Diaphragm Connection With Joists 2

[abusementpark]

It is pretty common to have a diaphragm (metal deck or concrete over metal deck) supported by steel joists over structural steel framing. For the case of steel joists spanning perpendicular to collector beams along a frame line, how do you normally assume the lateral load gets from the diaphragm to the steel collector beam, since there is a 2.5" gap (joist seat depth) thru which the lateral force needs to be transferred?

 

[slickdeals]

If the joist itself is not able to transfer the force before rollover force exceeds lateral force, then you can weld collectors (such as channels on top of the steel beam) to transfer the load from the diaphragm to the steel beam.

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[slickdeals]

Check page 35 of Designing with Vulcraft for shear collectors.


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[mmillerpe]

The joist seats can transfer a certain amount of the diaphragm load, but once you reach that limit you need to provide an alternate method. I typically use 2 1/2" square hss members welded to the girder between joist seats.

 

[SteelPE]

We typically use a number around 1700 pounds which was derived from the information similar to the reference provided by Slickdeals. The book I retrieved this information from is Designing with steel joists, joist girders and steel deck written by James Fisher, which appears to be the same book as referenced above, just under a different title.

When you exceed this capacity, the standard is to provide a 2-1/2" square tube in-between the seats as also referenced my mmillerpe.

 

[abusementpark]

I typically use 2 1/2" square hss members welded to the girder between joist seats.

How do you ensure that the deck flute (which will be parallel to the HSS2.5X2.5) will align such that a puddle weld connection can be made? For example, if the center to center spacing of the flutes is 4 inches (as in the 1.0C metal deck) it seems like there is a good chance the 2.5" wide tube could end up in between two flutes and then a connection can't be made.

 

[frv]

abusement..

You're thinking of this in the orthogonal direction; for the case noted, the flutes are perpendicular to the 2.5" tube.

For the case you're thinking of, the supporting member is typically directly supporting the deck.

 

[SteelPE]

abusement

We never said it was easy. You just need to make sure there is a note on your drawing about the required puddle welds. An experienced erector should be able to follow the plans and lay the deck accordingly.

 

[mmillerpe]

As SteelPE said, you have to make sure your intent is clear, and the deck erector is aware that he may have to cut his deck to make sure the flute aligns with the tube.

 

[frv]

Oops.. looks like I was thinking about this backwards.

 

[strguy11]

You should specify the rollover force on the plans for the joist designer to account for, or provide HSS blocks as specified above.

 

[msquared48]

I use two 2" X 2" angles welded back to back and staggered to get the 2.5" height, welding the deck to the top angle, and the bottom angle to the drag collector.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[nutte]

You should specify the rollover force on the plans for the joist designer to account for, or provide HSS blocks as specified above.

Come on... Is anybody doing this? I don't recall ever seeing this suggestion, or ever seeing this rollover force on a set of drawings.

 

[OHIOMatt]

nutte, I have been doing this for about the last 8 years. It was precipitated from an RFI from one of the major joist vendors. In resolving that question I was told that a 2 1/2" joist seat has a relatively small (something like 1.5 kips) capacity to resist rollover. I then found out that 5" joist seats had virtually 0 rollover capacity.

From that point on, I reported the value, and if exceeded 1 kip placed "blocking". I would use HSS 2 1/2 x 2 1/2 as well. In the event of the case were the deck flutes ran parralell with the blocking, I would show an "optional" angle welded to the tube to widen the bearing.

 

[abusementpark]

As SteelPE said, you have to make sure your intent is clear, and the deck erector is aware that he may have to cut his deck to make sure the flute aligns with the tube.

Has anyone ever had trouble with this getting done correctly in the field?

From what I have seen in workmanship on typical deck installation, I foresee some problems.

 

[SteelPE]

You do run into problems from time to time, but if your intent is clearly shown then it is no different than any other problem that may occur in the field. If they have to make corrections to make it work then they have to make corrections.

 

[bbartlet]

We have used HSS blocking in between joists for years. We've used this on both frames and above CMU shearwalls.

In cases where the joists have a high slope, the bearing seat can sometimes require more than the typical 2 1/2" (see SJI's spec) so we've shown details with a larger tube and noted to "Shim as needed" under the bottom of the tube to match the slope.

We have had some cases where the flutes did not align with the tube below, but as SteelPE mentioned, sometimes corrections need to be made. We have started clearly noting in a detail that the deck is to be attached to the tube and specifying a wider tube if we think it's going to be an issue. We haven't had any problems since.

 

[strguy11]

Nutte -

I have been doing this for about 5 years. In fact, this was due to a conversation i had with some engineers from a joist manufacturer. There are rough guidelines on the size of this force for a 2.5" seat (around 5K), and 5" seat (Around 3 kips). When the forces are higher than this, I use blocking between the joists.

 

[strguy11]

Oops. I had a typo earlier. I meant about 2k and 0.3 kips respectively. Sorry.

 

[abusementpark]

slick deals,

Thanks for the valuable link! However, one thing I noticed on page 35 as you referenced, was that the detail given in Figure 4.2.8 indicates that the number of puddle welds required to transmit the required diaphragm force to the shear collector is per the sidelap fastening requirements.

I'm not sure if I agree with that, because you can technically get a good diaphragm capacity with no sidelap connections. In this case, would no connection to the shear collector at all be adequate?? I don't think so. I would determine the required number of puddle welds based on the pounds/foot force to be transmitted over the length of my collector beams.

Does anyone else require the number of welds to be per the sidelap fastening requirements?