Parking Garage - Live Loads and Live Load Reductions 1

[WillisV]

Just wanted to pass along a good concise article describing what has become quite a mess in the codes the past few years - Parking Garage live loads and live load reductions:

http://www.gostructural.com/ME2/Aud...2C&tier=4&id=5E34C743493F4EEB83E0A8BE0E926A09

 

[rapt]

In cast insitu parking garages, and for that matter, any building where live load reduction has been used in the design of the horizontal members, has anyone ever checked the back propping requirements of the floors?

Though the floor is theoretically designed to carry 50psf, some members in the floor may have been designed to support only 30psf or, as this article has indicated, as low as 24psf. In this case, the whole floor can only be assumesd to be able to support the reduced live load for the design of the back-propping.

If it has been left to the builders engineer to determine the back-propping requirements, is he told that the floor can only support the reduced live load and they will therefore have to backprop twice as many floors?

For this reason, very few floors are designed using live load reduction in Australia. The columns, yes, and transfer beams but not the floors because the extra cost of the backpropping is more than the cost saving for the floor.

 

[archeng59]

thanks

 

[Brantlet]

I am a new member of Eng-Tips, and really appreciate this particular reference, since we have a current parking deck issue, that seemingly pertains to these same live loading requirements, as have recently been adopted into our State Building Code (Florida) document.

However, does the live load reduction also apply to the concentrated loading requirement, which on our deck would produce the critical moment? It would seem that instead of REDUCING live loads, that one might alternately want to INCREASE them for purposes of insuring conservative design?

Furthermore, do you know if the noted code requirement for the 3,000# concentrated live load is to be interpreted as meaning the consideration of just ONE single point load, or should this perhaps be interpreted and modeled as applying to ALL and MULTIPLE (let's say for sake of discussion a MAXIMUM number) of the available wheel loads that might exist in a span?

It would seem to me that the prudent thing to do (at least for a deck design), would be to determine the critical loading arrangement; in this case, by first estimatimg the influence area of deck, and the maximum loading that might be imparted from a string of vehicle wheels aligned at mid-span, which should produce the critical bending moment. After all, isn't this what I should really be estimating, which is the critical loading arrangment that might stand to occur upon the deck slab itself?

Nevertheless, from some of my PhD engineering school friends, I have also heard cases staring to be made for the consideration and use of load combinations to be used in better estimating and assimilating the design condition, to include both distributed as well as concentrated live load cases at the same time (I do not think this is presently being done by anyone out of concerns for overdesign and increasing the cost).

It presently appears to me that a lot of SE's in our area, are just selecting and applying the MINIMUM distributed live loading requirement from our building code, and are basing their entire parking structure deck design upon code minimum loading, without any consideration being made for other (higher/critical) loading arrangements, which in reality might lead to a failed deck. Isn't it true that the moment envelope from distributed loading case is not the critical case (at least for decks)? Isn't the point load always to be considered as the most critical case?

As a result of this, I have seen at least one collapse, a lot of punching shear, along with epoxy repairs and carbon fiber retrofit projects being pursued, as these structures encounter actual loading conditions that exceeds the engineers assimilated design loading, and structures are starting to distress and are failing rapidly.

I am obviously not an SE, but rather a mid-career general CE with PE who is employed in facilities management, but I am also structurally trained (and know better), and am seeing what appears to be a lot of UNDERDESIGNED elements.

 

[dik]

I've been involved in the design of several dozen parking structures and live load reduction shouldn't be an issue. With typical CIP concrete parking structures, a series of long beams at 15'+/- on centre pick up a concrete slab. The LL Red factor for the slab is 1, the LL Red factor for the individual beams is calculated on the trib area of the beam and the supporting columns are designed based on the total trib area the column supports... pretty straight forward.

For alternating loads, for slab design and column moment determination, have considered using the 'real' live load for a parking structure. A fully loaded parking structure if vehicles were parked side by side and in all the isles, etc. would be loaded to less than 20 psf and more likely 15 psf max. This adds assurance about any live load reduction you undertake...

Dik

 

[rapt]

Brantlet

Are these structures that are failing and being repaired prestressed flat plates or other structural systems?

If they are flat plates, a lot of the blame, in my humble opinion, can be attributed to the structural layout used in unbonded PT slabs and the design methodology being used by designers using ACI and PTI design methods for unbonded PT flat plates rather than the use of live load reduction by itself. Obviously designing for a significantly reduced live load makes the problem worse.

Regarding the point load, no it should not be reduced and the width of it's influence should be considered carefully.