Roof Anchor Connection In Existing Composite Slab 3

[Reno1986]

Hello,

I have to provide a design for a roof anchor on an existing composite slab roof. The building is old and the floor system consist of 4" slab with #3@14" and 10WF 25 steel beams carrying the slab along the 5.3 m span connected by four bolts connection to the existing steel columns.
The steel beams are fully encased by concrete and not visible.
My thinking is to provide a connection similar to what it will be on a steel deck new construction design.
The connection will be steel plate with four anchor M19 and clamping the bottom flange of the steel beam. After removing the cover on the bottom flange the steel plate will be placed under the bottom flange.

The design force is 22 kN applied at a hook elevated 300 mm from the top of roof slab and within any degree at the hook.

I would like to confirm the required checks for this design:
As a starting point the existing beam with the applied loads takes the 22 kN load where I applied at the end of the first third of the span upward and downward.
Check of bottom plate bearing on the bottom flange
Check of upper plate bearing concrete slab, (Would grout pad be necessary?)
Shear and Tension on bolts
Bearing pressure on bolts hole and bolts
Other than that I found that the generated moment could cause torsion on the beam below which I am planning to check but would not the concrete casing restrain that ? and how I might assess that?

Any thoughts or comments are appreciated.

https://files.engineering.com/getfi...a4af56b8af43&file=Roof_Anchor_Connections.jpg

sketch is attached
Thanks,

 

[Reno1986]

 

[BAretired]

The applied moment is 6.6 kN-m. The 4" slab, the torsional resistance of the 10WF25 and the concrete casing around the beam could each provide some resistance to rotation. The slab is reinforced with #3@14" in a single layer at unknown depth. A 10WF25 is weak and flexible in torsion. The casing around the beam will be chipped away on three faces, removing all continuity. The conclusion is that none of the above can be considered capable of resisting the applied moment.

I suggest adding a small wide flange beam between existing beams (see above).

Other than that I found that the generated moment could cause torsion on the beam below which I am planning to check but would not the concrete casing restrain that ? and how I might assess that?

You can check the torsion on the beam, but I don't think you will find it adequate. As mentioned above, the casing cannot be counted for any torsional resistance as it will be chipped on three faces. Best idea is to remove enough concrete to install a W6 welded to the 10WF25 to resist a moment of 6.6 kN-m (see above).



BA

 

[BAretired]

Upon further consideration, that detail is difficult to install. I think the following detail might be better.

BA

 

[Reno1986]

Thanks a lot BAretired for you advice. I will run the numbers and share the conclusion of the torsion capacity.

Meanwhile why you concluded that the three sides of the concrete casing should be removed. The anchor can extend below and be away from the two sides between the flanges and the bottom flange existing concrete cover will be chipped.

 

[Reno1986]

Also, in case the torsion did not work on existing beam , there are several utilities installed that will obstruct introducing a beam to resist the torsion. That being said, how about welding steel plates between the top and bottom flange to form a sort of HSS having the existing web in the center.

 

[BAretired]

Meanwhile why you concluded that the three sides of the concrete casing should be removed. The anchor can extend below and be away from the two sides between the flanges and the bottom flange existing concrete cover will be chipped.

I concluded three sides of the casing should be removed because that is the way I interpreted your drawing. The bolts could be placed each side of the casing so that only the bottom cover is removed. In that case, the casing is connected only by bond to the bottom flange. The torsional properties of the casing are not clear.

Also, in case the torsion did not work on existing beam , there are several utilities installed that will obstruct introducing a beam to resist the torsion. That being said, how about welding steel plates between the top and bottom flange to form a sort of HSS having the existing web in the center.

That would certainly work and seems to be worth pursuing but it means removing cover on each side of the beam for the full 5.3m span; however, it is a clean solution and more aesthetically pleasing.

BA

 

[spieng89]

BA I don't see how worker would be able to weld that end plate to the top flange of existing beam, but I agree with the principle. May be best to utilze the clamp metal somehow to connect W6 or angle bracing

 

[Reno1986]

Great input thank you all, but why we would need to extend the plates all along the span. My thinking is to calculate the combined stress in the flange and the web of the existing beam and to add the torsional stress at a location where shear and bending gravity stress is the least (Thinking at the end of the first third of the span). If the stress exceeded the capacity then will utilize the plates locally.

For welding into the existing steel beam I would ask for the concrete to be manually chipped to expose the steel flange and have its surface cleaned and wire brush before welding. Is there any other concerns?

 

[dhengr]

Reno1986:
Could you put BA’s W6 beam up under the bot. flg. of your W10 beam, and extend its length enough so that your clamping plate and bolts would tie both beams together. With this arrangement your welding clearance would be improved also.

 

[BAretired]

BA I don't see how worker would be able to weld that end plate to the top flange of existing beam, but I agree with the principle. May be best to utilze the clamp metal somehow to connect W6 or angle bracing.

You are correct. It is not ideal. Needs more thought.

Reno1986:
Could you put BA’s W6 beam up under the bot. flg. of your W10 beam, and extend its length enough so that your clamping plate and bolts would tie both beams together. With this arrangement your welding clearance would be improved also.

The problem is that if the beam is longer than the clear distance between flanges, it becomes impossible to raise it into place.



BA

 

[BAretired]

Probably easier to use a W10 between existing beams, bevel the flanges and get a decent weld to the existing beam flanges.

BA

 

[dhengr]

BA:
That’s why the longer W6 beam is ‘up under the bot. flg. of the W10 beam’ and then his four bolts will clamp the two beams together in a cruciform shape. Then only the bot. conc. has to be chipped away. And, the two beams could also be welded flg. to flg. Someone also needs to waterproof that roof anchor from above.

 

[BAretired]

Okay dhengr, I misunderstood your earlier suggestion. That would work but would encroach into the existing headroom.

In any of the options discussed to date, new fireproofing will be required to protect the existing beams where concrete has been removed.

It may be feasible to leave the existing concrete encasement in place both sides and bottom and provide solid blocking between beams each side of the proposed anchor? Solid blocking would prevent the existing beam from rotating and could be connected using concrete anchors without exposing the existing steel.

BA

 

[Geoff13]

Your subject line says "anchor" and your 22 kN load is the typical fall arrest anchor load, so I'm guessing that's what this is. If I've guessed wrong please ignore my posting.

Have you considered a "tip-over" anchor post like this Miller one (link). This Miller post needs a 6.5" thick slab, but perhaps other manufacturer's could work with a 4" slab like yours.

When the fall arrest occurs the post tips over, turning the load into a horizontal shear without the moment. As you can see in the brochure this post can be attached to something as light as a 24-gauge metal sheathing roof. For a concrete slab it's just 4 expansion bolts.

 

[BAretired]

The sketch above illustrates the solid blocking concept. It could be a steel WF, HSS or pneumatically placed concrete. Each end would be anchored into existing concrete without removing the fire protection of the existing W10x25 beams.

BA

 

[Reno1986]

Reno1986:
Could you put BA’s W6 beam up under the bot. flg. of your W10 beam, and extend its length enough so that your clamping plate and bolts would tie both beams together. With this arrangement your welding clearance would be improved also.
It is not possible at more than one location but if localized it might be possible. But I don't follow, if I used your arrangement with W6 underneath the bottom flange why I would need to weld referred in "your welding clearance would be improved also."

Geoff13
Indeed it is a fall arrest anchor. Thanks for the input and the interesting product but the loads seems less than the standards require, the following were listed in the pdf :
Performance Activation Force: 1000 lbs. (4.4 kN) Maximum Capacity: 310 lbs. (140.6 kg)
Also as for the concrete slab minimum 6" depth in required where we have 4" only. Is this product used in the states?

BAretired
Thanks for the input, I am calculating that we will have about 70 mm depth at best between the flanges at each side, not sure if that will be good for the blocking anchors.

 

[BAretired]

Indeed it is a fall arrest anchor.

That's good; it means the supporting hardware does not need to be fire rated.
Can the 300 mm height to the ring be reduced? That would reduce the applied moment.

I am calculating that we will have about 70 mm depth at best between the flanges at each side, not sure if that will be good for the blocking anchors.

"between the flanges"? Do you mean 70mm concrete cover over the flanges? Anchors would likely be installed between the top and bottom flange, so there should be a depth of about 150mm to install anchors.
If glulam blocking is used, anchors may not be necessary as the blocking is held in place by the bottom plate (see below).

BA

 

[Geoff13]

Indeed it is a fall arrest anchor. Thanks for the input and the interesting product but the loads seems less than the standards require, the following were listed in the pdf :
Performance Activation Force: 1000 lbs. (4.4 kN) Maximum Capacity: 310 lbs. (140.6 kg)
Also as for the concrete slab minimum 6" depth in required where we have 4" only. Is this product used in the states?

This tip-over post is an interesting device. First it tips over. As the tension load builds there is an internal "device" that tears and stretches to provide some additional energy absorption (over and above what the worker's lanyard has). The reference to 1,000 lb is the load this "device" starts stretching, not it's ultimate strength. A strength that low would be illegal as a fall arrest anchor anywhere in the world.

The Maximum Capacity listed in the brochure is the maximum worker weight. Many standard pieces of fall arrest gear are limited to 310 lb workers. Some companies have "heavy" gear for workers up to 400 lb, but they could not be anchored to this post. There's a long story behind how 310 lb got to be the standard weight, but this posting isn't the place for that much history.

If you read through all the Miller paperwork (not just the sales brochure I linked) you will find that the Ultimate Capacity is 5,000 lb (22 kN). I believe every fall arrest anchor you can buy in the U.S. or Canada would be rated the same.

Like many of the big fall arrest equipment manuacturer's, their stuff is easily purchased throughout the U.S. or Canada. I think most of them are U.S. companies.

Since the post base can be attached to 24-gauge metal sheathing, or 5/8" plywood, I expect the minimum slab thickness is somehow limited by the expansion anchors they provide rather than the 5,000 lb shear load. Perhaps another tip-over post manufacturer uses a diffent concrete anchor that works with your thin slab?