0. STEEL-TIMBER HYBRID STRUCTURES: PROBLEMS AND SOLUTION
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STEEL-TIMBER HYBRID STRUCTURES: PROBLEMS AND SOLUTION
For: Dr. Stiemer
CIVL 510
University of British Columbia
By: Johannes Schneider
and Carla Dickof
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1. Steel Timber Hybrid Structures: Problems and Solutions
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Hybrid Systems
Typical Hybrid Structures
Combine two or more material types that within a system or an element
Common between concrete, steel, and masonry or timber
Timber-steel hybrids are less common but gaining popularity
Johannes starts
- The aim of all the hybridization techniques is to optimally utilize each material
- hybrids can be monolithic as concrete and steel
Connection detail is the major challenge associated with timber steel hybrid structures.
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Steel Timber Hybrid Structures: Problems and Solutions

2. Steel Timber Hybrid Structures: Problems and Solutions
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Material Properties: Timber and Steel
Timber
Steel
Anisotropic material
Strong parallel to grain
Weak perp to grain
Stronger in compression than tension
Isotropic material
Same strength in tension and compression
Hygroscopic material
changing moisture causes swelling and shrinkage
High thermal expansion coefficient (ie. sensitive to heat)
Untreated wood can decay under certain environmental influences
Steel needs coating or galvanizing for durability
High resistance to chloride
Low resistance to chloride
Low resistance to rolling shear
High ratio Strength/weight
High strengths leads to small cross sections which are susceptible to buckling
Steel is considered isotropic, meaning that its properties are the same in any direction
wood has independent mechanical properties in the directions of three perpendicular axes: longitudinal, radial, and tangential
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3. Material Properties: Timber and Steel
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Material Properties: Timber and Steel
Material
Yield Strength (MPa)
Density
(kg/m3)
Modulus Elasticity
(MPa)
Compresion Strength (MPa)
Tensile Strength (MPa)
Steel
350
7800
200,000
Concrete
N/A
2300
20,000
20-40
Structural Timber
8,000-11,000
Parallel 30
Perp
Parallel 6
Perp
Big range in strength and elasticity for wood based on unpredictable quality defects.
Wood can be very flexible under loads, keeping strength while bending
Wood is much weaker in compression perpendicular to the grain than in compression parallel to the grain
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4. Engineered Wood Products
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Engineered Wood Products
Engineering products improve performance by spreading imperfections and locating them in regions of low stress. Some provide similar strength in two axes
CLT (Cross-Laminated-Timber)
Crosswise stacked board layers create a isotropic behavior in 2 directions
Plywood
Crosswise stacked veneer layers create a isotropic behavior in 2 directions
OSB (Oriented Strand Board)
layering strands of wood in specific orientations
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5. Micro vs. Macro Component Level Hybridization
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Micro vs. Macro Component Level Hybridization
Combine two or more material types within single element. A mechanism to transfer forces between materials is required
Flitch beam:
steel plate sandwiched between timber beams
Glued and/or pinned to transfer shear
Load sharing proportionally to relative stiffness of members
Built-in Steel Columns:
High fire protection
Wood as lateral support and preventing from buckling
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6. Steel Timber Hybrid Structures: Problems and Solutions
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Micro vs. Macro Component Level Hybridization
Post-tensioned CLT beam:
Low height of beam
Simple to fabricate
Glued-in rods:
Used for moment connections
Improvement of strength perpendicular to grain
Wood as lateral support and preventing from buckling
Problems of timber-steel hybrid (also advantages) – Johannes
Material properties (incompatibility)
Strength stiffness
Shrinkage/swelling
Glued in systems with shrinkage etc.
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7. Steel Timber Hybrid Structures: Problems and Solutions
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Micro vs. Macro System Level Hybridization
A combination of members of different material types within a system
Hybrid Trusses:
Combination according to their properties
Timber in compression
Steel in tension members
Less corrosive exposure of the truss
Careful design of connections is required. Avoiding of water in connections
SAP Arena – Mannheim Hybrid Space Truss
Hybrid Bridge – Kössen / Switzerland
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8. Steel Timber Hybrid Structures: Problems and Solutions
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Micro vs. Macro Component Level Hybridization
Vertical Mixed Systems:
Lower levels in concrete or steel
upper levels in timber
Code height limitation for timber structures
Big difference in stiffness is major design challenge
“flexible” wood storeys
“rigid” ground-storeys
9-story concrete-timber Hybrid building – London GB
Reduced floor weights = reduced seismic
5-story concrete-timber hybrid building Vancouver BC.
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9. Steel Timber Hybrid Structures: Problems and Solutions
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Micro vs. Macro Component Level Hybridization
Steel frame and timber floor
diaphragm:
Moment resisting frame with timber floor or timber joist and plywood diaphragm to transfer lateral loads
Utilizing of Hybeams
Reducing overall weight of building
seismic benefits
high degree of prefabrication and less time for erection
14-story hybrid building steel frame with hybeams
Cargolifter Office building - Berlin
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10. Steel Timber Hybrid Structures: Problems and Solutions
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Micro vs. Macro Component Level Hybridization
Steel frame, timber shear walls
and timber floor diaphragms:
Using the high strength of steel for gravity loads
Timber shear walls in CLT or Midply taking the lateral loads
Distribution of load carrying between steel and timber elements
Prefabrication and light structure are advantages
7-story residencial building with steel frames and laminated board stack slabs - Berlin
EXPO 2000 roof - Hannover
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11. Steel Timber Hybrid Structures: Problems and Solutions
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Connections
Capacity design at member connections
Nails, pins, and bolts
All dowel type connections
Ductility is introduced by balancing the max plastic steel deformation while maintaining min wood crushing and no wood fracture
tensioning creates clamping force
Screws
higher strength and lower ductility
Connections are the most complicated thing about hybrid design. Seismic design mandates capacity design which means that must be designed to resist the capacity of the member. With wood it is also important to design the connection’s mode of failure. It is also important to yield that steel to develop ductility in the system. Crushing failure in the wood is preferable to splitting failure to avoid brittle conditions. Its important not to design a connection to create force perpendicular to the grain as this will cause splitting.
Nails, pins, and bolts are considered dowel connections that work by bearing on the wood. Screws work by creating bearing on the wood in the direction of load, but they also have tensile capacity. The tensile capacity is developed by the screw threads as each one bears on the wood. As nails and pins are loaded, they begin to deform and pull out of the wood due to minimal tensile resistance. For this reason, bolts and screws are stronger. Bolts ends prevent the bolts from pulling out of the wood by bearing on the sides of the members. Screws work by using each thread to internally bear on the wood to prevent pull out.
When small diameter connectors are used, steel yields and wood crushes locally. When large diameter connectors are used, they are much more likely to cause splitting. This is partially due to issues with wood shrinkage around the bolt and partially due to the bolts stiffness prying the grains apart.
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Steel Timber Hybrid Structures: Problems and Solutions

12. Steel Timber Hybrid Structures: Problems and Solutions
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Connections
Brackets
Steel plate elements connecting to wood members
Post-tensioned systems
Post-tensioned tendons are not bonded to the timber
Mild steel is bonded or glued to the timber
Steel and cabl;e strength must be balance for ductility and self-centering ability
Brackets are steel plate pieces that use dowel or screw connections to connect two or more members together. The placement of the connectors, like screws and bolts, in these types of connections is very important. You have to be careful not to restrain the member perpendicular to the grain or splitting can occur as a result of shrinkage. There is not the same concern parallel to the grain as the shrinkage in this direction is considerably less.
Post-tensioned systems are a newer type of system that has the advantage of being self centering. It works by having these post-tensioned tendons through the beams. These are un-bonded and move independently from the wood. There are also glued in place rods that are meant to yield under high seismic load. This brings ductility to the system during a seismic event. When a large seismic event occurs, deflection will happen as shown. The bonded mild steel will yield, dissipating energy, and then the tension in the un-bonded high strength tendon will pull the system back to its original configuration –ie. self center
Designing this type of system is complex as it is important to balance the strengths of the two types of steel in the system. You want to maximise the amount of steel the yields to maximise the ductility of the system, but it is important to have enough strength in the tendon to overcome the strain hardened strength in the mild steel after yielding has occurred so the system can self center. It is also important to make sure that the wood beams don’t crush while the deflection is taking place. Typically a steel plate is placed at the end of the beam to distribute the loads somewhat.
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Steel Timber Hybrid Structures: Problems and Solutions

13. Ductility & Energy Dissipation
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Ductility & Energy Dissipation
CLT shear walls:
No energy dissipation in panel. Possible dissipation through
Friction in step joints
Deformation energy in connectors
Difficulty to find right balance to get right failure mode
Block-failure
Here are some examples of failure mechanisms of connections for CLT panel shear walls during cyclic loading. In the top you will see block failure where the wood fractures. Thiss is undersireable and leads to brittle failure. The middle option is failure of the bracket. In this case there was failure due to fatigue and exceedence of loading. This is also not desirable. The last option is pull out of the connectors. This is the best option as it leads to the most energy dissipation in the system. As you can see the bracket has yielded resulting in energy dissipation there, as well as pulling out the connectors, resulting in further energy dissipation.
Another option would include placing a step at the various points along the shear wall. As you can see here the center of this is not one continuous CLT panel. If the connection between the 2 were stepped so as to overlap, they could be connected to develop both friction and connector yielding. This would allow further energy to be dissipated during cyclic loading.
Fracture in bracket
Pull-out failure
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Steel Timber Hybrid Structures: Problems and Solutions

14. System Study: Steel Frame with Wood Infill Shear Walls
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System Study: Steel Frame with Wood Infill Shear Walls
5 story moment frame with infill shear walls placed in the central bay
Look at one frame of the building
Model with Vancouver Seismicity
Steel moment frame
Wood shear walls
The frame will have external bays of 9m and an internal bay at 6m. The internal bay will contain the infill wood shear wall.
The first storey will be 4.5m tall and all the subsequent stories will be 3.65m tall. The frame will be assumed to carry 6m of tributary width within a building and we will ignore torsion. The building will be assumed to be in Vancouver on site class C soil
The purpose of this study was to determine the effect that the shear walls have, and how much the deflection is reduced as a result of the shear wall addition.
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15. Timber Shear Wall Design
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Timber Shear Wall Design
Midply Shear Walls:
Improved strength (more than twice that of typical shear walls)
Nails in double shear instead of single
Nail head can no longer pull through sheathing
Increased nail edge distance on studs
Risk of buckling out of plane
Energy dissipation through nail pull-out and deformation
Midply shear walls were chosen for the system study we performed. They perform better that regular shear walls with respect to strength and are equally ductile. We decided against CLT due to its extremely high stiffness and the resulting compatibility issues.
The reason the perform so well is due to the connections of the studs to the sheathing. Midply walls use their nails in double shear. Also, the heads of the nails are not able to pull through as they are at the stud instead of the sheathing. And finally because the studs have changed orientation, the nails placed at the center of the stud have a much larger edge distance than those in a typical shear wall.
On the whole, test results have shown midply walls to be approximately 3 times stronger than regular shear walls, but the code recommendation was for twice as strong. Although this additional strength will be beneficial for wind and general design, the overstrength must be accounted for in the seismic design. Wood shear walls are generally given an over-strength factor of 1.7 according to the national building code. This is likely insufficient based on the test results. Despite this, we will continue to use this factor in our study.
Midply wall buckling
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16. SAP2000 Model: Steel Moment Frame
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SAP2000 Model: Steel Moment Frame
A base steel frame was modeled in SAP2000 to provide a basis for comparison
Type D Ductility
Static pushover analysis performed using NBCC 2005
Vancouver Seismic Hazard Index Used
Initially we modelled a 5 story, 2D frame. The frame was designed to be type D ductility which means that hinge points at failure will all occur in the beams, not the columns. The design was based on a design completed on by a guy name Yousuf.
Static analysis according the NBCC 2005 was used and the building was placed in Vancouver.
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17. SAP2000 Model: Steel Frame with Wood Shear Walls
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SAP2000 Model: Steel Frame with Wood Shear Walls
Midply Shear walls were used
Multi-linear links between plywood and studs
Connection between the shear wall and the steel frame
Lateral connections at top and bottom with multi-linear links
Vertical connections on sides with multi-linear elastic links
The frame was left the same for this model, and we added a midply shear wall at the central bay over the entire height of the building
The wall itself was modeled with the sheathing and the studs in place. The sheathing was modeled as an orthotropic member and the studs were simplified to isotropic. The were connected with non-linear springs. The springs or links were defined the have stiffness parallel to the face of the sheathing and to be rigid parallel to the sheathing. These springs were developed to model nailed connections. This non-linear-linear shape was determined and has been compared with test data of midply walls by a masters student here at UBC in 2004
The connection of the wall to the frame made with rigid links in the direction of load transfer. The links at the top and bottom of the walls were given fixed is the horizontal direction. On the sides of the walls the springs were fixed in the vertical direction. They were also connected out of plane of the frame.
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18. SAP2000 Model: Comparison between Models
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SAP2000 Model: Comparison between Models
Ductility (Rd) reduces seismic shear force that the system need resist. Energy is absorbed through permanent deformation
Steel Moment Frame
Frame w/ Shear Wall
Ductility (Rd)
5.0
3.0
Building Period (s)
1.32 s
1.07 s
Total Base Shear
114 kN
197 kN
Increase in results in higher loads by lowering the period of the structure which increases the spectral acceleration
The ductility is based on the amount the building can deflect after is has started to yield and results in a decrease in seismic shear it must resist. It is denoted by the factor Rd here, where a larger number represents more ductility. The values from ductility of each seismic resisting system is taken from the code.
The building period is determined based on the mass of the building and its stiffness. The additional of the shear walls results in and increase in stiffness and the mass remains essentially the same resulting in a shorter period (we ignored the increase in mass from the walls). This results in a higher spectral acceleration – ie. the accel the building experience. I should also mention that the building period shown here is that determined by the SAP2000 model. There is likely some error in this due to modeling error and both will likely be somewhat lower once adjusted by the code. Regardless, the ratio should remain about the same.
It is clear that the increase in spectral acceleration and the decrease in ductility results in a much larger base shear for the model with the wood shear wall. The increase nearly 75%.
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Steel Timber Hybrid Structures: Problems and Solutions

19. SAP2000 Model Results: Steel Frame with Wood Shear Walls
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SAP2000 Model Results: Steel Frame with Wood Shear Walls
Steel Moment Frame
Frame w/ Shear Wall
Total Defl’n
28.7 mm
21.5 mm
Storey
Defl’n
7.08 mm
5.4 mm
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References
Khorasani, Y., (2010). Feasibility Study of Hybrid Wood Steel Structures. Thesis, (M.A.Sc). University of British Columbia
Maloney, T.M., (1996), The Family of Wood Composite Materials, Forest Products Journal. Vol. 46 Issue. 2: pg.19-26
Varoglu, E. et. al. (2006) Midply Wood Shear wall System: Concept and Performance in Static and Cyclic Testing, Journal of Structural Engineering, Vol.132 Issue 9:pg
Yousuf, M., and Baghchi, A. (2009) Seismic design and performance evaluation of steel-frame buildings designed using the 2005 National building code of Canada, Canadian Journal of Civil Engineering, Vol. 36 Issue 2: pg
Clarke, C. (2004). Midply Shear Walls use in Non-Residential Buildings. Thesis, (M.A.Sc). University of the West Indies
Schneider, J. (2009). Connections in Cross-Laminated-Timber Shear Walls Considering the Behaviour under Monotonic and Cyclic Lateral Loading. Thesis, (Diploma). University of Stutgart
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Steel Timber Hybrid Structures: Problems and Solutions