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RE: steel joist web member K value

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Gary,
Thanks for the replies and all the help.  All your points are valid and if I
were designing the joists, I would typically look at all these things.  I
too have designed a few trusses in my time.  There are some joists in a
portion of the building that will need to be redesigned for a "fix" of some
sort.  At that time, if I am asked to help with that, all those points will
definitely be taken in to account.  As I have said, web buckling appears to
be the limiting factor as that was what was seen throughout. And the webs
that "should have buckled" as compared to a software model are generally the
ones that did.  The basic point that I have been asked to do is run some
parallel calcs, to my clients, to the basic joist configuration as a check
for him.  Just basic member checks show the joists have very little "code
designed" snow load capacity beyond the dead load and self weight of the
truss.  Obviously, from what I was told the snow amounts were, the capacity
went far beyond a design value for allowable so the joists performed way
into safety factors.  I'm not sure beyond this how much involvement I will
have.  Thanks again you for your time.  If I am asked to help with a design
for retrofit I may have some additional questions.
Joe Grill


-----Original Message-----
From: Gary L. Hodgson and Assoc. [mailto:design(--nospam--at)hodgsoneng.ca] 
Sent: Thursday, January 21, 2010 5:54 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: steel joist web member K value

Joe,
This just raises more questions which may or may not help. The effective 
length should be taken from neutral axis of each chord.  As the weld is 
at the face of the HSS, then the weld is subject to a moment in addition 
to the axial forces because of the eccentricity from the NA of the 
chords. Were the welds sized for that moment?  Was the bar also sized to 
take account of the induced moment?  There is a requirement here to 
treat connections (to HSS) such as these as possible fixed or moment 
connections. This introduces warping or distortion into the face of the 
HSS, depending on its thickness. The other thing that bothers me is that 
if the round bars are diagonals as is likely, then it will be awkward to 
get a good all-around fillet weld, i.e.on the acute angle, you probably 
have less penetration and then on the obtuse side you will have a 
smaller effective throat.
There was a publication up here quite a few years ago of HSS 
connections.  It had several recommendations for trusses and joists 
using HSS in combinations with other shapes, but none of them were your 
joist configuration.  I hope some of this helps.
I have been designing joist on and off since 1967 and have had my share 
of growing pains.
Gary

On 1/20/2010 8:19 AM, Joseph R. Grill wrote:
> Gary,
> Thanks for the reply.  The joists have a single HSS at the top chord and a
> single HSS at the bottom chord.  The web members are solid round steel
rods.
> I understand this is not a traditional steel joist.  The round rods are
> welded to the face of the HSS chords.
> Joe
>
>
> -----Original Message-----
> From: Gary L. Hodgson and Assoc. [mailto:design(--nospam--at)hodgsoneng.ca]
> Sent: Wednesday, January 20, 2010 5:54 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: steel joist web member K value
>
> Joe,
> I hope this helps. The Canadian specification for Steel Structures
> states that "for buckling in the plane of the web, the effective length
> factor shall be taken as 0.9 if the web consists of individual members.
> For all other cases, the effective length factor shall be taken as 1.0."
> You stated that the web members are round rounds welded to the _web_
> members which are HSS members. I think you meant that the HSS members
> are the chords. The first question that comes to mind are the top and
> bottom chords double or single HSS? Traditionally, joists are made with
> double angles in the chords. If your case had double HSS in the top and
> bottom, the joist would be similar to the traditional joist. If the
> joist had a single HSS as the chords, were there double web bars, i.e.
> one bar on each side of the HSS? Or were there single web bars welded
> into the flanges of the HSS chord members? I am asking these questions
> just to get a better picture of how the welds were placed.
> Gary
>
> On 1/19/2010 2:52 PM, Joseph R. Grill wrote:
>    
>> Harold,
>>
>> Thanks for the reply. If I was doing the design, I would use K=1.0. In
>> this case it's more of why the joist failed and at what superimposed
>> snow load it failed in addition compare to what would have been a more
>> realistic "code" level design. There isn't a decking system to
>> restrain the top chord, but it appears that the web members were the
>> first to go by compression web buckling. I will note that the
>> restraint at the top chord (1/3 points) appears a bit shaky, but must
>> have worked, because without it, it would appear the joists would have
>> failed at the top chord much sooner. Also, there had to have been some
>> end condition, "fixity" at the web members, or they would have buckled
>> much sooner. Which "K" value is picked is kind of a backwards judgment
>> to kind of fit the failure at the approximate load at failure. Hope
>> that all "makes some sense"
>>
>> Joe Grill
>>
>> *From:* Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com]
>> *Sent:* Tuesday, January 19, 2010 11:48 AM
>> *To:* seaint(--nospam--at)seaint.org
>> *Subject:* RE: steel joist web member K value
>>
>> Joe,
>>
>> Historically, joist manufacturers used a simple truss model and would
>> assume a k=1 and design the bridging to provide out of plane stability
>> for the bottom chord. The top chord needs bridging for erection, but
>> once the deck is installed, it is the deck that stabilizes the chord
>> and the web members at the top. This is true even with hat section
>> chords like the old Armco / Sheffield joists.
>>
>> Figure it both ways just to determine the delta if for no other reason.
>>
>> Regards, Harold Sprague
>>
>>
>> ------------------------------------------------------------------------
>>
>> From: vveng(--nospam--at)cableone.net
>> To: seaint(--nospam--at)seaint.org
>> Subject: steel joist web member K value
>> Date: Tue, 19 Jan 2010 10:57:59 -0700
>>
>> I am helping another engineer do an investigation of some steel
>> joists. They are not from a major mfg. such as Vulcraft probably shop
>> built.
>>
>> The web members are round rods welded to the web members which are HSS
>> sections. There was a failure with the joists due to heavy snow
>> loading and the webs failed (buckled out of plane) as an analysis
>> would indicate. I will have to contact my associate, but I think in
>> most cases the welds did not fail. He did mention one in particular,
>> but I think the majority of the welds were OK and if anything that one
>> failure was probably to the weld but at the HSS wall at the weld.
>>
>> I would like to check the joist using a K=.85 at these web members. I
>> think that is more that reasonable. Does anybody think otherwise?
>>
>> Thanks,
>>
>> Joe Grill
>>
>> Joseph R. Grill, PE
>>
>> Verde Valley Engineering, PLLC
>>
>> 2220 Sky Drive
>>
>> Clarkdale, AZ 86324
>>
>> Ph. 928-600-5459
>>
>> Fax 928-649-3659
>>
>> email: VVEng(--nospam--at)cableone.net
>>
>> ------------------------------------------------------------------------
>>
>> Hotmail: Trusted email with Microsoft's powerful SPAM protection. Sign
>> up now.<http://clk.atdmt.com/GBL/go/196390706/direct/01/>
>>
>>      
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