Discussion: View Thread

Adding a stiffener is preferred for several reasons.
1. They reduce the effective bending / buckling spans along the main member without increasing the overall weight much
2. Stiffeners serve as intermediate supports for the compression flange, often very desirable to provide
3. Using a heavier, bigger main member is an alternative too as it increases the moment of inertia. But the downside is excess weight and absence of support mentioned in "2" above.
4. However, stiffeners mean extra welding, either in shop or in the field. That means additional cost.

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Sriram Kalaga Ph.D., P.E., F.ASCE
Senior Structural Engineer
Glen Burnie, MD
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Original Message:
Sent: 11-09-2017 18:28
From: Akintunde Alade
Subject: Stiffener

hello,

For construction sake,  when steel member has been designed and a stiffener is required due to buckling. Which of the method is prefareable,

1) To design for the stiffener 

2) To increase the section of the member. 

       

An intermediate stiffener that is connected to the compression flange of a beam within the span of the member does not provide a brace point for lateral-torsional buckling of the beam.  To prevent lateral-torsional buckling of a beam, one needs to either restrain lateral displacement of the compression flange or restrain twisting of the entire cross section of the beam at the location of the restraint.  Both brace strength and brace stiffness needs to be addressed for restraining lateral-torsional buckling of a flexural member. 

Brace stiffness is dependent on many factors, not all of which are addressed in the AISC Specification.  For example, when cross bracing is used between girders of a bridge to restrain lateral-torsional buckling of all bridge girders, the number of girders and the spacing of the girders affect the brace stiffness.  If the girder spacing of a two-girder bridge is small, the two-girders will behave similar to a single girder with twice the cross-sectional properties of an individual girder the unbraced length will be the entire span for the girders.

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Robert Abendroth P.E., M.ASCE
EMRT ASSOC PROF IOWA STATE UNIVERSITY
Ames IA
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Original Message:
Sent: 11-15-2017 01:14
From: Sriram Kalaga
Subject: Stiffener

Adding a stiffener is preferred for several reasons.
1. They reduce the effective bending / buckling spans along the main member without increasing the overall weight much
2. Stiffeners serve as intermediate supports for the compression flange, often very desirable to provide
3. Using a heavier, bigger main member is an alternative too as it increases the moment of inertia. But the downside is excess weight and absence of support mentioned in "2" above.
4. However, stiffeners mean extra welding, either in shop or in the field. That means additional cost.

------------------------------
Sriram Kalaga Ph.D., P.E., F.ASCE
Senior Structural Engineer
Glen Burnie, MD

Original Message:
Sent: 11-09-2017 18:28
From: Akintunde Alade
Subject: Stiffener

hello,

For construction sake,  when steel member has been designed and a stiffener is required due to buckling. Which of the method is prefareable,

1) To design for the stiffener 

2) To increase the section of the member. 

       

That is a very situational question and unfortunately there is not a one and done clear cut answer.
Generally you can add the stiffeners without substantial cost increase to fabrication. The exception being, primarily, in high seismic situations where some of the fit up requirements are more severe.
The other consideration is what is framing into the stiffened member. Do the stiffeners interfere with framing angles or plates for members framing in perpendicular? If you're unsure, lay out the whole connection yourself. if you have to come up with an elaborate connection for a simple shear connection, maybe bumping the member size isn't a bad idea.
Of course, percentage of weight increase for that is a consideration. Steel is sold by the pound still :-) And, with that you have to consider how often the connection occurs.
other thoughts:
If you're going to increase the member sizes, be sure to do it before the job is bid. If the change is substantial, that is a back charge waiting to happen.
Avoid doubler plates if at all possible. I know you didn't mention them but it bears mentioning. If you need doubler plates it is almost always more economical to increase the member size.
And finally, Kudos to you for actually looking at this instead of simply sending out the drawings and requiring the fabricator to design all the connections. It still amazes me how many engineers send out construction documents and rely on someone else to design the part of the building where failure has the most severe consequences.

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Nathan Smith P.E., M.ASCE
Engineer / Salesman / Detailer / and All Around Nice Guy!
Engineered Steel Products,LLC
Randleman NC
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Original Message:
Sent: 11-09-2017 18:28
From: Akintunde Alade
Subject: Stiffener

hello,

For construction sake,  when steel member has been designed and a stiffener is required due to buckling. Which of the method is prefareable,

1) To design for the stiffener 

2) To increase the section of the member. 

       

Stiffeners prevent buckling of member elements under concentrated loads.  Do not mistake stiffeners as a substitution to prevent member buckling, see requirements for nodal or relative bracing for this consideration.

It is almost always more economical to add stiffeners instead of upsizing the member.  This is not always the case for stiffeners on bridge/plate girders, which is also a different design process.

As mentioned above, always avoid web doubler plates on columns by upsizing, it is worth it.  For moment connections to columns, consider the actual design moment, using the full capacity of the beam can result in excessive welding and oversized stiffeners.  Consider designing for the elastic capacity of the beam.

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Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI
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Original Message:
Sent: 11-09-2017 18:28
From: Akintunde Alade
Subject: Stiffener

hello,

For construction sake,  when steel member has been designed and a stiffener is required due to buckling. Which of the method is prefareable,

1) To design for the stiffener 

2) To increase the section of the member. 

       

If the stiffener is required for a one-time load on the beam or column, you can typically install the stiffener (or a few of them) cheaper than upsizing the beam or column to avoid the need.  If this is a repetitive installation, you may find it more economical to upsize the member.

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Andrew Holmes P.E., L.S., M.ASCE
Consulting Engineer
Melbourne FL
------------------------------

Original Message:
Sent: 11-09-2017 18:28
From: Akintunde Alade
Subject: Stiffener

hello,

For construction sake,  when steel member has been designed and a stiffener is required due to buckling. Which of the method is prefareable,

1) To design for the stiffener 

2) To increase the section of the member. 

       

Original Message------

If the stiffener is required for a one-time load on the beam or column, you can typically install the stiffener (or a few of them) cheaper than upsizing the beam or column to avoid the need.  If this is a repetitive installation, you may find it more economical to upsize the member.

------------------------------
Andrew Holmes P.E., L.S., M.ASCE
Consulting Engineer
Melbourne FL
------------------------------

One aspect of modeling a deep girder in the computer software, (when using transverse and longitudinal stiffeners) needs to be addressed.

Single Line element input in computer model geometry do not normally reflect any stiffener properties but only that of the web.
Won't the analysis results become approximate especially for deep girders?
Should surface elements be used instead of line elements, for modeling the girder?

Dr. M. Ali Khan

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Mohiuddin Khan M.ASCE
CHF BRIDGE ENGR
Moorestown NJ
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Original Message:
Sent: 11-23-2017 18:03
From: Andrew Holmes
Subject: Stiffener

If the stiffener is required for a one-time load on the beam or column, you can typically install the stiffener (or a few of them) cheaper than upsizing the beam or column to avoid the need.  If this is a repetitive installation, you may find it more economical to upsize the member.

------------------------------
Andrew Holmes P.E., L.S., M.ASCE
Consulting Engineer
Melbourne FL

Original Message:
Sent: 11-09-2017 18:28
From: Akintunde Alade
Subject: Stiffener

hello,

For construction sake,  when steel member has been designed and a stiffener is required due to buckling. Which of the method is prefareable,

1) To design for the stiffener 

2) To increase the section of the member.