Joist Camber Along Adjacent Wall

[Bdon88]

I am confused on cambering joists or a steel beam that runs adjacent to a wall. For example, I have seen discussions on only cambering the joist 50% to prevent ponding. However, I seen another post discussing the differential deflection of a wall to a adjacent joist and since your wall wont deflect that you should limit your joist deflection to 2*(distance from joist to wall)/Limit. So, when I try to meet this differential deflection limit, I find that I need to camber the adjacent joist more then the typ. joist.

This seems to contradict the other post about only cambering the joist 50%?

 

[dik]

Can you support on the wall? I try not to run joists parallel to an adjacent wall.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[sbisteel]

The camber limit for bar joist adjacent to walls is to facilitate deck erection. Typically the deck is supported by a flat ledger angle that follows the roof line, if you have a cambered joist not far away it is very difficult to get the deck to lay properly between the 2 members.

 

[phamENG]

Not having read the other post, I'm guessing the suggestion to use 2*d/limit is to set a relative camber. In other words, make sure the camber at the center is no more than that. How far is your joist from the parallel wall, what's the span, and what is your standard camber?

 

[Bdon88]

https://www.mitek-us.com/uploadedFi...icles/design-tips/Differential Deflection.pdf

Above is the link to the differential deflection. I’m thinking this might not apply to joists since they are steel and this is about wood trusses.

 

[r13]

If the distance from the beam to wall is less than half of the interior beam spacing, then you shall proportion the cambering amount for that beam accordingly.

 

[phamENG]

Deflection is deflection - the fact that it's a different material only means the deflections will be more or less than the other with a comparable section. Some of the specific technical direction is geared more toward metal plate connected trusses, but the fundamentals are applicable across materials.

I'd say your best bet is to draw a section through the floor that cuts through a few joists and the wall at midspan of the joists. Draw the joist in the proper location based on the specified camber. Then, for the joist closest to the wall, modify the camber until you have a shape in the roof that would allow the deck to be installed without significant bending/warping (the 2*distance to wall/deflection limit equation).

 

[XR250]

Why is it standard practice to install a bar joist adjacent to the wall? Why not leave the last one 5 - 6 ft. away and let the wall support the deck? Seems like it would mitigate this issue.

 

[Bdon88]

I guess what I meant by different material was really the application of it. In the example they were attaching dry wall to the bottom chord of the truss. In my situation there will be nothing attaching to the bottom chord of the joist. So no dry wall should crack. However, my main concern is ponding on the roof. I have been looking at the total deflection (DL + LL) of my joist and trying to make that max deflection less then (the 2*distance to wall/ deflection limit). My joist span is about 60 ft and that differential deflection is requiring me to limit my deflection to about 0.6" (5 ft from wall) and with a 60 ft span it forcing me to oversize the joist an possibly try to camber out the DL deflection to meet that 0.6". Is this even something I should worry about?

Thanks for all the replies everyone.

 

[phamENG]

Ponding tends to be a more global issue. I wouldn't worry so much about the shape of a single joist, but rather the entire roof. SJI has a good technical guide for designing joists to avoid a ponding instability. Have you gone through that? AISC also has a section on ponding.

Back to your original question, though. The limits in camber for the first joist adjacent to a wall (whether it's 2" away, which is a bad idea, or 5' away) is related to deck erection as mentioned by sbisteel. At mid-span, the deck crew is going to have to bend the deck from the first joist down to the wall.

If your ponding analysis indicates that camber is needed, then you need to be careful along the side walls just to be sure they can build it.

 

[r13]

I don't know what is my word you are quoting, as there is nothing in your response related to what I was saying. In the sketch below, the bay next to the end wall is quite stiff, because of the short distance between the wall and the joist. Also, the camber for the first joist should be less than the interior joists, as it carries less load. So a proper slope can be set and maintained to allow free flow towards the wall.

 

[sbisteel]

Just want to point out that SJI camber is based on length only, the camber does not change with loading or stiffness. Without otherwise specifying something on your drawings, you will get the same camber on all 60' long joists.

 

[BAretired]

I agree with sbisteel. Camber will be consistent for all joists. Placing a joist immediately adjacent to a masonry wall is not recommended. In my opinion, it's better to maintain standard joist spacing throughout, using an adjustable steel angle anchored to the wall to support the deck.

BA

 

[r13]

Yes, you need to specify the chamfers. But note the SJI recommendation is not a mandate, you shall calculate in according to your own situation, and smooth out the transition from high to low, otherwise you might run into other issues.

 

[dik]

I would keep a uniform level for the attachment to the wall...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

Joist spacing is not to be specified arbitrarily. Quite often, the first joist has to be placed close to the wall to be able to run supports for equipment, or piping, to satisfy operation needs. Then you will need to proportion the deflections from the standard deflections charted on the supplier's catalog to determine the amount of chamber, if you wish the bottom chords end up on the same level.

 

[KootK]

For example, I have seen discussions on only cambering the joist 50% to prevent ponding.

In general, cambering any joist will reduce the likelihood of ponding within the span of that joist resulting from progressive deflection. To my knowledge, however, this is not unique to joists next to walls. In fact, joists next to walls are surely less prone to ponding than joists in the field owing to the tendency of the wall to aid in supporting the deck and the tendency of the joist to get "hung up" as a result.

However, I seen another post discussing the differential deflection of a wall to a adjacent joist and since your wall wont deflect that you should limit your joist deflection to 2*(distance from joist to wall)/Limit.

In my experience, limitations of that sort are usually about the restricting differential deflection (not camber) between a joist and an adjacent wall so that finishes are protected and so that the joists don't get overly "hung up" by the wall's participation in holding up the deck. I've seen numerous plans where the EOR specified that the first joist or two next to a wall should be designed to double the deflection limits of the joists in the field. And, sometimes, that results in a doubled up joist (go figure).