Phd Student (Civil Engineering)

Peter BK Mbewe

PhD Student (Civil Engineering)

Email:

Telephone:

Fax:

Read my CV here

1.0 Current research topic: Structural system performance evaluation towards sustainable residential buildings

Status: Current student (2014-2017)

Project description:

Current methods for structural design and analysis do not consider building sustainability as a key component in selecting reliable structural systems. There is lack of an acceptable common measure for structural system performance evaluation and this has made it difficult to incorporate structural performance in building sustainability and vice versa. This research aims at developing a structural system performance evaluation methodology based on a combination of structural system stiffness, strain energy and structural damage principles, and then integrating the structural system performance with overall building sustainability through building life cycle

1.1 Research goals and objectives

1.2 Research concepts and proposed methodology

1.3 Expected research contribution

2.0 Previous research topic: Development of analytical flexural models for SFRC beams without steel bars

Status: MSc. Research (2009-2011), completed

Project description:

The use of any new structural materials understanding its material behaviour under structural loads and develop models that can be used to predict its behaviour under use. SFRC offers better tensile properties as compared to normal concrete and the tensile strength of SFRC can be considered in design thereby reducing the amount of steel bars that may be required.

2.1 Research objectives

2.2 Research concepts and proposed methodology

2.3 Development of proposed analytical models

2.4 Material characterisation

2.5 Model verification

2.6 Conclusion

3.0 Other research projects

3.1 Laboratory for an integrated African Network for the Built Environment (LIANE) project

Status: Junior researcher (2014-2015)

Project description:

The research project aims to improve the quality and sustainability of building and settlement planning in agglomeration areas of greater cities in the region of the Great Lakes in East Africa – in cooperation with the African partner universities. The main focus is on energy and resource saving in the building sector. With expertise in the fields of urban planning, architecture and civil engineering, this project uses an interdisciplinary approach. The universities involved are Augsburg university (responsible for project management and knowledge management and transfer), Jomo Kenyatta University of Agriculture and Technology (responsible for conferences and training workshops), University of Rwanda (responsible for contact and interaction with local stakeholders), and Stellenbosch university (responsible for practical research).

Project objectives

i. appropriate settlement structures and settlement densities

ii. Resource conserving water supply and wastewater disposal

iii. Energy production (based on regenerative resources)

iv. Improvement of local building typologies and technologies

v. Use of sustainable and locally available materials

3.2 JENGA project

Status: Junior researcher (2014-2016)

Project description:

JENGA project aims at academic capacity building and knowledge transfer for energy access and efficiency and low-carbon technologies in the field of sustainable housing, addressing the fact that the building sector accounts for 40% of global energy consumption and 30% of greenhouse gas emissions. By bringing together an interdisciplinary team from five different countries including Higher Education Institutions from Kenya (Jomo Kenyatta University of Agriculture and technology), Uganda (Uganda Martyrs University), Rwanda (University of Rwanda), South Africa (Stellenbosch University) and Germany (Augsburg University of Applied Sciences) as well as UN Habitat as an important stakeholder, JENGA aims at enhancing regional integration of these issues within higher education in East Africa. Using a holistic approach to develop courses and disseminate respective knowledge on energy efficient and sustainable housing in Africa, JENGA project aims at tackling two major mutually reinforcing challenges, which culminate in a rising need for energy: fast growing populations and rapid urbanisation. By addressing the building sector with its exceptional potential to mitigate the environmental impacts of energy and resource consumption, the project will reach out to achieve sustainable development (MDG 7).

Project objectives:

i. Academic capacity building at the participating four African universities

ii. Involvement of stakeholders and wide regional dissemination of knowledge on energy-efficient and sustainable housing

iii. Development of joint programmes at BA/MA level in the field of architecture / environmental design / construction

Publications:

1) Mbewe, P. (2011). Development of analytical flexural models for steel fibre-reinforced concrete beams with and without steel bars, MScEng-thesis, University of Stellenbosch.

2) Van Zijl, G., & Mbewe, P. (2012). Towards a design model for steel fibre reinforced concrete in bending. In G. J. Parra-Montesinos, H. W. Reinhardt, & A. E. Naaman (Ed.), High Performance Fibre Reinforced Cement Composites 6 (pp. 221-229). New York: Springer.

3) Van Zijl, G., & Mbewe, P. (2013 ). Flexural modelling of steel fibre-reinforced concrete with and without steel bars. Elsevier, 52-62

Prizes and bursaries

· ANSTI/DAAD Post Graduate Fellowships

· Departmental bursary

1.1 Research goals and objectives

Goal one: Development of structural system performance evaluation method

In order to develop a unified structural system performance evaluation methodology that can generate comparable structural performance indicator(s) the following tasks will be performed:

(a) Development of analytical models for evaluation of structural system performance based on system's stiffness reduction to model progressive damage of the structure due to seismic effects and subsidence caused by flooding, actions prone to specific regions in the Western Cape;

(b) Numerical modelling and evaluation of key subsystem’s structural performance;

(d) Verification of analytical and numerical models for structural performance evaluation.

Goal two: Building’s environmental and economic sustainability evaluation

This aims at identifying an appropriate sustainability assessment approach from state of the art research that can be integrated with structural performance parameters of the building structure through building life cycle evaluation. The following tasks will be performed:

(a) Identification of common building models which represent residential building topologies for single storeys and multi-storey buildings for a selected climatic region;

(b) Evaluation of state of the art building sustainability assessment methods; and

1.2 Research concepts and proposed methodology

A systematic procedure for research execution is shown in Figure 1.1.

Figure 1.1: Research design

1.3 Expected research contribution

Using system performance evaluation procedure developed, it is envisioned that an integrated structural design approach that incorporates building sustainability would be proposed as shown in Figure 1.2. Development of structural system performance evaluation methodology would allow system’s comparison with regards to structural performance and building sustainability performance.

Figure 1.2: Proposed integrated building design

The following are expected research outcomes and contributions to the field of structural engineering and building sustainability:

(a) A novel structural system performance evaluation methodology that simulates computational damage approach by systematic stiffness reduction. This will be an iterative structural system analysis tool for structural engineers that incorporates non-linear behaviour of both the structural members and connections;

(b) For the specific case studies (with specific structural behaviour of structural elements and their connections), dominant factors influencing structural system performance will be established and can be used by engineers as guidelines when deciding the optimal capacity of structural elements and connections; and

(c) Through evaluation of structural system’s behaviour under various loading conditions within its life span and state of the art methods for building life cycle analysis, the research findings will work towards establishing a fundamental linkage between structural performance, building life span, building maintenance demand and the overall building sustainability.

2.1 Research objectives

i. Material characterization for SFRC, especially the post peak tensile behavior;

ii. Development of analytical models for flexural design of SFRC with and without steel bars; and

iii. Verification of proposed analytical models based on laboratory experimental tests and available data from literature.

2.2 Research concepts and proposed methodology

A methodological process for development and verification of the proposed flexural models for SFRC beams is shown in Figure 2.1.

Figure 2.1: Research plan

As stated in Figure 2.1, the research project was broken down into three specific activities, namely, material characterisation, development of analytical models and verification of analytical models. Material characterisation and verification of analytical models required data from laboratory experiments and the whole experimental data acquisition and data analysis is summarised in Figure 2.2.

Figure 2.2: Experimental data acquisition and analysis

2.3 Development of analytical models

The proposed model is based on use of stress blocks in both tension and compression. For compression behaviour, a bilinear stress distribution is used to derive an equivalent stress block as shown in Figure 2.3. Elastic drop-down constant stress distribution is assumed for tensile behaviour and it is used to derive an equivalent stress block as shown in Figure 2.4. It should be noted that the equivalent stress blocks are flexible (can vary in size) based on perceived stress and strain states.

(a) Bilinear compression stress distribution /
(b) Conversion of a stress block for compression stress distribution

Figure 2.3: Compression stress block conversion

(a) Elastic drop-down constant stress distribution /
(b) Conversion of a stress block for tension stress distribution

Figure 2.4: Tension stress block conversion

With respect to Figures 2.3 and 2.4, By defining ω and ω’ to represent ratios of yield to post yield strain in compression (ɛcy/ɛcp) and, cracking to post-cracking strain in tension (ɛty/ɛtp) respectively, factors that convert stress distribution to stress block (λc, ηc, λt, ηt) are derived based on force equilibrium principles and maintaining the same position of resultant forces for the two force systems and are given below:

, , (1)

, , (2)

Flexural resistance for a SFRC beam without steel bars can be derived from Figure 2.5.

By considering the simplified rectangular stress block (Figure 2.5), the flexural capacity of the section can be derived consistent with strain compatibility and force equilibrium principles. Based on practical values of strains, the moment capacity for a SFRC without reinforcement will be dictated by the tensile capacity of the section. With this assumption, the beam may fail under two distinct strain states, namely, strain state where ultimate tensile strain is reached while compression stress is still elastic state (stage 2 failure) and strain state where tensile stress is reached with compression in post-cracking state (stage 3 failure). The depth of the neutral axis and the lever arm between compressive and tensile forces are determined using equations given Table 1.1.

Stage 1 Stage 2 Stage 3 /
Design stress blocks
Figure 2.5. Left: Three stages of stress-strain states. right: Rectangular stress blocks for design

Table 1.1. Neutral axis and curvature formulas using bilinear and drop down models

Parameter / Stage 1 / Stage 2 / Stage 3
(neutral axis ratio) / / /
(curvature) / / /
(lever arm ratio) / / /

Equivalent compressive strains at failure are determined as follows:

(3)

An appropriate value for equivalent strain calculated from eq. (3) is chosen if it meets the criterion ec2eq £ ecy £ ec3eq£ ecu.

With the overall depth of the section known, the depth of the neutral axis and the lever arm between compressive and tensile forces are determined using equations in Table 1 and the moment resistance of the section is determined as follows:

(4)

2.4 Material characterisation

Materials characterisation tests were conducted for both normal concrete and steel fibre-reinforced concrete. Further tensile tests on steel bars used in the experimental program were conducted. Figures 2.5 and 2.6 show the test set-up and results from material characterisation tests.

Compression tests set-up /
Compression test results

Figure 2.5: Material characterisation tests-compression tests

Splitting test set-up /
Splitting test results

Figure 2.6: Material characterisation tests-Splitting tests

2.5 Model verification

Model input parameters (derived from characterization experiments) for batches A and B are shown in Table 2.2

Table 2.2: Model input parameters from material characterisation test.

Batch Name / fc,cube (MPa) / ft (MPa) / μ (= fteq/ft)
Batch A (mean values) / 25.60 / 3.88 / 0.480
Batch A (characteristic values) / 24.10 / 3.57 / 0.378
Batch B (mean values) / 41.20 / 5.17 / 0.485
Batch B (characteristic values) / 39.23 / 4.82 / 0.475

Using data from Table 2.2 as input to SFRC model, predicted flexural capacity is determined and compared to experimental data. Comparison of model predictions with experimental data is shown in Figure 2.7.

(a) Mexp vs. Mpred (mean values of input model parameters) /
(b) Mexp vs. Mpred (Characteristic values of input model parameters)

Figure 2.7: Comparison of model predictions with experimental data for SFRC beams without reinforcement

2.6 Conclusion

The following conclusions can be made about the research:

i. An analytical model has been developed based on stress blocks

ii. Conversion factor, λt ranges from 0.95 and 1.03

iii. The proposed models have been verified using experimental data.

iv. The models fairly predict the bending moment capacity of the beams.

Challenge:

i. Use of splitting test data instead of direct tension

Future work:

i. Need for improved assessment of direct tensile behaviour of steel fibre reinforced concrete

ii. Need for a systematic reliability study