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Re: steel joist web member K value

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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 then 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

------------------------------------------------------------------------

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