[TowerTalk] Which Vertical is More Robust ?

[Jim Thomson]

Date: Wed, 1 Jan 2020 11:13:16 -0500
From: Gedas <w8bya at mchsi.com>
To: TowerTalk at contesting.com
Subject: [TowerTalk] Which Vertical is More Robust ?

##  Im  sure  aluminum  EMT  tubing is  pretty  soft  stuff. 
EMT  is  designed  to  be  easily  formed,  bent,  etc..with  conduit  benders.

##IF  you  know  the  yield  strength  of  the  proposed  EMT,
plus  the  yield  strength of  the   upper  tubing, 6061-T6 is 40  ksi,
6063-T832 is  39  ksi.   Then  just  stuff  the  proposed  design
into  DXEs  handy ...  yagi  mechanical  software.   It  will  spit  out
exactly  how  far  it  will  deflect.....at  any  given  wind speed,  and
also  what wind speed  it  will  break  at......and  also  WHERE  it  will
break.     

##  Ok,  just  checked,  AL-  EMT  is   6063-  T1.   Thats   super  soft,
with  a low yield  strength.   It  will  fold  like  an  accordion.  Dont  use  it. 

##  design the  entire proposed vert,    using   .120  wall   6061-T6
for  the  lower  sections.   Start  with  at  least  2  inch  OD  at  base,
and  build  the  bottom  sections  using    6 ft  lengths.
Like  2 inch, 1.75  inch, 1.5 inch,  1.25 inch   Allow 3-6  inches  for
overlaps,  and  use a pair  of  .25-20  SS  bolts  and  nylocks..at
right  angles  for the  splices. That  gets  you  up  to  23.25 ft .  
Use 1  inch x .058 wall x 3  ft,  then same  with  .875, 750, .5 inch. 

##  DXEs   ..yagi  mechanical  is  dirt  cheap,  easy  to use,  and  dead
on  accurate.    Dont  guess.   It  will  also spit  out  deflection  with
guys,  like  say  1  set  of  guys at 23 ft.   Use  the...  no-spec windload   option,
thats  what u   get  in  a  wind  tunnel.     U  can  also  enter  the  yield  strength
for each  individual section. 

##  section  modulus  x  yield strength  = bending  moment,  its  that  simple.

##  software  will  spit  out  the  weak  points,  which  can  easily  be  beefed  up
using  doublers, triplers  etc.   

##  Dont  mess  with  galvanized steel  EMT.

Jim  VE7RF 





<A) Starts out with a 10' length of 1-1/4" EMT followed by tapering 5' 
<sections of aluminum tubing (4' long exposed, 1' inside the previous 
<lower section) until the very top length section which will be 3/4" OD 
<dia. These will be standard DX engineering aluminum tubes.

<B) Starts out with a 10' length of 1-1/2" EMT, then a 5' length of 
<1-1/4" dia of EMT (4' long exposed, 1' inside the previous section of 
<EMT) then followed by tapering 5' sections of aluminum tubing as above. 
<The only difference being this choice would end up using 1" aluminum 
<tubing as the very top piece.

<So the obvious difference between the two are that choice A) uses just 
<one length of thinner EMT and overall thinner aluminum tubing sections 
<to the top while choice B) uses 2 lengths of larger dia EMT and then 
<larger dia aluminum tubing to the very top.

<I do not have the software to analyze choice A) vs. choice B) in a high 
<wind situation. I am uncertain if the stronger, larger dia tubing with 
<it's higher wind load will be a plus or a minus in the end to it's 
<overall strength. Any thoughts or comments on which one will survive 
<better during heavy wind conditions? TU

<Gedas, W8BYA