Existing Concrete Pan Joist System - Bridging Joist Removal

[Robcat71]

Hello,

I am currently investigating the removal of a bridging joist located at the mid span of the joist. The span is roughly 15 ft. and the joists 5" X 10" spaced 25" on center. Looking at current ACI design for one way joist systems I have not encountered any bridging requirement, and the examples I have reviewed are for longer spans and much deeper joists.

I have not worked much with pan joist systems and not sure how and if their design has evolved over the years.

Are these bridging joist critical members or more to assist with load distribution?

 

[ajk1]

I recall that when I was starting out in 1964 that we used what were called "Tin pan joists" which were concrete joists formed with removable steel pans.

Eventually Joist systems were abandoned as a system because they were not as economical as other systems.(Before 1964, the joist system used in my area of Canada had clay tile forms in which the clay tiles which acted as forms stayed permanently in place, and the finished plaster ceiling was trowel directly onto the clay tiles, so there was no need for bridging in that system).

I know that does not answer your question, but if I find anything I will let you know.

 

[Robcat71]

Thank you ajk1. I did learn about the clay tile forms as well. To me it seems like the "bridging/blocking joists" may have been driven more by the old form work layout based on some of the information and photos I have found. But they do make sense for stability I guess. More recent designs/construction typically don't have the bridging or blocking joist.

I still do not have any solid information if they can be removed without effecting the joist capacities, but will continue to research. There is not much information regarding the older pan joist systems that I can find.

 

[HouseBoy]

It seems to me that if only ONE bridging joist is "removed" then there would still be a bridging joist on the opposite side of each of the affected joists.
On that basis, I would GUESS that such a move is acceptable. (Not sure of the rules on that though.)
Full disclosure - I HAVE allowed the contractor to drill a large hoe in the bridging joist in 1940's era concrete floor construction (for a large single family home)

 

[HTURKAK]

I have experience with Eurocode 2 and in order to give an idea i have copy and pasted clause 5.3.1(6) ;

(6) Ribbed or waffle slabs need not be treated as discrete elements for the purposes of analysis, provided that the flange or structural topping and transverse ribs have sufficient torsional stiffness. This may be assumed provided that:

- the rib spacing does not exceed 1500 mm
- the depth of the rib below the flange does not exceed 4 times its width.
- the depth of the flange is at least 1/10 of the clear distance between ribs or 50 mm, whichever is the greater.
- transverse ribs are provided at a clear spacing not exceeding 10 times the overall depth of the slab.

Apparently one transversal rib/ distribution rib would be necessary to comply with EC-2 in order not to treat as discrete elements..















I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[dik]

They were common back a bit... they even had special pans that provided wider joists at the ends to accommodate shear. As part of a system, I'd be reluctant to remove any 'bridging' elements, without knowing why they were placed in the original construction. I really like the 'waffle' slabs produced at the time; I find them quite attractive.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

Distribution ribs are not required by code (ACI) but are typically used in most all concrete joist construction for the intended purpose of unifying deflections from local point loads.
They are not required for strength/safety but help minimize cracking in the top slab due to differential joist deflections.

Taking one out, or cutting a large hole in it, shouldn't be a worry. I'd avoid taking multiple, and adjacent, ribs out to allow at least one distribution rib to connect to any one joist.