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Patterned Loading in FEA Slabs

Posted on August 14th, 2020 in Analysis

Summary

The source material provides a detailed explanation of the importance of pattern loading in finite element frames, particularly when analysing slabs.

Structure and Setup

The demonstration uses a structural model consisting of a steel and concrete frame, a slab, office space, and industrial warehouse space with differing piling layouts.

To enable pattern loading in the program, surface loading on the slab is applied to live alternate instead of L1, live one. This setting instructs the program to use the applicable loading patterns. Dead wall loads are also designated as alternate to facilitate patterning.

Creating Load Patterns

The process for defining these patterns involves going into alternate load patterns within the software and creating strips across the structure.

Sets and Directions: Four load sets are required. Load sets 1 and 2 create the alternating load patterns (max, min, max, min and vice versa) in one direction (X direction). Load sets 3 and 4 perform the exact same function in the opposite direction.
Load Groups: The creation of these load groups is described as straightforward. Dummy members are added solely to facilitate the creation of these load groups and allow the program to split up the structure for alternating patterns.
Pattern Creation Tools: Tools such as auto pick can be used to automatically define the growing panel. The patterns do not need to be 100% accurate, only indicative. Combining two sets, such as set one (one and two) or set three (three and four), provides a blanket coverage over the entire structure.

Loading Combinations

The structural analysis uses loading combinations and cases based on the created patterns (D1 to D4 and L1 to L4).

Example load combinations include:

The first load factor using D1, D2, L1, and L2, all set to maximum.
A subsequent case involving maximum on the ones and minimums on the twos.
The opposite case, where maximums are applied to the D twos (and corresponding L twos) and minimums to the ones.

It is noted that although the Euro code typically dictates that dead loads are not patterned, it is better practice to include them as D1 and D2 within the framework. Service cases follow the same method, stripping the load in the opposite direction.

Impact and Results of Pattern Loading

Pattern loading is essential because it reveals critical moments that are missed when simply loading all spans simultaneously.

Initial results, viewed using shell elements, showed moments ranging from 63 down to 30s. When comparing results from the default "all spans loaded" case to the "alternate" loading cases, significant increases in moments are observed, particularly in sagging moments between supports:

Moment Increases: Alternate loading causes moments to increase significantly. One example showed an increase from 20 to 36, representing approximately a 40% increase in moment.
Hogging Moments: Pattern loading can reveal a hogging moment where none existed previously, resulting in an infinite increase in the hogging moment.
Other Increases: Increases in moments were also observed when comparing 50 (alternate) to 42 (original), which is nearly a 20% increase, and 58 (alternate) compared to 51 (original), which is nearly a 15% increase.

By using pattern loading, the envelope of ultimate cases captures all these larger values.

Although the peak moments over the column heads generally do not change, the alternative loading cases are crucial because all spans loaded may not be the dominant case for moments between spans. While more complicated patterning methods, such as those based on Muller-Breslaw (e.g., max, max, min, max), exist, they offer virtually no significant benefit to the design and result in only minuscule increases.

Summary with Timestamps

The importance of pattern loading in finite element frames is demonstrated through the analysis of a steel and concrete frame and slab. Applying pattern loading is essential as it reveals critical moments that are missed when all spans are loaded simultaneously.

The following table summarises the process and findings, including the relevant timestamps:

Timestamp Range	Key Concept / Action	Details and Findings
00:00:07,680 – 00:00:36,080	Introduction and Model Setup	The video examines the importance of pattern loading in finite element frames. The model includes a steel and concrete frame, a slab, office space, and industrial warehouse space with differing piling layouts.
00:00:36,440 – 00:01:08,400	Enabling Pattern Loading	To activate pattern loading, surface loading on the slab is applied to live alternate instead of L1, live one. Dead wall loads are also designated as alternate to allow the patterning to take place.
00:01:08,720 – 00:01:42,320	Creating Load Patterns (Strips)	Patterns are defined in the alternate load patterns section by creating strips through the structure. Four load sets are required. Sets 1 and 2 create max, min, max, min alternating load patterns in the X direction. Sets 3 and 4 perform the exact same function in the opposite direction.
00:01:43,280 – 00:02:17,440	Using Dummy Members	Load set 4 corresponds to D4 and L4 loads. The creation of these load groups is very straightforward. Dummy members are added solely to allow the program to split up the structure and facilitate the creation of these load groups and alternating patterns.
00:02:28,640 – 00:03:29,200	Pattern Definition Tools	The patterns do not need to be 100% accurate, only indicative. Tools like auto pick can be used to automatically define the growing panel. Combining two sets (e.g., set one and two, or set three and four) provides blanket coverage over the entire structure.
00:03:35,600 – 00:04:52,880	Loading Combinations	Loading combinations and cases use the defined patterns (D1 to D4 and L1 to L4). An example load combination sets D1, D2, L1, and L2 all to maximum. Subsequent cases involve maximums on the "ones" and minimums on the "twos". Although the Euro code typically dictates not patterning dead loads, it is better practice to include D1 and D2. The opposite case involves maximums on the D twos (and L twos) and minimums on the D ones (and L ones).
00:04:54,320 – 00:05:16,480	Service Cases and Preparation	Service cases follow the same method, stripping the load in the opposite direction. The results are then examined graphically.
00:05:37,120 – 00:06:07,440	Initial Moments (Shell Elements)	Viewing results using shell elements, initial wooden armour moments are seen ranging from 63 down to 30s. Sagging moments are observed.
00:06:44,840 – 00:08:43,120	Analysis via Strips (Baseline Case)	The effects are viewed easier by looking at strips across the frame. When viewing the baseline case (all spans loaded), moments seen are 51, 42, 42, and 37, with hogging under the columns.
00:08:44,080 – 00:09:21,680	Impact of Alternate Loading (Case 1)	Switching to alternate one loading causes moments to jump significantly. A moment increases from 20 to 36, which is approximately a 40% increase in moment. Alternate loading also revealed a hogging moment where none existed previously.
00:09:21,520 – 00:09:51,280	Moment Comparison	The unrevealed hogging moment represents an infinite increase. Other observed increases include: 50 compared to 42 (nearly a 20% increase) and 58 compared to 51 (nearly a 15% increase).
00:09:52,160 – 00:10:08,720	Capturing Critical Values	The envelope of ultimate cases captures all these larger values generated by patterning. The peak moments over the column heads generally do not change.
00:10:32,880 – 00:10:59,360	Alternative Patterning Methods	More complicated patterning methods (like Muller-Breslaw: max, max, min, max) exist but are complicated and offer virtually no significant benefit to the design, resulting only in minuscule increases.