Concrete anchors

[WinterDeere]

I am looking to set some drop-in achors as deep as possible in a 5" thick decorative concrete slab, for the propose of mounting the base plate of a large cantilever umbrella. The mounting plate is drilled for 3/8" hardware, which would be plenty strong, except that all of the 3/8" stainless drop-in anchors I find are only 1-9/16" long. I'm worried that any anchor set so shallow in this high-aggregate decorative concrete could tear out and leave a large divot in the concrete under high wind load.

Does anyone make a nice 4" or 5" long drop-in stainless anchor?

Yes, I know there are stud anchors that are longer, and would hold deeper into the concrete. But I don't want several studs left standing out of the concrete any time I move or remove the umbrella base.

 

[otlski]

So it seems your main concern is that your decretive concrete is not say 3000 psi but something weaker? Correct? Otherwise, the load rating would be valid and you would not have to worry.

Two alternatives perhaps. One, use epoxy anchors to spread the shear load more intimately. Two, go up two sizes in anchor bolt size and Loctite in a threaded insert.

 

[caver]

May not be what you want but I was supplied with Simpson Strong-Tie Titen HD 1/2 x 6" for the brackets on my timber frame. I'm sold on their product. I put them in with a 1/2" impact and never stripped one out.

 

[4570Man]

You could epoxy in all thread or set wedge anchors but these concrete screws are my preferred method.

 

[Dryland Farmer]

The concrete screws hold better than a wedge anchor. Keep in mind if you do use them and you screw them in and out often enough in the same hole, they will start holding less and less because it wears the sides of the hole. If you need to take them out often a threaded insert is your best option.

 

[EddieWalker]

I use galvanized wedge anchors for everything. I've never tried the screw in ones since I've had such good luck with what I'm familiar with.

https://www.lowes.com/pd/Red-Head-10-Pack-5-1-2-in-x-1-2-in-Wedge-Anchors/1000775748

One of my biggest issues with stainless steel is how week the threads are. I've stripped the threads while tightening stainless steel bolts just about every time I've used them. I'm at the point that I just throw them away and use galvanized bolts instead.

 

[WinterDeere]

yep, Dryland Farmer nailed it, the concrete screws are more a once-and-done solution. I don't think I'll be moving this thing often, but I like keeping the option open to remove it at least occasionally, if possible.

I'm leaning toward buying the usual 1-9/16" drop-in anchors for 3/8" bolts, but just drilling and setting them deeper. So, I might have to make my own longer setting tool, that's not a huge obstacle. I can drop a plain un-threaded sleeve in on top of the anchor, to prevent the bolt from wobbling in the drill hole, that could even be epoxied in place.

otlski named the problem correctly, I'm not sure what the cohesive strength of this exposed-aggregate concrete really is. There's a very high amount of aggregate, and it's smooth pebble to boot, so I'm guessing the cohesive tear-out strength is probably quite low. I think any shallow-set anchor is likely to just pull a nice 6" round divot out of the concrete surface, as it's torn out by high wind load on the umbrella. Getting the expansion sleeve set as far down into the slab as practical is my goal.

 

[WinterDeere]

I use galvanized wedge anchors for everything. I've never tried the screw in ones since I've had such good luck with what I'm familiar with.

https://www.lowes.com/pd/Red-Head-10-Pack-5-1-2-in-x-1-2-in-Wedge-Anchors/1000775748

One of my biggest issues with stainless steel is how week the threads are. I've stripped the threads while tightening stainless steel bolts just about every time I've used them. I'm at the point that I just throw them away and use galvanized bolts instead.

Your post came in as I was typing. These would work, in fact I have my shop compressor and pool pump equipment mounted using these. But they leave a stud standing out of the patio if the umbrella is ever moved or removed, and the only way to deal with that is permanent removal with a grinder. Not ideal.

As to stainless, I work with stainless hardware every day, for both work and hobby. From long experience, what you're describing is almost definitely thread galling due to heat, and not tensile failure. The UTS of 18-8 stainless is 70,000 psi, and the thread pull-out at 2 diameters typical thread engagement length is 47,500 lb. on a 3/8"-16 internal thread. You're not exeeding that, because the bolt tensile strength is only 5400 lb. on a 3/8"-16 UNC bolt.

Your issue is galling due to friction and heat while screwing it in on dry threads, made worse by load as fastener draws tight. Use antisieze, or really any form of thread lubrication, to resolve that issue. Dry galling aside, failures of threads in stainless simply cannot happen, as the bolt will always fail first (by 9:1 tensile force ratio).

 

[Dryland Farmer]

Your post came in as I was typing. These would work, in fact I have my shop compressor and pool pump equipment mounted using these. But they leave a stud standing out of the patio if the umbrella is ever moved or removed, and the only way to deal with that is permanent removal with a grinder. Not ideal.

As to stainless, I work with stainless hardware every day, for both work and hobby. From long experience, what you're describing is almost definitely thread galling due to heat, and not tensile failure. The UTS of 18-8 stainless is 70,000 psi, and the thread pull-out at 2 diameters typical thread engagement length is 47,500 lb. on a 3/8"-16 internal thread. You're not exeeding that, because the bolt tensile strength is only 5400 lb. on a 3/8"-16 UNC bolt.

Your issue is galling due to friction and heat while screwing it in on dry threads, made worse by load as fastener draws tight. Use antisieze, or really any form of thread lubrication, to resolve that issue. Dry galling aside, failures of threads in stainless simply cannot happen, as the bolt will always fail first (by 9:1 tensile force ratio).

Very interesting. I would have agreed with EddieWalker before I read your post.

 

[ponytug]

Your post came in as I was typing. These would work, in fact I have my shop compressor and pool pump equipment mounted using these. But they leave a stud standing out of the patio if the umbrella is ever moved or removed, and the only way to deal with that is permanent removal with a grinder. Not ideal.

As to stainless, I work with stainless hardware every day, for both work and hobby. From long experience, what you're describing is almost definitely thread galling due to heat, and not tensile failure. The UTS of 18-8 stainless is 70,000 psi, and the thread pull-out at 2 diameters typical thread engagement length is 47,500 lb. on a 3/8"-16 internal thread. You're not exeeding that, because the bolt tensile strength is only 5400 lb. on a 3/8"-16 UNC bolt.

Your issue is galling due to friction and heat while screwing it in on dry threads, made worse by load as fastener draws tight. Use antisieze, or really any form of thread lubrication, to resolve that issue. Dry galling aside, failures of threads in stainless simply cannot happen, as the bolt will always fail first (by 9:1 tensile force ratio).

I would have to agree with both you and @EddieWalker. While I know that stainless threads should not fail with anti-seize, and good ones don't in my experience. That said, I have experienced a rash of stainless failures that I put down to cheap stainless not being properly heat treated, or the alloy not actually being what was claimed. FWIW: I have reasonable success with fasteners from Fastenal, and McMaster, not so much from other box store vendors or online sellers, which is why I think it is a quality or specification issue.

All the best,

Peter