.
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