The Shed Pic Thread (losh's request) - please read the rules.

[chrisp]

But whatever fastener is used, isn‘t the strength of the concrete itself the weakest link?

It’ll either be the concrete or how well the fastener is fitted to the concrete. I’ve seen quite a few expansion sleeve anchors (aka dynabolts) that are loose or come up pretty easily. The fastener itself doesn’t usually fail, but rather it was never fitted right or the concrete was weak.

The thing about concrete is it’s really strong for compression loads (when the forces are squeezing to together) but it’s quite weak for tensile loads (when it’s being pulled apart). Its tensile strength is only about 10% of its compressive strength (so 3.2 MPa using your example). Hence, steel reinforcements are used to overcome the tensile weakness.

The point that is the weak link in my view is the tensile forces developed on an unevenly loaded two-post hoist. The vertical upright and the support pad essentially form a lever a bit like a claw hammer being used to pull a nail. The front-back imbalance is amplified by this leverage so 100s of kg imbalance on the hoist might turn into several tonnes at the fasteners and this force will be compressive on one side (no problem for concrete) but tensile on the other side (which will soon pull out and poorly affixed fasteners). It’s quite scary to think that the fastener is hanging in purely by the huge expansive forces asserted on the walls of a small hole in the concrete.

I like those two-posters that have a elongated pad to reduce the tensile forces. I see that some manufacturers offer an extended base as an option. I reckon that would be a pretty good investment in the integrity of the safety of a two-post hoist. Or, consider a 4-post design where the tensile forces will be considerably lessened by the nature of the design.

 

[Immortality]

Ultimately yes the concrete that the anchor goes into plays a huge role but I've seen to many dyna-bolt type anchors pull out and I've seen concrete fail.

When a press went in they would poor a new concrete slab, the big presses had somewhere in the area of 4ft+ of concrete beneath them with a **** ton of re-bar.

Ultimately, something will always fail when pushed hard enough.

 

[VS 5.0]

Exhibit A

 

[chrisp]

Exhibit A

I’ll just let the customer know that their ute running a little behind schedule!

 

[losh1971]

Have to be fair though, that hoist barely looks good enough to lift a Might Boy and clearly looks like the bolts are way too short.

 

[Skylarking]

I've seen to many dyna-bolt type anchors pull out and I've seen concrete fail.

Many non trades people use wrong drill size or poor drilling techniques and their expansion type anchors wont fit nicely and won’t go really tight… yet some just think “meh it’ll be right”... some tradies are not so removed from this mindset but they usually get pulled into line by the bosses who have to guarantee their work.

And on the home front, how many bother to blow out the crap within the hole they’ve drilled before pushing in an anchor (be it a chemical, expansion or screw type). Not all ensure a clean hole before shoving it in

Same issues of poor technique can impact chemical anchor systems.

https://www.gooduse.com.tw/en/news/news-020.html

Always worth following the manufacturers processes religiously when lives can hang in the balance because of a fastener failure.

 

[Skylarking]

I’ll just let the customer know that their ute running a little behind schedule!

Or it’s a drop mic moment and it’s stage right…

 

[Immortality]

Have to be fair though, that hoist barely looks good enough to lift a Might Boy and clearly looks like the bolts are way too short.

It's hard to tell as you can't see very well under the top part of the concrete that has lifted but it looks like the hole goes deeper under the center where the bolts would have been. That concrete doesn't look very good.

 

[VS 5.0]

Not all ensure a clean hole before shoving it in

 

[Skylarking]

Purposely phrased that way to add some humour and see if anyone actually read my post