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Re: steel joist web member K value

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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.

On 1/19/2010 2:52 PM, Joseph R. Grill wrote:


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)]
*Sent:* Tuesday, January 19, 2010 11:48 AM
*To:* seaint(--nospam--at)
*Subject:* RE: steel joist web member K value


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)
To: seaint(--nospam--at)
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?


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)


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