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*TO DO (see labels for assignee):* Provide numerically solved example: 3 span continuous beam with uniform and point loads, check mispan moment.
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The purpose of Pattern Live Load Factor (PLLF) is to enable modeling of moment redistribution due to yielding. When nonzero PLLF is specified for a frame object, the program treats the member as if it is simply supported for resisting the applied live loads (this applies only for calculating the design moments; the moments obtained from the analysis are not affected). The PLLF then enables to specify what percentage of the live load should be applied to the frame object for which no continuity is assumed. This enables to model a condition when negative moments are redistributed and thereby increasing the positive mid-span moments. You can specify zero PLLF if you wish not to consider this moment redistribution.
The Pattern Live Load Factors can be assigned to individual elements by using "Design > Steel Frame Design (or Concrete Frame Design) > View/Revise Preferences" menu command:
!Pattern_live_load_factor.png!
h1. Example
The maximum midspan moment due to uniform live load would be calculated as:
{math}M_{max} = {fwL^2}/{8}{math}
where
* f ... the pattern live load factor
* w ... applied uniform live load
* L ... length of the frame object
* {math}M_{max}{math} ... maximum midspan moment (calculated assuming no continuity; simply supported condition is assumed)
For point loads, the span positive moments would be calculated similarly as for uniform loads: the frame object would be assumed as simply supported and the specified factor would be applied to the live loads.| Show if |
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TO DO: - Provide example with solutions and midspan-moment checks for a 3-span continuous beam subjected to uniform and point loading.
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The pattern live-load factor (Pllf) is intended to account for variability in the live-load arrangement on a structure. Since live-load pattern is not automated, Pllf approximates its design effect.
When nonzero Pllf is assigned to a frame object, positive design moments are calculated assuming a simply supported condition under live-load application. While this does affect design moments, analysis moments are not affected. Pllf enables users to specify the live-load percentage applied to a frame object for which no continuity is assumed. Negative moments are then redistributed in a manner similar to that of moment redistribution from yielding. As a result, positive midspan-moment values increase. Zero Pllf may be specified to avoid moment redistribution.
Pattern live-load factors may be assigned to individual elements using the Design > Steel Frame Design or Concrete Frame Design > View/Revise Preferences command, as shown in Figure 1:
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Figure 1 - Pattern live-load factor
Example
The maximum positive midspan design moment induced by uniform dead- and live-load application would be calculated as follows:
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where:
- f1 = code-based dead-load factor (1.2)
- f2 = code-based live-load factor (1.6)
- Pllf = Pattern live-load factor (0.75) specified in SAP2000 or ETABS
- LL = distributed live load (force/length)
- MDL = positive dead-load moment with actual boundary conditions due to dead load only
- Mpos MAX - maximum positive midspan design moment assuming a simply-supported condition with no continuity for live load
Given point loading, Mpos MAX would be calculated similarly as for uniform loading, in which a simply supported condition is assumed and the live-load factors specified are applied.
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Related Emails: - Email "Wiki entry on pattern live load": (ok → rs, 10/25/2010, ID 7635351)
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