Double Angle Compression Members

[ron9876]

In AISC 9th Edition the discussion for double angle compression members on page 3-53 seems to indicate that the tables are based on adding a minimum of two connections between members.

In a condition where truss diagonals and verticals are constructed with double angle members with no connections between angles it appears these tables do not apply even for buckling about the X-X axis. Am I reading this correctly? Thanks.

 

[structSU10]

I think that is correct; the intermediate connectors modified the effective length factor. The tables use the adjusted effective length, so would not be applicable. I would think you could use the table for single eccentrically loaded angles, and multiply by 2.

 

[humanengr]

Are the truss double angle members designed based on the strength of a double angle shape?
If so, they must be connected. Otherwise, as mentioned above, all you have are two single angles.
Why are you using AISC 9th edition? Is the 13th or 14th in effect for your project?

 

[ron9876]

I am reviewing an existing condition. It seems the same criteria applies for the 13th Edition tables also but it has more information about using angles for this condition.

 

[BAretired]

I don't have the AISC document, but I agree with humanengr.

BA

 

[amec2004]

We call this double angle connecting plate as "batten plate". Normally it 4’ c/c distance and it’s required for built-up section.
When design the double angle compression capacity, modified slenderness ratio is used as per AISC 360-05 E6-1 to account for interconnection bolt at spacing = 4’ = 1.2m

anchor bolt design per ACI 318-11 crane beam design

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[ron9876]

Thanks for the input. amec2004 is it your understanding that the AISC tables are based on two connections even if the length of the member is say 5'?

 

[amec2004]

>>Thanks for the input. amec2004 is it your understanding that the AISC tables are based on two connections
>>even if the length of the member is say 5'

I normally don't use table and use spreadsheet to make my own table and chart so that everything is under my control. Anyhow you can verify the table with your own calc.

For BOLTED double angle member under compression or tension there is an eccentricity between bolt group center line and built up section neutral axis line. That creates a eccentric moment and bring down the member's resistance a lot up to 60% less. Someone may think that applies to hor. brace only similar to WT section, but actually for vertical double angle brace, it has larger eccentricity than hor. brace in many double angle sections. Please check it. None of the table consider the connection eccentricity case and only give you the resistance of pure compression case, which is not the actual condition as eccentricity exists in the BOLTED double angle brace connection.

Back to the batten plate issue, normally the steel fabricator default to use 4’ as the batten/connecting bolt spacing in the shop drawing. For design engineer we put max 4’ batten/connecting bolt spacing in the steel general notes. That’s the conventional way throughout US and Canada.

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[nutte]

Ron, for short double angle members, the tables make no distinction. There was a question about this in the Steel Interchange section of an issue of Modern Steel Construction in the last two or three years. AISC's answer was along the lines of "well, for really short members, I guess you could check them as two single angles."

 

[ron9876]

nutte that is what I did. I have an existing condition that I am checking and I am trying to determine why truss diagonals don't work. It looks like the original design may have missed the part about the tables assuming a minimum of two coonectors. Just wanted to be sure I was reading it right. Thanks.

 

[DaveAtkins]

amec2004,

I believe unless fatigue is a consideration, the eccentricity you mention can be ignored (AISC J1.7).

DaveAtkins

 

[amec2004]

Eccentricity in WT & Double Angle Connection
1. In AISC LRFD 1999 Section J1.8 the eccentricity is allowed to be neglected.
2. In AISC LRFD 2005 AND 2010 the LRFD1999 Section J1.8 “… negligible effect on the static strength of such members” has been taken out
3. In AISC LRFD 2005 AND 2010, the wording doesn’t NOT mean the eccentricity can be neglected in terms of static member strength

Attached AISC EJ went through the actual load test and draw the following conclusions
1. In AISC LRFD 1999 Section J1.8 the eccentricity is allowed to be neglected.  That’s not true compared to real load test
2. For WT and Double Angle section with eccentricity > 1.5” = 38mm, axial compression/tension + compression interaction design is required.

Please also check AISC EJ 2002 Q3 Influence of Bolt-Line Eccentricity on WT Tension Member Capacity

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[amec2004]

Re-post as the attached pdf file link doesn't work

Eccentricity in WT & Double Angle Connection
1. In AISC LRFD 1999 Section J1.8 the eccentricity is allowed to be neglected.
2. In AISC LRFD 2005 AND 2010 the LRFD1999 Section J1.8 “… negligible effect on the static strength of such members” has been taken out
3. In AISC LRFD 2005 AND 2010, the wording doesn’t NOT mean the eccentricity can be neglected in terms of static member strength

Attached AISC EJ went through the actual load test and draw the following conclusions
1. In AISC LRFD 1999 Section J1.8 the eccentricity is allowed to be neglected.  That’s not true compared to real load test
2. For WT and Double Angle section with eccentricity > 1.5” = 38mm, axial compression/tension + compression interaction design is required.

Please also check AISC EJ 2002 Q3 Influence of Bolt-Line Eccentricity on WT Tension Member Capacity

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[ron9876]

AISC 13th Edition section E5 allows the neglect of the eccentricity with certain conditions. That is what started my questioning since I calculated capacity of two single angles and it was less than the values in Table 4-8 for double angles.

 

[amec2004]

DaveAtkins , it seems you are correct.

Thanks



anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[DaveAtkins]

I am looking at the Fourteenth Edition of the AISC Manual, and Section J1.7 says the eccentricity can be ignored for statically loaded members.

DaveAtkins

 

[amec2004]

>>I am looking at the Fourteenth Edition of the AISC Manual, and Section J1.7 says the eccentricity can be ignored for statically loaded members.

You are right.

How about WT section, any thought ?

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[nutte]

Amec, I'll agree that a careful reading of AISC 2005 section J1.7 doesn't explicitly state that this eccentricity can be neglected for statically loaded members. But it should. It's poorly written.

But don't take my word for it. The Commentary for this section does state explicitly that this eccentricity has long been ignored and has a negligible effect on the static strength of such members.

 

[humanengr]

OK, really getting to the nitty gritty of J1.7 Commentary:
A view in the plane of the member per Fig. C-J1.3 shows welds balanced about the neutral axis of the angle.
If you cut a section through the plane of the member, there is still an eccentricity between the longitudinal axis
of the member and the welds. The question; should this latter eccentricity be considered in the member design?

 

[BAretired]

Probably should. What would be the argument for not considering it?

BA