Existing Steel Joists - Automatic Add Load applied?

[Eng456123]

Is there anything in the SJI standard that requires all joists to be designed for a ___# add load, or does it have to be designated in the structural drawings for that to happen?

Looking at adding a new exhaust fan to an existing roof (designed in 1983, H series joists) and am seeing that we will need joist reinforcing unless we can say the joists were designed for some sort of add load.

Thanks.

 

[WesternJeb]

Exhaust fans are usually very lightweight, like less than 100#.. How large is this sucker?

 

[CDLD]

There may be an allowance for the collateral loads included in the uniform dead load.
Whatever is leftover from the dead weight of the roof can be assumed as an allowance for collateral loads.
If say 15 psf of the total dead load is an allowance for collateral loads, you can multiply 15 psf by the tributary area of one panel point, and you may get the capacity you need without reinforcement.

 

[Eng456123]

2'x2'x10'tall - 444# ; Greenheck VK-H-10-5

 

[JAE]

SJI joists simply have a maximum capacity listed in the load tables (assuming you are talking about standard designated joists).

There's no "collateral" load or such secretly included beyond what the capacity is for a given span.

The tables generally give you the total load capacity along with a live load limit based on a set deflection (usually L/360 for the live load only condition).

If you are dealing with standard "H" joists, you'd find the appropriate load table for that time period of 1983.
Based on the span - there will be a TL and LL capacity provided.

You would then take your dead load, live load, collateral load - as applied actually to the joist - and check that against the max. TL capacity.

 

[Eng456123]

@CDLD - Not sure I follow, but I am not dealing with the existing design loads. I know the capacity of the joist from the historical load tables so that is what I am comparing to.

 

[Eng456123]

@JAE - ahh I see. Original dead load was given as 30psf for bare metal deck roof. I was using this design load, in addition to the RTU point load to check against the V&M diagrams determined using the TL capacity from the tables. Seems like if we could confirm there is not 30psf up there, but more like 20psf, we could be within the envelope and be in the clear...

 

[CDLD]

I see. I was referring to an allowance for collateral loads on the design drawings.
Original designer may have bumped up the dead load to account for collateral loads.

 

[JAE]

Seems like if we could confirm there is not 30psf up there, but more like 20psf, we could be within the envelope and be in the clear...

Yes - that's the issue.

You need to do an inventory of the existing dead loads there now and tally that up with the currently required roof live load (or snow load) and your new RTU.

One thing to note is that the original design snow loading may have been higher than what is now required by the code.
In my area the historical snow load for many years was 30 psf (up until perhaps 1990-2000).
Today it can be as low as 22 psf...gives you some "additional" capacity that isn't required now.

So if you verify the actual dead loads, take the 30 psf snow load and see where your existing joists land with respect to that. In other words, try to mimic what the original engineer did in arriving at that particular joist size. This essentially calibrates you to the original design.

Then take your current required snow load, your added RTU loading and see how it does.

Watch out for snow drifting if you are near a step in the roof elevation - or if your RTU is longer than 15 feet in length where you need to include drifting around the unit as well (see ASCE 7).

 

[Eng456123]

@JAE - got it thanks, I'll see what we can do to verify. Luckily we are in a desert with no snow Sounds like it would make this task a little more difficult.

Was already able to back-engineer the joists as well; (30psfDL+20psfLL)(6.6')+11.4plf(SW) = 342plf - Original 16H8 TL = 359plf

One thing I just discovered while doing this exercise was that in the old catalogs, they list Resist. Moment and Max. End React. If you do wl^2/8 with the total uniform load capacity, that value is about 5% lower than the values given for Resisting Moment. Brings my original TL capacity from 359plf to 378plf.

 

[JAE]

I believe the end reactions were the maximum value based more on the seat not crushing (vs. end shear due to a uniform load). Stay with the lower value to be safe.

 

[lexpatrie]

Strengthening existing open web joists - Fisher, Eng. Jour, 4Q2005

This is one of those areas where the standard of care floats wildly, some engineers look up the history and try to figure out an original design and check it for any revised loads, some slap in some cross bracing to share the load between two-three joists and don't (I suppose) analyze, still others will mark it up for field-spliced joists and pretty much skip any design calculations (a KCS joist maybe, to avoid any shear issues), and some won't bother with any analysis at all or any reinforcement, and then some will give four pages of supplemental welding.

Best case scenario, you can either ID the joists in the field, or make a viable surmise based on the span, depth, spacing, and existing construction and/or plans. from there a shear strength envelope (H series has 50% minimum at the center, K has 25%), and a moment envelope can be drawn for new and existing. Back when AOL was a bigger deal, I think NJ Bouras had an online joist calculator that made pretty pictures (Vulcraft had an MS-DOS program I've never quite gotten the hang of)...

Anyway then you can devise reinforcing/bypasses, etc, if needed.

The risk with identifying in the field based solely on span, depth, and spacing, is that it's a proprietary Non-SJI joist and you come up with a pretty unestablished set of design values that you then reinforce based on. I've only had one project so far that I couldn't ID the joist as an SJI product, and that's where I went with the spliced joists. I was dealing with HVAC units, a heavy hung hood and a MUA (it's for a restaurant going into the space), and the joists were locally produced proprietary, while I "found" section sizes for the joist layout, the problem is the capacity of the connections/welds. How beyond building a full analytical model, oh, and the snow load increased.

Documentation varies so wildly from project to project.

I don't recommend this article, much, as there is so little connective tissue for a lot of these assertions, but it does give you an idea of how someone else approaches the issue:

https://www.edtengineers.com/blog-post/roof-collapse

Charles (or I guess Mr. Whitley) never responded to any of my questions so enjoy the typos and various invalid specs that were "violated". It took me almost a week to find that 600,000 limit, and that's for Steel, steel joists aren't in the specification for steel construction by AISC and that limit doesn't strictly apply.

Regards,
Brian