www.innovationandresearchfocus.org.ukInnovation & Research Focus Issue 101 MAY 20154

BUILDINGS, STRUCTURES, MODELLING & TESTING

Dynamic response of tall timber buildingsTimber buildings have well established economic, social and environmental advantages over other structural options. In particular, their low carbon footprint and high strength to weight ratio makes tall timber construction an attractive solution for satisfying the pressing housing demands in densely populated areas at minimum environmental costs. However, due to the low mass and flexibility typical of tall timber construction, concerns regarding their dynamic behaviour have been raised.

An MSc research project conducted at Imperial College London by Ishan Abeysekera, funded by the

Institution of Structural Engineers, has examined the dynamic response of tall timber buildings of different configurations subjected to Tornado and Downburst wind loading. Four buildings incorporating: i) solid cross-laminated-timber (CLT) shear-wall systems, ii) glued-laminated (Glulam) frames, and iii) hybrid systems, were studied.

Buildings of 7, 11, 20 and 30 storeys were modelled and analysed in the Finite Element software SeismoStruct. Particular attention was given to the accurate representation of the connection behaviour as well as material and structural damping. Wind velocity series were generated following typical Tornado and Downburst distributions and extensive response history analyses were performed. The results were discussed in terms of peak acceleration levels attained at selected locations throughout the height of the building considering varying frequency ranges. The software SeismoSignal was employed for post-processing.

A comparison against international performance criteria for user comfort as set out in ISO10137 revealed the inability of the buildings studied here to satisfy the codified acceleration limits. Therefore, additional analyses employing tuned-mass-dampers (TMD) tailored to the most demanding frequency response in each case were carried out. It was found that TMDs with an active mass of 5% of the total building mass are able to produce up to a 50% reduction in peak acceleration levels. The models proposed as part of the present study constitute a fundamental step towards the assessment of alternative response modification strategies as well as the development of numerical tools for the future optimisation of TMD designs.

A poster giving further details of this project and details of the Institution of Structural Engineers Grant Scheme are available at: http://www.istructe.org/education/scholarships-grants-and-bursaries/msc-research-grants. Diagrams and their explanations featured in this article have been taken from the poster.

For further information about this research please contact the project supervisor, Dr Christian Málaga-Chuquitaype, Imperial

Dynamic Response of Tall Timber Buildings Ishan K Abeysekera

Supervisor: Dr Christian Málaga-Chuquitaype

1.Introduction • Tall timber buildings are gaining importance as a sustainable alternative to concrete and steel buildings.

• Due to their low mass and high flexibility the performance of tall timber buildings under transient wind loads are under question.

• This study aims to assess the performance of 4 configurations of timber buildings under transient wind loads by means of numerical analysis against the comfort criteria set out in ISO10137[1].

2.Modelling

3.Wind Loading Two types of wind profiles are considered: i) downburst, and ii) tornado (see Figure on the right). Generated wind load profiles were applied to all 4 building under consideration in the direction parallel to the strong axis (i.e. causing bending about the weak axis).

Based on experimental data the analysis was run for various values of damping ranging from 3 to 13% of the critical value.

5.Results The floor acceleration histories were post-processed in order to compare the building response with codified criteria. Results are presented below for the top floor in all buildings under study with approximately 10% viscous damping.

Building 1

• Cross Laminated Timber (CLT) building with steel connections.

• The CLT panels were modelled as rigid frames and the connections as equivalent springs. This is justified by the fact the most of the flexibility of the building comes from the connections (shown below).

• Floor modelled as rigid constraints with concentrated masses at floor levels.

Building 2

• This building employs Glued-Laminated (GluLam) beams and columns, CLT cores and a CLT slab flooring system.

• Floors are modelled as rigid constraints, with all the building mass concentrated at the floor levels.

• CLT cores modelled with equivalent elastic beam-column sections.

Buildings 3 and 4

• Based on the research project by Skidmore Owings and Merrill, SOM [2].

• These buildings use a composite framing system with concrete and timber elements.

• Walls and columns are modelled as elastic frame elements.

• Full-strength connections assumed for beam-column joints.

• Floors are modelled as rigid constraints with floor masses and vertical loads simulated as lumped mass elements.

6.Conclusions • Downburst loading appears to be more critical for lower buildings

(Buildings 1 to 3) whilst tornado loading is more critical for tall buildings (Building 4).

• When subjected to downburst action, all buildings fail to meet the ISO10137 criteria for the full range of frequencies analysed.

• TMDs with an active mass of 5% the total building mass are able to reduce in nearly 50% the peak accelerations. Further studies should be performed leading to optimized designs of TMDs.

Building 1 7-storey

Building 2 11-storey

Building 3 20-storey

Building 4 30-storey

Natural modes along weak axis – Period (s)

Mode 1: 0.68s Mode 3: 0.16s

Mode 1: 1.17s Mode 4: 0.28s

Mode 1: 1.24s Mode 4: 0.38s

Mode 1: 1.76s Mode 4: 0.54s

Rigid panel

Rigid panel

Link elements

Node Rigid bar

Figure: Modelling of CLT panels

4. Tuned Mass Damper (TMD) The effectiveness of Tuned Mass Dampers (TMD) to mitigate the high levels of floor acceleration was examined. Up to a 50% reduction in the peak acceleration values was possible in most cases by employing TMD with a mass equivalent to 5% of the building mass. No design optimization was performed.

References [1] ISO 10137:2007 Bases for design of structures - Serviceability of buildings and walkways against vibrations. [2] Skidmore Owings & Merrill. Timber Tower Research Project. Available from: http://www.som.com/ideas/research/timber_tower_ research_ project [3] SeismoStruct v6.5 available from http://www.seismosoft

0.0

1.0

2.0

3.0

4.0

0 0.5 1 1.5

Nor

mal

ized

hei

ght

Normalized wind speed

Normalized Downburst wind velocity profile

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5

Nor

mal

ized

heig

ht

Normalized wind speed

Normalized Tornado wind velocity profile

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 2 - Tornado

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 3 - Downburst

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 3 - Tornado

w/o TMDwith TMDOffice CriteriaResidential Criteria

Acknowledgements The financial support of the IStructE through a MSc Research Grant for the research described in this poster is gratefully acknowledged.

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 1 - Downburst

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 1 - Tornado

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Bulding 2 - Downburst

w/o TMDwith TMDOffice CriteriaResidential CriteriaDynamic Response of Tall Timber Buildings

Ishan K Abeysekera Supervisor: Dr Christian Málaga-Chuquitaype

1.Introduction • Tall timber buildings are gaining importance as a sustainable alternative to concrete and steel buildings.

• Due to their low mass and high flexibility the performance of tall timber buildings under transient wind loads are under question.

• This study aims to assess the performance of 4 configurations of timber buildings under transient wind loads by means of numerical analysis against the comfort criteria set out in ISO10137[1].

2.Modelling

3.Wind Loading Two types of wind profiles are considered: i) downburst, and ii) tornado (see Figure on the right). Generated wind load profiles were applied to all 4 building under consideration in the direction parallel to the strong axis (i.e. causing bending about the weak axis).

Based on experimental data the analysis was run for various values of damping ranging from 3 to 13% of the critical value.

5.Results The floor acceleration histories were post-processed in order to compare the building response with codified criteria. Results are presented below for the top floor in all buildings under study with approximately 10% viscous damping.

Building 1

• Cross Laminated Timber (CLT) building with steel connections.

• The CLT panels were modelled as rigid frames and the connections as equivalent springs. This is justified by the fact the most of the flexibility of the building comes from the connections (shown below).

• Floor modelled as rigid constraints with concentrated masses at floor levels.

Building 2

• This building employs Glued-Laminated (GluLam) beams and columns, CLT cores and a CLT slab flooring system.

• Floors are modelled as rigid constraints, with all the building mass concentrated at the floor levels.

• CLT cores modelled with equivalent elastic beam-column sections.

Buildings 3 and 4

• Based on the research project by Skidmore Owings and Merrill, SOM [2].

• These buildings use a composite framing system with concrete and timber elements.

• Walls and columns are modelled as elastic frame elements.

• Full-strength connections assumed for beam-column joints.

• Floors are modelled as rigid constraints with floor masses and vertical loads simulated as lumped mass elements.

6.Conclusions • Downburst loading appears to be more critical for lower buildings

(Buildings 1 to 3) whilst tornado loading is more critical for tall buildings (Building 4).

• When subjected to downburst action, all buildings fail to meet the ISO10137 criteria for the full range of frequencies analysed.

• TMDs with an active mass of 5% the total building mass are able to reduce in nearly 50% the peak accelerations. Further studies should be performed leading to optimized designs of TMDs.

Building 1 7-storey

Building 2 11-storey

Building 3 20-storey

Building 4 30-storey

Natural modes along weak axis – Period (s)

Mode 1: 0.68s Mode 3: 0.16s

Mode 1: 1.17s Mode 4: 0.28s

Mode 1: 1.24s Mode 4: 0.38s

Mode 1: 1.76s Mode 4: 0.54s

Rigid panel

Rigid panel

Link elements

Node Rigid bar

Figure: Modelling of CLT panels

4. Tuned Mass Damper (TMD) The effectiveness of Tuned Mass Dampers (TMD) to mitigate the high levels of floor acceleration was examined. Up to a 50% reduction in the peak acceleration values was possible in most cases by employing TMD with a mass equivalent to 5% of the building mass. No design optimization was performed.

References [1] ISO 10137:2007 Bases for design of structures - Serviceability of buildings and walkways against vibrations. [2] Skidmore Owings & Merrill. Timber Tower Research Project. Available from: http://www.som.com/ideas/research/timber_tower_ research_ project [3] SeismoStruct v6.5 available from http://www.seismosoft

0.0

1.0

2.0

3.0

4.0

0 0.5 1 1.5

Nor

mal

ized

hei

ght

Normalized wind speed

Normalized Downburst wind velocity profile

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5

Nor

mal

ized

heig

ht

Normalized wind speed

Normalized Tornado wind velocity profile

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 2 - Tornado

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 3 - Downburst

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 3 - Tornado

w/o TMDwith TMDOffice CriteriaResidential Criteria

Acknowledgements The financial support of the IStructE through a MSc Research Grant for the research described in this poster is gratefully acknowledged.

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 1 - Downburst

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Building 1 - Tornado

w/o TMDwith TMDOffice CriteriaResidential Criteria

0.01

0.1

1

0.1 1 10Peak

acc

elra

tion

top

floor

(m/s

2)

Frequency (Hz)

Bulding 2 - Downburst

w/o TMDwith TMDOffice CriteriaResidential Criteria

• Downburst loading appears to be more critical for lower buildings (Buildings 1 to 3) whilst tornado loading is more critical for tall buildings (Building 4).

• When subjected to downburst action, all buildings fail to meet the ISO10137 criteria for the full range of frequencies analysed.

• TMDs with an active mass of 5% the total building mass are able to reduce the peak accelerations by nearly 50%. Further studies should be performed leading to optimized designs of TMDs.

Results and conclusions

The floor acceleration histories were post-processed in order to compare the building response with codified criteria. Results are presented below for the top floor in all buildings under study with approximately 10% viscous damping.

Buildings modelled

Wind loading and Tuned Mass Damper (TMD)Two types of wind profiles are considered: i) downburst, and ii) tornado. Generated wind load profiles were applied to all 4 buildings under consideration in the direction parallel to the strong axis (i.e. causing bending about the weak axis). The effectiveness of Tuned Mass Dampers (TMD) to mitigate the high levels of floor acceleration was examined.

College London (02075 946003; E-mail: c.malaga@imperial.ac.uk).