Stress Reversal in Steel Joist

[Stenbrook]

I am trying to determine what exactly the stress reversal is in a steel joist. When I use Vulcraft's shear diagram assistant program on their website, it gives me the stress reversal as a %. I want to be able to calculate that percentage on my own but can't find anywhere that tells me exactly how to do that.

Any help you can give me would be much appreciated!

 

[KootK]

Usually, stress reversal will refer to webs that are normally in tension that wind up being in compression under a new/special load case. I expect that the %reversal would be:

Magnitude of reversed shear at location / Magnitude of original shear at location * 100.

I haven't tinkered with Vulcraft's app though so you'll want to confirm that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jike]

Please describe the load conditions that your joist is subjected to.

If the joist has only uniform load on it, the SJI specs require that the webs to be designed for 1/4 of the max. end shear. This comes from the stress reversal in the webs that would occur due to a partial uniform load (from one end to the centerline of the joist).

You should easily be able do this by hand to check the program.

 

[Stenbrook]

I have an 805 lb RTU unit that is sitting on the joist length side perpendicular to the span of the joist. The RTU is 4.5 feet wide and has a wind pressure of 68.4 psf on the side of the unit. This results in a T/C as the support of 163 PLF. The joist is 20'-0" span (14K1) with an allowable uniform load of 284 PLF. Using 20 psf dead load and 20 psf live load in addition to the RTU weight and overturning wind load, I can come up with an "equivalent" uniform load required to achieve the reaction and moment. However, this does not take into account the shear capacity being reduced at the mid span of the joist as compared to the end. With all that being considered, I can determine the shear diagram for the loading and compare it to the "shear envelope" (which would consider the minimum 25% of the end reaction for a 14K1 joist). This does not tell me anything about the "stress reversal" that is mentioned as a criteria in the vulcraft manual. They say 5% max or use a KCS joist, but I don't know how to calc that out by hand.

KootK, I looked into what you were saying but I can't get my hand calc'd number to match that of Vulcraft's % from their in web app. :/

 

[jike]

As KootK says, stress reversal is simply the change from tension to compression in the webs. The shape of the shear diagram (above and below the axis) will tell you this.

Perhaps, another engineer in your office can help you with your hand calc. Breaking it down to different load conditions may help [dead load, live load, wind load (each direction)] you determine which combinations produce the maximum in Vulcraft's program.

 

[mike20793]

Can you just use a constant shear joist (KCS) and save yourself some headache or is this existing construction? FWIW, I always use KCS joists under RTU's since the exact location and weight of the RTU may change from the original design, assuming you are dealing with new construction. Would your percent reversal come from the difference between the equivalent uniform load shear diagram and the actual shear diagram?

 

[Stenbrook]

It is an existing condition unfortunately.From what I have been able to tell, the stress reversal (in terms of percentage) is the amount of shear at the mid span (where theoretically for a uniform load the shear would be zero) divided by the maximum end reaction (from the maximum allowed uniform load) and then multiplied by 100. How would you strengthen the joist if the stress reversal is exceeded?

 

[KootK]

SJI has a ton of documents on this that you'll want to look into. If the former tension web can handle the new compression, you're good. If not, you may need to reinforce the web to bring its slenderness down. Seriously though: google steel joist reinforcement and you'll find oodles of ideas.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Triangled]

It's my understanding that Vulcraft's stress reversal percentages are used for joist selections in new construction and is, IMO, a bit unwieldy, and not useful for reinforcement situations,
unless of course, you are in possession of the original joists designs in which such a stress reversal percentage is provided as original joist design criteria.

As I understand it, your (OP) statement at 14:08 today is almost correct except that the midspan actual shear is divided by theoretical end reaction of a joist selected for which its uniform load shear diagram completely encompasses your actual shear diagram; and it extends for a distance from midspan to the place of actual 0 shear.
Unwieldy for me anyways, and I cannot see any value in reinforcement scenarios, unless again you are in possession of original design criteria.

Agree with the rest of the above comments.

Normal web reinforcements for a 14K1 would be 2 angles per web, each welded to the vertical leg of the top and bottom chord. Design new double angle reinforcement to take total force requirement at that web.

 

[jike]

An alternate if you cannot make existing joists work by reinforcing the webs is to install new joists alongside the existing. Be sure you either field splice at the centerline or one end bearing. Be careful to match remaining camber in existing joists and provide lateral bracing for new joists at bracing lines.

 

[jike]

A simpler way to analyze is to compare the shear and moment diagrams for the original joist capacity with the shear and moment diagrams for the new load conditions.

 

[jike]

My previous comment on a simpler analysis was referring to "analysis as a beam" rather than a truss. Obviously, the local effects (of truss webs and chords) are ignored so it is imperative to take an extra conservative design approach. This approach will help you more clearly understand stress reversals, etc.