Are roof rod braces required? 1

[Devstruct]

Hi Everyone,
I wanted to get some additional opinions on my situation.
I am inspecting x-bracing in an 1930s warehouse. Roof bays are 20 ft wide, supported by trusses spanning 100'. The roof bays alternate from a 'high hat' at the top of the trusses to a low roof at the bottom.
It the high hats, many of these braces have been bent or cut entirely over the years. See below for an example bent around a fire pipe.
My thought is that these braces are the roof diaphragm, and are thus needed for the lateral system. However, another engineer in our group seems to think these were only used for construction purposes during the erection of the truss, and once the decking is on are no longer needed.

Do you think these braces are required?

 

[KootK]

1) My first guess would have been the same as yours: the braces may have been intended to function as the diaprhagm.

2) I do agree with your colleague that, if the decking product and it's connections justify a diaphragm capacity, that capacity can be mobilized. It may well be the stiffer load path relative to the rod bracing given the realities of connection slop etc.

3) I've not heard of rod bracing like this being used for temporary erection stability. That said, I'm no expert in that department. Hopefully one of our industrial / PEMB specialists can chime in on that.

 

[DayRooster]

I have worked on similar structures from the same era. These are tricking questions since they are not straight forward. In my opinion, both you or your colleague could be correct. Best way to know would be a review of the existing drawings (if they are available). As an example, one of the structures I worked on had rod bracing for the roof lateral force system in some areas of the facility with a "bearing only" concrete plank roofing system; while in other areas the roofing had been replaced with metal deck (where the structure had been modified). It should be noted that areas with new metal deck had proper fastened for diaphragm action (not simply for uplift loading). Another question too, is this deck the original deck or was it replaced at any point? If it is original then keep in mind that the construction practices of the 1930s were much different than modern structural steel framing.

https://www.aisc.org/publications/historic-shape-references/#28368

 

[r13]

I suggest to go over SDI roof deck design guide to understand your roof deck, and see whether the rod bracing is required or not. I don't want to say one way or the other, but I've never designed the rod bracing ever, but specify for practical concerns to square and hold roof beams in place.

 

[Devstruct]

Thanks for the input!

Dayrooster, the decking appears to be original. in a couple locations it has been replaced with modern B deck. There are no screws through the beam flanges, so I am assuming it has been puddle welded. The issue that remains is there isn't a way to confirm the spacing without tearing the roofing off.

R13, I'll have to do some googling to try and find a historic decking guide. If you happen to have a reference I'd love to take a look?

Another suggestion I was given this morning was to just model the whole thing and remove the braces. These old buildings tend to have a lot of redundancy, so I may be able to get it to work without them.

 

[DayRooster]

I was unaware that high-hat metal deck shapes where readily available back in the 1930s. Keep in mind that the Steel Deck Institute was not founded until 1939. You may be able to contact SDI about it and they probably have an expert on staff that could provide more insight.

In addition welding was not overly popular at this time period. Most connections I saw on these old structures were rivets style with minimal welding (usually only shop welding). If the deck is original then they could have puddle welded the deck down but that would be a new one for me too (in this time period). But then again I was not alive and engineering back in the 1930s

In the 1940s I saw much more bolting and welding being performed in lieu of rivets (even though I still saw rivets being used still). (Edit: when I say I "saw", this is referring to review of historical design drawings. Again I not over 100 years old )

https://www.aisc.org/globalassets/modern-steel/archives/2002/09/2002v09_steel_deck.pdf

https://www.aisc.org/globalassets/a...-data-pertaining-to-structural-steel-1934.pdf

(Page 425 of the book and 436 of the PDF)

From the structures I worked on, I noticed that the rod bracing was critical. Personally, I noticed that in my programs if I did not stabilize the structure at the rod bracing points then it would collapse when I ran stability cases. The structure I was working on was some big steel but all for gravity loading. Without sending the load to outside brace bays it wasn't cutting it. That being said, I would interested to see how yours turns out.

Finally, I would recommend brushing up on 1930s design and zeroing in on the exact year in 1930s. Quite a bit was changing back then with regards to material strengths, rivet strength (hand driven/power driven) and weld strength. Also, the AISC standard at the time was pretty short and most of the true design information was region based on steel manuals provided by the steel companies. Just a heads up if you are not already looking at this type of stuff.

 

[r13]

DayRooster said well. But I suspect the deck has been replaced, also the roof beams looked modern. I don't have any information on historical roof decks, but here is the diaphragm design manual from SDI that may assist you in the evaluation. Link

 

[DayRooster]

IMHO, the roof beams do look 1930s era. The end connections appear to be riveted which would led me to think the steel is original but has been re-coated over the years. I agree that the roofing appears to be more modern than 1930s (if it is metal deck). One more thing to add to the discussion: Once I was looking at 1930s era roof section from a similar vantage point and initially thought it was painted ribbed metal deck only to find out later it was a painted precast concrete bearing only channel slab (back-to-back channel flanges). Just something to consider for what it is worth. I am assuming you were able to look at it with more clarity than me. But if it is the original roof then it might be worth a double check...also for what it is worth, here is another helpful guide to a variety of interesting framing technologies from the past:

https://www.structuremag.org/wp-content/uploads/2015/02/StuartDictionary1.pdf

 

[bones206]

Could you elaborate on the roof system a little more? I'm having trouble picturing the alternating high hats. It's not a sawtooth roof system is it?

 

[r13]

Yeah, the connection seems riveted, but the beam shape does not like that produced in the 1930's.

 

[DayRooster]

R13 - CB sections were also common in the 1930s. Check them out in the historic steel manuals I linked previously...

 

[r13]

Sketch below is taken from the last page on your reference. A CB beam is shown after. And your are correct, as there is no taper on the flanges.

 

[lexeng18]

Just jumping in here to expand a little on DayRooster's post-- keep in mind that while steel decking use back be dated back to the 30s, it wasn't used as a diaphragm until the mid 60s.

Between the 30s and 60s steel decking was just a vertical load transferring element to span between secondary roof members and as an underlayment for roofing materials.