Is this connection doable? 4

[DanAndrews]

Hi fellows,

I have this 3.8m cantilever canopy to design. I am going to use 150SHS @ 1m spacing by using the fix connection to slab. There is 60 set down.

I am proposing the following connection to do the moment connection work. The design moment is 7.5kNm per SHS. Then I am assuming the level arm 75mm which gives me 100kN T/C to design to. Do you see any problem with this design? Is there any other way you would recommend? (note that I cannot extend the SHS to the back of the slab). Thank you.

 

[dik]

I would think the force would be closer to 7.5kN-m and maybe a 10 kN shear to produce that moment. Transferring that magnitude of load to a slab edge is not difficult. The original 100kN may be a consequence of the initial connection.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Tomfh]

The 100kN refers to the tension and compression couple that the moment applies to the slab edge.

The couple has to be significantly less than 150mm apart due to the slab only being 150 thick. So it will be 100kN or thereabouts.

 

[dik]

5.6K-ft (7.5kN-m) approx x 12/4 (d distance) = 16.8K (approx 75kN) OK... I see it. still an easy force to connect for... 2 - 15M rebars (60 grade, one on each side of the weld plate at the top) looks like lots... all loads being factored.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Tomfh]

Yes you could design those anchors to carry it.

But it seems wrong to me to put the opposing 10 tonnes of force onto that concrete edge.

How wide is the bearing width? Say 250mm? And say it bears on the outermost 20mm, that gives 20MPa bearing on the concrete edge.

That is getting into the zone where concrete edges start failing.

Maybe the bearing stresses are fine.. maybe it’ll work ok. But my engineering judgement says no.

 

[dik]

You don't have that force on the slab edge... the force of 75kN has to be resisted by the reinforcing steel which is easy... 2 - 15M or 2#5 bars does the trick. It's easy to develop that force in the rebar. When I found out the moment was 7.5kN-m, I immediately thought of a 'nuisance connection', ie, not really high enough to get worried about.

I changed my mind about headed studs and rebar is the way to go... the headed stud approach could work, but would require a 'silly' cage of some sorts.

The initial connection looked wierd because there was so little depth to the moment connection and the angle provided wasn't very stiff. A solid plate backed up by concrete is a lot stiffer.

It's easy to develop 7.5 kN-m in the 6" conc slab... and if you use an 8" slab it's even easier. Unless I'm really missing something this is not a difficult connection. With a 12" wide plate (3" on ea side of the HSS), the depth of the conc compression block at the bot is a little over 1" or 1/2" to the centroid (3ksi conc)... so rebar is approx 0.33*4/3.5 = 0.40... still 2 - 15M approx (even with prying action). You have to forgive me... I'm a bit of a dinosaur. I only function in Imperial, but can design for metric (SI). The tube can easily be connected to the slab... whether it addresses deflection and serviceability is another issue, but the connection can safely be made.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[NorthCivil]

The weakest link here is the connection, right at the hinge. Thats where the deflections will originate, the canopy will rotate about.

if just held with a couple runs of rebar, the concrete surface is still prone to crushing at the top and bottom edges (see my sketch).

not sure how we can fit 2 rows of rebar practically within a 150 slab? with covers and tolerance, i would think only 1 row would be practical.

how about something like this sketch?

 

[Tomfh]

Thicken the slab. Give the connection something decent to hang on to.

 

[dik]

have to make sure you get concrete into the corner... way too congested... as noted, an 8" slab works better. There's no lacking of material to get the connection developed; don't know if def'l, etc. is OK...that rebar in the middle could be better placed.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[hokie66]

When constructing a building, it is best not to try to use furniture or piano building techniques.

 

[MIStructE_IRE]

Can you just fix the box sections to the underside of the slab and give them a nice big backspan?