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[SEAOC] RE: [SEAOC] NEED OPINIONS: Design of "Multibeam" One Way Slab System

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The problem proposed is more universal than one might think.  A similar 
problem presents itself when designing top level parking decks that can be 
accessed by fire fighting equipment.  This situation occurs often where a 
building has subgrade parking in which the parking garage footprint exceeds 
the footprint of the building.

The UBC and NFPA provide for clear markings on the windows "FD", but these 
windows can only be accessed by some sort of areal platform.  Most 
structural engineers simply design for AASHTO maximum truck loading.  When 
the fire apparatus arrives on the scene the outriggers are put down on a 
structural deck that can be overloaded by an enormous factor.  The failure 
mode is most often beam or punching shear.

The only limitation to outrigger loading is 75 psi as provided by the NFPA. 
 I performed a cursory survey of equipment manufacturers and found maximum 
outrigger service loads of 30,000 to 60,000 pounds on a single outrigger.

We work with the fire departments to determine potential fire apparatus and 
maximum loads which then become a design load case.  The outrigger loads 
invariably govern by quite a margin, yet non of the model building codes 
provide for this.

Lacking any better information source we distributed these loads we used the 
AASHTO methodology.  I too would like to know if there is a better way.

Harold Sprague
Black & Veatch
From: seaoc
To: seaoc
Subject: [SEAOC] NEED OPINIONS: Design of "Multibeam" One Way Slab System
Date: Friday, October 04, 1996 10:09PM

I would like to know if anyone has any "hard/fast" data on the
distribution of loads among the beams of a multibeam platform structure.

This is a type of construction commonly seen in bridges, where you have
PS Concrete box beams laid right next to each other, and a composite
(for LL) slab poured over them.  AASHTO has its formulae for
distribution of loads in this case, but I don't think it would apply for
a one-way slab type of construction.

In my case, I'm trying to apply multibeam to a wharf, which will have
very large, static live loads representing operating a crane for
unloading ships.  If you have loads from, e.g., truck crane outriggers,
or the tracks of a crawler-crane, how do you determine the effective
width of beam+deck that assists in supporting the load?

I've seen an old (30+ years) set of design notes from a local
engineering firm that utilizes a formula like 0.6S+1.5T, where S is the
span between simple supports, and T is the width of the (assumed square)
loaded area on the deck.  But I'm not sure if this is rational, or just
something someone came up with for the heck of it.

Anyone here got any ideas?