Bar Grating on Concrete Beams Detail 1

[bookowski]

See attached sketch. This is rooftop, within a screen but otherwise exterior. A series of flying beams forms a screen and various intermediate levels and at one of these levels there will be bar grating supporting mechanical equipment. A couple of beams will be steel to allow future removal/access but otherwise all concrete.

Any suggestions for a detail for the grating to concrete beam? Seems like it will require an embed of some type to which angles/clips can be welded?

When using embeds at exposed conditions what precautions are taken to limit water ingress around the plate into the beam?

(I'm stuck with the concrete beams - so going all steel isn't an option)

 

[SlideRuleEra]

Bookowski - Suggest that hot-dip galvanized angles be anchored to one side of each concrete beam using either galvanized embedded bolts or stainless steel epoxy anchors. If possible, locate the angle on the side of the concrete beam that will not interfere with the planned future access (see below). The bar grating can now be anchored to all beams, both steel and concrete, using the same grating G-clips (shown below). When the grating is removed the entire acess area is now clear, even the fasteners are gone.

Having small embedded anchors or studs that protrude from the beams is not a good idea. They are easily damaged, can be a safety concern, and often require the grating be precisely and carefully placed before it can be tied down. We started using the G-clip fasters at electric generating stations in the early 1990's - they have proven to be quite satisfactory.

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[msquared48]

Agree with SlideRule for the most part. Those G-clip fasteners look, well, wicked... I would opt for J-bolts to the leg of the angles if possible.

Mike McCann, PE, SE (WA)

 

[bookowski]

Thanks SlideRule. I was thinking of something along those lines but I started convincing myself that a post installed anchor into the side of a beam may lead to durability issues long term by creating an moisture entry point into the beam.

 

[jdcollins]

bookowski - at a previous job I worked on some hospitals in Iowa that commonly had these roof gratings for mechanical chases and what not. The detail was usually a small embed plate installed at around 4'-0" o.c. or so with the grating support angle then welded to the embed. That way the angle didn't have to be attached with any post-installed anchors, possibly cracking the concrete.

 

[hokie66]

I like SRE's approach. As long as your drilled anchors are below the beam top bars, there should not be an issue with concrete distress around the bolts. The alkalinity of the concrete will protect the bolts, in addition to the galvanizing or the stainless steel.

 

[SlideRuleEra]

Hokie bring up a good point, that I forgot to mention: Unequal leg angles are preferred, the "short" outstanding leg is all that a G-clip needs. The "long" leg makes it easier to get the embedded anchor bolts or epoxy anchors inside the rebar cage.

Here is some background info that may be of interest:

Embedded plates, anchored with Nelson studs, are used for all sorts of purposes at electric generating stations, including bar grating support. In our experience, as time goes by (measured in years) a disturbing percentage of the embedded plates come loose - then you have a "perfect storm" of trouble. Often the reason they came loose was honeycomb concrete directly below the embedded plate. No matter how diligent the Contractor, getting the concrete properly consolidated under the plate is an elusive goal - and you don't know there is a problem, the plate blocks view of the honeycomb. At this point, the (loose) Nelson studs make things worse - they insure that concrete BELOW the shallow honeycomb is disintegrating, too. Now, removing the compromised embedded plate, with Nelson studs attached, causes more damage. Finally, you have to try to come up with a way to replace the embedded plate, and all you have for support is concrete that has been more or less pulverized.

We found it best to avoid using embedded plates when possible - easily done with many bar grating supports.

Where you do have to use embedded plates, we doubled the number of Nelson studs and put numerous small holes in the plates to help get trapped air out during concrete placement.

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