Performance Based Building Design 1

From Below Grade Construction to Cavity Walls

Hugo S. L. Hens

• Publisher: Ernst & Sohn
• Year: 2012
• Print ISBN:
  9783433030226
• Edition: 1
• eBook ISBN:
  9783433601976
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Just like building physics, performance based building design was hardly an issue before the energy crises of the 1970s. With the need to upgrade energy efficiency, the interest in overall building performance grew. The term "performance" encompasses all building-related physical properties and qualities that are predictable during the design stage and controllable during and after construction. The term "predictable" demands calculation tools and physical models that allow evaluating a design, whereas "controllable" presumes the existence of measuring methods available on site. The basis for a system of performance arrays are the functional demands, the needs for accessibility, safety, well-being, durability, energy efficiency and sustainability and the requirements imposed by the usage of a building.

As the first of two volumes, this book applies the performance rationale, advanced in applied building physics, to the design and construction of buildings. After an overview of materials for thermal insulation, water proofing, air tightening and vapour tightening and a discussion on joints, building construction is analysed, starting with the excavations. Then foundations, below and on grade constructions, typical load bearing systems and floors pass the review to end with massive outer walls insulated at the inside and the outside and cavity walls. Most chapters build on a same scheme: overview, overall performance evaluation, design and construction.
The book is absolutely recommended to undergraduates and graduates in architectural and building engineering, though also building engineers, who want to refresh their knowledge, may benefit. The level of discussion assumes the reader has a sound knowledge of building physics, along with a background in structural engineering, building materials and building construction. Where and when needed, input and literature from over the world was used, reason why each chapter ends listing references and literature.

1

Performances

1.1 In general

This chapter starts by providing some definitions and the performance arrays. It then gives an analysis of the interaction between a rigorous application of performance metrics and building, followed by the possible impact of performance formulation on the construction process.

1.2 Definitions and basic characteristics

The term ‘performance’ encompasses all building-related physical properties and qualities that are predictable during the design stage and controllable during and after construction. Typical for performances is their hierarchical structure with the built environment as highest level (level 0) followed by the building (level 1), the building assemblies (level 2) and finally layers and materials (level 3). Relation between the four levels is typically top-down. ‘Predictable’ demands calculation tools and physical models that allow evaluating a design, whereas ‘controllable’ presumes the existence of measuring methods available on site. In some countries, the selection of building performance requirements had legal status. That coupled with a well-balanced enforcement policy guarantees application. One could speak of must and may requirements. Must is legally required, whereas may is left to the principal.

1.3 Advantages

The main advantage of a performance-based rationale is the objectification of expected and delivered building quality. For too long a time, designers juggled with ‘the art of construction’ without defining what kind of art was involved. With a rigorous application of performance metrics, the principal knows the physical qualities he may expect. In forensic cases, performance requirements provide a correct reference, which is not the case with the art of construction. A performance approach may also stimulate system based manufacturing. And finally, performance metrics could steer the building sector in a more research based direction.

1.4 Performance arrays

The basis for a system of performance arrays are the functional demands, the needs for accessibility, safety, well-being, durability, energy efficiency and sustainability and the requirements imposed by the usage of a building. For the arrays, see Table 1.1 and 1.2.

Table 1.1. Performance array at the building level (level 1).

¹ In countries like The Netherlands, Germany and Austria fire safety belongs to building physics. In other countries, it doesn’t.

Table 1.2. Performance array at the building assembly level (level 2).

¹ In countries like The Netherlands, Germany and Austria fire safety belongs to building physics. In other countries, it doesn’t.

1.5 Design based on performance metrics

1.5.1 The design process

‘Designing’ is multiply undefined. At the start, information is only indefinitely known. Each design activity may produce multiple answers, some better than others, which however cannot be classified as wrong. That indefiniteness demands a cyclic approach, starting with global choices based on sparse sets of known data, for buildings listed as project requirements and design intents. The choices depend on the knowledge, experience and creativity of the designer. The outcomes are one or more sketch designs, which then are evaluated based on the sets of imposed or demanded level 0 and 1 performance requirements. One of the sketch designs is finally optimized and the rest not meeting the performances are discarded. The result is a pre-design with form and spatiality fixed but the building fabric still open for adaptation.

With the pre-design, the set of agreed-on data increases. During the stages that follow, refinement alternates with calculations that have a double intent: finding ‘correct’ answers and adjusting the fabric to comply with the performance requirements imposed. That last phase ends with the final design, encompassing the specifications and the construction drawings needed to realize the building.

1.5.2 Integrating a performance analysis

Designing evolves from the whole to the parts and from vaguely to precisely known data and parameters. These are generated by the design itself, allowing performance analysis to become more refined as the design advances.

During the sketch design phase only level 1 performance requirements such as structural integrity, energy efficiency, comfort and costs receive attention. As most data are only vaguely known, only simple models facilitating global parametric analysis can be used. This isn’t unimportant as decisions taken during sketch design fix many qualities of the final design.

At the pre-design stage along with level 1, the level 2 performance requirements also have to be considered as these govern translation of form and spatiality into building construction. As more parameters and data are established, evaluation can be more refined. The load bearing system gets its final form, the enclosure is designed and the first finishing choices are made. Options are considered and adjusted from a structural, building physical, safety, durability, maintainability, cost and sustainability point of view.

Detailing starts with the final design. Designing becomes analyzing, calculating, comparing, correcting and deciding about materials, layer thicknesses, beam, column and wall dimensions, reinforcement bars and so on. The performance metrics now fully operate as a quality reference. Proposed structural solutions and details must comply with all level 2 and 3 requirements, if needed with feedback to level 1. That way, performances get translated into solutions. Performances in fact do not allow construction. For that, each design idea has to be transformed into materials, dimensions, assemblies, junctions, fits, building sequences and buildability, with risk, reliability and redundancy as important aspects.

Performance requirements also should become part of the specifications, so contractors may propose alternatives on condition they perform equally or better for the same or lower price.

1.6 Impact on the building process

For decades, the triad <principal/architect/contractor> dominated the building process. The principal formulated a demand based on a list of requirements and intents. He engaged an architectural firm, which produced the design, all construction drawings with consultant’s help (structural engineers, mechanical engineers and others), and the specifications on which contractors had to bid. The lowest bidder got the contract and constructed the building under supervision of the architect.

That triad suffers from drawbacks. The architect is saddled with duties for which he or she is hardly qualified. Producing construction drawings is typically a building engineering activity. Of course, knowledge about soil mechanics, foundation techniques, structural mechanics, building physics, building materials, building technology, and building services was procured but always after the pre-design was finished, that means after all influential decisions had been made. The split between design and construction further prevented buildability from being translated into sound construction drawings, which today, still, hardly differ sometimes from the pre-design ones. Details and buildability are left to the contractor, who may lack the education, motivation and resources for that. The consequences can be imagined. No industrial activity experiences as many damage cases as the building sector.

A performance rationale allows turning the triangle into a demand/bidder model. The demand comes from the principal. He produces a document containing the project requirements and intents. That document is much broader than a list of physical performances. Site planning, functional requirements at building and room level, form, architectural expression and spatiality are all part of it. Based on that document, an integrated building team, which includes the architect, all consulting engineers and sometimes the contractor is selected based on the sketch design it proposes. The assigned team has to produce the pre- and final drawings, included structure, building services, all energy efficiency aspects and, if demanded, an evaluation according to LEED, BREEAM or any other rating systems. If the contractor is part of the team, the assigned team also has to construct and decommission the building. Otherwise, a contractor is chosen based on a price to quality evaluation.

1.7 References and literature

[1.1] VROM (1991). Teksteditie van het besluit 680 (Text edition of the decree 680). Bouwbesluit, Den Haag (in Dutch).

[1.2] Rijksgebouwendienst (1995). Werken met prestatiecontracten bij vastgoedontwikkeling, Handboek (Using performance based contracts for real estate development, handbook). VROM publicatie 8839/138, 88 p. (in Dutch).

[1.3] Stichting Bouwresearch (1995). Het prestatiebeginsel, begrippen en contracten (The performance concept, notions and contracts). Rapport 348, 26 p. (in Dutch).

[1.4] Australian Building Codes Board News (1995). Performance BCA, 14 p.

[1.5] CERF (1996). Assessing Global Research Needs. CERF Report #96-5016 A.

[1.6] Lstiburek, J., Bomberg, M. (1996). The Performance Linkage Approach to the Environmental Control of Buildings. Part 1, Journal of Thermal Insulation and Building envelopes, Vol. 19, Jan. 1996, pp. 224–278.

[1.7] Lstiburek, J., Bomberg, M. (1996). The Performance Linkage Approach to the Environmental Control of Buildings. Part 2, Journal of Thermal Insulation and Building envelopes, Vol. 19, April 1996, pp. 386–402.

[1.8] Hens, H. (1996). The performance concept, a way to innovation in construction. Proceedings of the 3^rd CIB-ASTM-ISO-RILEM Conference ‘Applications of the Performance Concept in Building’, Tel Aviv, December 9–12, p. 5-1 to 5-12.

[1.9] Hendriks, L., Hens, H. (2000). Building Envelopes in a Hollistic Perspective. Final report IEA-Annex 32, IBEPA, Task A, ACCO, Leuven, 101 p. + add.

[1.10] ANSI/ASHRAE/USGBC/IES (2009). Standard 189.1 for the design of high-performance green buildings except low-rise residential buildings.

[1.11] ANSI/ASHRAE/USGBC/IES (2010). 189.1 User’s manual.

[1.12] Hens, H. (2010). Applied building physics, boundary conditions, performances, material properties. Wilhelm Ernst und Sohn (a John Wiley Company), Berlin.

2

Materials

2.1 In general

The second chapter first reviews materials used for thermal insulation. It then considers vapour barriers, also called vapour control layers, and air barriers, more generally known as air control layers. The last part examines joints between building components.

2.2 Array of material properties

Each knowledge field evaluates materials according to their properties. The storage and transport of heat, moisture and air in and across materials is also quantified that way, with density ρ and porosity Ψ – the weight per unit volume of material and the volume taken in by the pores in a unit volume of material – as basic characteristics. Even the consequence of heat, air and moisture presence is described using properties, with some combinations of properties mirroring unique physical features, see Table 2.1.

Table 2.1. Array of thermal, hygric and air-related material properties.

2.3 Thermal insulation materials

2.3.1 Introduction

Thermal insulation materials were developed in order to minimize heat transport. That requires reducing thermal conductivity (λ) to the utmost, an objective that could not be reached without knowing how heat is transferred across a porous material.

2.3.2 Apparent thermal conductivity

2.3.2.1 In general

The property ‘thermal conductivity’ stands for the ratio between the vector ‘heat flow rate’ somewhere in a material and the vector ‘temperature gradient’ there. In isotropic materials, that ratio is a scalar whereas in anisotropic materials it is a tensor with a value along the x-, y- and z-axis: λ_x, λ_y and λ_z. For those materials, Fourier’s second law becomes:

(2.1)

But this definition does not apply for a highly porous insulation material. Their apparent thermal conductivity is described as the heat passing a 1 m³ large cube with adiabatic lateral surfaces per unit time for 1 K temperature difference between top and bottom face. That condition is met per m² in an infinitely vast, 1 meter thick layer with 1 °C difference between both isothermal faces. Measurement of the apparent thermal conductivity is based on that description. A material sample of thickness d meter is mounted between a warm and a cold plate. Once steady state is reached, the temperature difference (Δθ) over and heat flow (Φ) across the central part of the sample is logged. When the test apparatus is wrapped adiabatically and the central area A is small compared to the sample area, heat flow develops perpendicularly to thickness and the apparent thermal conductivity becomes:

(2.2)

2.3.2.2 Impact of the transport modes

Heat flow across a dry porous material combines four transport modes (Figure 2.1): (1) conduction along the matrix, (2) conduction in the pore gas, (3) convection in that gas and (4) radiation in all pores between the pore walls. If humid, two additional modes intervene: (5) conduction in the adsorbed water and (6) latent heat transfer.

Apparent thermal conductivity as measured is not a fixed material property but a characteristic whose value depends on factors directly linked to these transport modes.

(1) + (2) Conduction along the matrix and in the pore gas (λ_c)

If only these two intervened, the equivalent thermal conductivity should be:

(2.3)

Figure 2.1. Heat transfer in a porous material.

where Ψ is...

Table of contents

• Cover
• Title page
• Copyright page
• Dedication
• Preface
• 0 Introduction
• 1: Performances
• 2: Materials
• 3: Excavations and building pit
• 4: Foundations
• 5: Building parts on and below grade
• 6: Structural options
• 7: Floors
• 8: Outer wall requirements
• 9: Massive outer walls
• 10: Cavity walls
• 11: Panelized massive outer walls