Load everywhere!
It is probably tempting to assume that, by applying live load
on every beam span and slab panel, we can produce the greatest moment and shear
everywhere. After all, structural engineers always want some conservativeness
to sleep soundly at night.
Take a simply supported beam as an example. The more you
load it, the higher its response. Everything is predictable.
𝘞𝘩𝘦𝘯 𝘉𝘦𝘢𝘮
𝘚𝘱𝘢𝘯𝘴
𝘢𝘳𝘦
𝘕𝘰
𝘓𝘰𝘯𝘨𝘦𝘳
𝘰𝘯
𝘛𝘩𝘦𝘪𝘳
𝘖𝘸𝘯
Now connect several spans together. Each span can no longer
behave independently. What happens in one span affects the supports and
neighbouring spans.
For a continuous beam, the loading arrangement that produces
the maximum hogging moment at a support is not necessarily the same arrangement
that produces maximum sagging moment within a span.
Imagine the beam as a flexible plastic ruler resting over several supports. Pressing down on different spans changes how the ruler bends over each support. Loading one span while leaving neighbouring spans unloaded can increase sagging within that span because the restraining effect provided by loading elsewhere is reduced.
𝘉𝘦𝘸𝘢𝘳𝘦 𝘧𝘰𝘳
𝘈𝘯𝘺𝘵𝘩𝘪𝘯𝘨
𝘛𝘩𝘢𝘵
𝘋𝘪𝘴𝘳𝘶𝘱𝘵𝘴
𝘵𝘩𝘦
𝘉𝘢𝘭𝘢𝘯𝘤𝘦
Imagine carrying two shopping bags. Two equally loaded bags
may feel balanced. Try carrying only one bag and your body must twist and
redistribute effort, even if the total weight is lower.
Interestingly, the same idea applies to a multi-bay frame.
If neighbouring bays carry different loads, the imbalance can produce unequal
column moments, joint rotation and sway in flexible frames.
Flat slabs can also be sensitive to uneven loading. When adjacent panels carry different loads, demand around particular column zones can increase. The resulting unbalanced moment transfer may also increase punching shear demand even though the overall floor appears satisfactory.
𝘐𝘯𝘵𝘦𝘨𝘳𝘢𝘵𝘦
𝘒𝘯𝘰𝘸𝘭𝘦𝘥𝘨𝘦
𝘪𝘯𝘵𝘰
𝘠𝘰𝘶𝘳
𝘗𝘳𝘰𝘧𝘦𝘴𝘴𝘪𝘰𝘯𝘢𝘭
𝘞𝘰𝘳𝘬𝘧𝘭𝘰𝘸
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