In the early 1970’s J.P. Colaco Ph.D. developed the steel stub girder system. This innovative structural system aimed at providing construction savings through the integration of mechanical, electrical and plumbing service ducts into the part of the building volume that is only occupied by the floor framing system. It was noted that overall depth of floor systems could be large, leading to significant increases in the overall height of buildings, the steel tonnage for the project and the associated cost of façade sys-tems with greater areas. The stub girder system made extensive use of fairly simple shop fabrication techniques and the composite action of the concrete slab and steel load carrying capacity members. This composite floor system had significantly more strength, stiffness and ductility, while the amount of structural steel was reduced. The stub girder system was widely used for a variety of steel-framed buildings in North America (Canada, U.S.A. and Mexico) ranging from 2-70 stories for about two decades. However, due to the relative complexity of the analysis required, labor intensive construction nature and transformations suffered in the international steel market, the use of the system has become non existent.

1. STRUCTURAL
UPGRADE
OF STUB GIRDERS
Kevin Vazquez1
Speaker: Kevin Vazquez
1 Meyer Borgman Johnson, Inc.
Minneapolis, MN
U.S.A.
kvazquez@mbjeng.com
Institution
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2. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Outline
• Introduction
• Motivation
• Methodology
• Analysis
• Strengthening
• Conclusions
One Allen Center Houston, Texas
(American Institute of Steel Construction, 1972)

3. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Introduction
- Developed in the late 1960’s by J. P Colaco. Ph.D.
- Aimed at providing construction savings by integrating building systems into
part of the building volume typically occupied by the structure.
- The system relies in the compressive capabilities of the composite slab atop
and the tensile strength of the steel beam at the bottom. The structural
behavior of the system resembles the one of a Vierendeel truss.
- The vertical openings/windows of the girder allowed to run floor beams or
purlins continuously, thus taking advantage of the continuous beam
behavior.
- Relative complexity of structural analysis, labor intensive nature, difficulties
to inspect & retrofit have made the system become obsolete and seldomly
used.

4. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Introduction
Full elevation of Stub Girder system (Bjorhovde R., 1999)

5. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Introduction
Components of a Steel Stub Girder system
(David C. Stringer, 1982)

6. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Introduction
Cut-away schematic view of the Stub Girder system (David C. Stringer, 1982)

7. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Motivation
This built-up system was widely used for about two decades in North America
(Canada, USA and Mexico) to build steel-framed buildings ranging from 2 to 70
stories in height.
Constructions of this vintage and built with this structural solution are due for
renovations and adaptive reuse. Structural engineers of today are required to
work in projects where the stub girder system is present.
There is very limited practical information about the steel stub girder system and
neither design guidelines nor building codes or engineering books include
information about it.
The concept of a historical structure is relative to the community being served.
Methodologies and approaches used in the study of historical constructions are
also applicable to the study of more contemporary structures.

8. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Methodology – Case study key aspects
- 5 story building built in 1985 in Minnesota, USA
- Client and Architect based on the west coast of the USA
- 3rd floor of an office building renovation including relocation of
sizable data room/center (servers, modems, etc.)
- Live load capacity anticipated increase of 3.3 times (from 2.4 to 8
kN/m2)
- Minimal disruption to lower floors required
- Tight schedule to develop the design phase of the project
(4 months)

9. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Methodology – Case study
• Project meetings to define scope and client’s expectations
• Historical research (original construction documents, building codes, papers
describing the stub girder system)
• Site observation & condition assessment
• Testing program
• Structural analysis
• Design of strengthening
• Constructability related issues
• Construction administration

10. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Case study – Original plans

11. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Elevation survey and concrete core locations

12. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Crack mapping

13. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
GPR Concrete coring
Connections survey Welds survey

14. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Structural analysis
- Approximate hand calculations
- Stick model based on recommendations by Dr. Colaco with the aid
of the commercial SE software ETABS

15. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Analysis – live loads
Original
New

16. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Analysis – internal forces and stresses
Axial
Bending moment
Shear

17. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Traditional strengthening

18. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Traditional strengthening

19. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Traditional strengthening

20. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Traditional strengthening

21. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Traditional strengthening

22. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Traditional strengthening

23. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Conclusion
The methodology used in the development of the case study enabled
a streamlined communication with the project stakeholders, an
understanding of the condition of the structure, and management of
risks and expectations.
The structural analyses performed permitted us to first understand the
behavior of the stub girder system, to identify weaknesses, internal
forces, and stiffness, to predict the behavior under different loading
conditions and to devise the strengthening techniques needed.
Due to the relative complexity of the structural analysis required, labor
intensive construction nature and transformations suffered in the
global steel market, the use of the stub girder system has become
obsolete and seldomly used. Today, structural engineers will only
need to study this system when modifications or repairs are
necessary.

24. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Minnesota Structures

25. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Minnesota Structures

26. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Minnesota Structures

27. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Minnesota Structures

28. Paper Number: 378
Kevin Vazquez
SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11th
Minnesota Structures

29. SAHC 2018INTERNATIONAL CONFERENCE ON STRUCTURAL
ANALYSIS OF HISTORICAL CONSTRUCTIONS11
th
Speaker: Kevin Vazquez
kvazquez@mbjeng.com
THANK YOU