Multiple Presence Factor in AASHTO LRFD

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Swimfastalex PE

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I am a little confused by the wording in AASHTO LRFD C3.6.1.1.2 and how it relates to the Multiple Presence Factor.

Here it is as follows:

If a component supported a sidewalk and two lanes of vehicular live load, it would be investigated for:

  • One lane of vehicular live load, m = 1.20;
  • The greater of the more significant lanes of vehicular live load and pedestrian loads or two lanes of vehicular live load, m = 1.0, applied to the governing case; and
  • Two lanes of vehicular live load and the pedestrian loads, m = 0.85.
I understand bullet 1 and 3.  Bullet 2's wording I think is a bit confusing.  Can anyone maybe help me understand this one a little?

Thanks.

 
the multiple presence factor addresses the probability of multiple fully loaded lanes occurring simultaneously.  so the mp goes down as more load is considered. 

Case 1 - Looking at a single loaded lane uses the maximum mp=1.2,

Case 2 - considering the greater of:  1)largest single lane vehicle load and pedestrian load or 2) two vehicle lanes uses mp=1.0

or Case3 - consider two vehicle lanes and the pedestrian load using mp = 0.85

Hope that helps.

 
Thanks it does.  I think what was confusing me was the word "lanes".  And the other additional wording they have.  But it make sense.

 
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