maths question, need help please ..

signs · 15 Nov 2018 at 10:33

adolf hamster said:
damnit, you mean i coulda got paid for saying that?

you don't need money ! have a heart felt thanks from me

signs · 15 Nov 2018 at 10:37

Jonnybmac said:
How much is burried and how much isn't? Where on the post is the sign pinned? Is the centre 300 down from the top or at the top?

Afaik you need to work out where the load is concentrated on the post to work out the max bending moment (would be found at the base) then compare that with the failure stress of the wood in that axis. You'd also need to work out an average force from wind pressure acting on the sign. Do you really need to do this?

Hi Jonny

the sign is going to be set in a concrete base 475 mm diameter x 250mm height ( smaller pipe now but more robust ) the sign panel (600mm x 600mm) will be mounted at the top of the post (post 2.5m ) so 1.9m from floor level to bottom of sign.

so just trying to work out the "topple " factor

cheers

Jonnybmac · 15 Nov 2018 at 11:00

BM = F * L = F * 1.9 m
Stress = google or ask supplier material property
Compare two above

To aquire the bending monent force input use
F = 1/2 * air density * windspeed^2 * Area
F = 1/2 * 1.2(at sea level) * speed^2 * 0.36 m^2 (0.6 m * 0.6 m)

Assuming no wind effects on the post itself. Can be done as a UDF over the post but resulys would be negligible.

I could well be wrong as all of this is from memory of doing bending moments earlier this year

Edit just read Adolfs post. I was assuming a bent pole and not it lifting out of the ground

wolfie138 · 15 Nov 2018 at 13:18

Syla5 said:
It does entirely depend on the concrete density, but consider comparing it to a strong man Atlas stone that has been squashed in to a disc, it really isn't hard to comprehend that something that doesn't seem overly large can weigh that much.

yeah, as signs said, it's something that never twigs at you until you see a calculation, and it seems to outrageously OTT that it couldn't be true. wonder how much my drive weighs! :-D

touch · 15 Nov 2018 at 13:47

wolfie138 said:
yeah, as signs said, it's something that never twigs at you until you see a calculation, and it seems to outrageously OTT that it couldn't be true. wonder how much my drive weighs!

It's even more scary to calculate how much the concrete tiles on your house roof weigh. 4-5 tons? :eek:

dirtychinchilla · 15 Nov 2018 at 14:07

signs said:
this is about what i got it too also

Which is wrong! My answer is the correct one

adolf hamster · 15 Nov 2018 at 14:10

Jonnybmac said:
Edit just read Adolfs post. I was assuming a bent pole and not it lifting out of the ground

a bent pole is a possibility, but seeing as he didn't give us a wall thickness or specific material properties for the tube i went with the toppling angle.

there's also the possibility of it sliding along the ground or the possibility of the sign itself breaking before the pole bends or the base tips/slides.

asim18 · 15 Nov 2018 at 14:49

Is this something to do with that flag-gate thread where people were like "that's not a real pole".

Seems to me like someone wants to erect a real pole using real maths.

Jonnybmac · 15 Nov 2018 at 14:54

adolf hamster said:
a bent pole is a possibility, but seeing as he didn't give us a wall thickness or specific material properties for the tube i went with the toppling angle.

there's also the possibility of it sliding along the ground or the possibility of the sign itself breaking before the pole bends or the base tips/slides.

So maby variables

adolf hamster · 15 Nov 2018 at 15:04

Jonnybmac said:
So maby variables

yep, it's at this point i'd give up and be like "**** it she'll be grand"

signs · 15 Nov 2018 at 15:37

dirtychinchilla said:
Which is wrong! My answer is the correct one

its not mate sorry . one bag of post fix is 20k alone

TwsT · 15 Nov 2018 at 15:41

[FnG]MUGnolia said:
IS THE DRAIN PIPE ON A TREADMILL?

it would absolutely ta....

signs · 15 Nov 2018 at 15:43

update (just in case anyone else needs to do this !!!! ) i got a structural engineer report ... the concrete at the new smaller size weights around 13.5 stone and the sign "topple" is 50mph still standing at 70% of that

cheers all

dirtychinchilla · 15 Nov 2018 at 16:11

signs said:
its not mate sorry . one bag of post fix is 20k alone

Then your dimensions were wrong!

AndyT · 16 Nov 2018 at 10:09

The sign will fall over in a 50mph gust, which is a Beaufort Scale Force 9 - (strong gale) - branches break off trees, shingles blown from roofs, high crested waves etc.

It might start rolling around / shifting slighty on its edge a little bit before that, but tha'll probably be reet.

EDIT:

signs said:
update (just in case anyone else needs to do this !!!! ) i got a structural engineer report ... the concrete at the new smaller size weights around 13.5 stone and the sign "topple" is 50mph still standing at 70% of that

cheers all

Arrrgh, beaten to it. Never mind.

Syla5 · 16 Nov 2018 at 10:16

signs said:
update (just in case anyone else needs to do this !!!! ) i got a structural engineer report ... the concrete at the new smaller size weights around 13.5 stone and the sign "topple" is 50mph still standing at 70% of that

cheers all

Odd that a structural engineer provided a weight of the concrete in stones lol.

So new size would be roughly 86kg in weight. Which means your structural engineer used a weight per litre of concrete of roughly 1.94kg

Even using the original dimensions your engineer would have suggested 137kg.

Interesting to see how much weight a perceived small area can take up.

signs · 16 Nov 2018 at 13:13

AndyT said:
The sign will fall over in a 50mph gust, which is a Beaufort Scale Force 9 - (strong gale) - branches break off trees, shingles blown from roofs, high crested waves etc.

It might start rolling around / shifting slighty on its edge a little bit before that, but tha'll probably be reet.

EDIT:

Arrrgh, beaten to it. Never mind.

wow that's impressive !!! thank you , make me wish i hadn't paid for the report now lol

I've gone back to the original size of base now 600mm x 280mm (ish) deep , any idea of the "topple" factor now please (if you have time that is ?)

AndyT · 16 Nov 2018 at 13:24

signs said:
wow that's impressive !!! thank you , make me wish i hadn't paid for the report now lol

I've gone back to the original size of base now 600mm x 280mm (ish) deep , any idea of the "topple" factor now please (if you have time that is ?)

No worries. With a 600mm diameter base, 280mm height, the sign won't topple over until you have a 74mph gust. See below, things that have changed are highlighted blue.

Out of interest, how much did your engineer charge?

signs · 16 Nov 2018 at 13:32

AndyT said:
N

it was a weird conversation with the engineer Andy , not used one before so i gave him the spec then he asked me how much i wanted to pay !!!! i said "you tell me" he said £150 , i said thats more than i wanted to pay ! he said how about £100? , i said how about £50 ? he said £75 . so i went with that

AndyT · 16 Nov 2018 at 13:34

signs said:
it was a weird conversation with the engineer Andy , not used one before so i gave him the spec then he asked me how much i wanted to pay !!!! i said "you tell me" he said £150 , i said thats more than i wanted to pay ! he said how about £100? , i said how about £50 ? he said £75 . so i went with that

Haha, come to me next time and we'll see if we can sort something out!