Straw bale building Training for European Professionals: Load bearing by Herbert Gruber

CONTENTS

U3 – LOAD-BEARING STRAW BALE BUILDING

U3 TIME U3 Learning Outcome

PAGE 5

U3 Session 1 : Design & Planning 5 hrs Info presentation 1 : Design Basics Info presentation 2: Drawing Basics Tipps: Pros and Cons of Load-Bearing SBB: 1 2–1 3, Weather protection: 1 4–1 5, Planning: 1 6, Preparatory Works: 1 7, Building Schedule: 1 8–19, Cutting List: 20, Costings: 21

7 8 10

U3 Session 2: Construction & Statics 2 days Info 1 : Construction – Overview and Examples Tipps: Dressing Bales: 26, Notching: 27, Compressing 28–29 Info 2: Statics – Structural Behaviour (Virko Kade) Info 3: Compressing & Settlement Info 4: Legislation Info 5: Characteristics of the Planning Process Info 6: Remarks on Building Physics

23 24 38 42 44 45 46

U3 Session 3: Tools & Practice 1 day 31 Tipps: Bale Needles: 32, Hedge Saws: 33, Persuader: 34, Gap Filler: 35 Info 2: Strings and Knots 36 Credits and Impressum

SHORT VERSION OF STEP HANDBOOK U3 The complete version is exclusively available as part of the STEP training:

http://baubiologie.at/strohballenbau/step-strohballenbau-lehrgang-mit-zertifikat-201 7/

U3 – LOAD-BEARING STRAW BALE BUILDING

LEARNING OUTCOMES

U3 Level 3 (ECVET credit points: 1 5) / Level 4 (12)

Knowledge

Skills

Trainees know … • about the existing national building regulations related to straw bale building. • the specific problems organising a load bearing straw bale building site and their solutions ( weather protection, safety, logistics). • the symbols to be able to understand plans and construction drawings. • the necessity and the techniques to avoid and to close gaps (between bales and between bales and construction). • the reasons and the techniques to fill in straw in the required density and the specific methods of compressing and their application. • the techniques to fix the bales in the construction (corner detail). • the techniques to adapt the straw bales layers to the requirements of load bearing straw bale building. • the pros and cons of the load bearing technique and its requirements related to schedule, planning, budget, and resources. • the details for adjacent building elements (floor and roof plates, ring-beam and openings) according to load-bearing straw bale building. • the details for openings (windows, doors) and for intersections (chimneys, pipes, wires). • the reasons and the techniques to prepare the substrate with a plane and gap free surface.

Trainees can … • control the quality ofexisting constructions in relation to its suitability for load bearing straw bale building. • handle the tools and machines which are used generally on straw bale building sites and for simple wood constructions. • choose appropriate bales for the load bearing techniques, resize, reknot and fix the bales together. • adjust the bales to the length of the construction elements in a suitable assembly. • do basic carpentry to make additional wooden construction aids in order to erect straight walls. • compress the assembled bales in different techniques in an adequate way. • fill gaps and holes with straw in a required compression. • fabricate base and top ring beam and fill it with insulation material and fix it adequately. • calculate building costs related to the straw bale constructions. • prepare all surfaces for the successive crafts people (plaster, cladding, wind- and air tightness) or execute these tasks in mutual agreement. • assemble the supporting wooden structures and the frames for openings.

Competence Trainees can … • organize and attend the building site at all stages and adapt the working process, the use oftools and adequate techniques related to straw bale building (planing, preparation, execution, additional crafts). • co-ordinate and communicate the special needs of load-bearing construction with other professionals. • explain different methods ofload-bearing construction with reference to advantages and disadvantages. • inspect and select good quality bales for load-bearing construction during the whole building process. • control the general quality of the bales during the whole building process.

SESSION S1

U3 – LOAD-BEARING STRAW BALE BUILDING

U3-S1 : Design and Planning Objectives:

Trainees … … have the ability to read and understand architectural plans and construction drawings … know different structural options … know the dis-/advantages of every solution

Knowledge and Skills: -

Making a building schedule Organization and execution of the work within schedule Making a cutting list Calculation of the building costs

Lecture/Talk Explanations Practice

Trainer:

Location:

Classroom

Duration: 5 hours

Equipment:

Projector, Flipchart

Theory

Methods:

Basics of architectural plans and construction drawings Different structural options, characteristics and bale requirements Advantages and disadvantages of different techniques How to prepare a building schedule How to make a cutting list Calculate the building costs (material, labour, etc)

Practice

Study and develop a case-study comparing the results with other trainees

Organisation: Prepare a case study for every group of trainees (2–3 persons), study it in order to be able to compare it with the results of the participants.

Unterlagen:

Info Sheets: I1 Drawing basics Text Sheets: X1 Straw bale and legislation X2 Advantages and disadvantages of loadbearing X3 Characteristics of different options X4 Organisation of building site and schedule X5 Design basics

Evaluation:

Practical Test Final: Multiple Choice Oral Test

U3 – LOAD-BEARING STRAW BALE BUILDING

U3-S1-I1 : Design Basics

SESSION S1 – DESIGN – INFO 1

U3 – LOAD-BEARING STRAW BALE BUILDING

U3-S1-I1 : Design Basics

SESSION S1 – DESIGN – INFO 1

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 – DESIGN – INFO 1

U3-S1-I1 : Drawing Basics Understanding architectural plans Architectural drawings are a way of representing three dimensional shapes in two dimensions. When drawn accurately and to scale they can be used for a variety of purposes; from calculating quantities of materials needed to helping workers to construct the building to the right size and shape with doors and windows in the correct positions.

Plan View: The plan view is the most basic drawing and is a “top-down” view of all or part of the building. Various information is shown in plans. A floor plan is typically used to show layouts of rooms, stairs, doors and windows and can be visualized as what you’d see if you looked down after cutting through the building at about eye level (1 500 mm above finished floor level), with everything above the cut removed. Typically a separate floor plan is included for each floor in the building. Other plans may show specific parts of the building, e.g. sole plate plan, bale plans, wall plate plan.

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 – DESIGN – INFO 1

U3-S1-I1 : Drawing Basics

Elevations: Elevations are views of the front, back and sides of the building. Elevations are used to convey an impression of how the building will look from outside but are also used to show positions and dimensions of doors and windows, roof pitch and sometimes detail such as external finishes etc.

Sections: Sections are used to show additional information. A section is, a bit like a plan view, a slice through the building. Sections are generally a vertical slice through the building and are typically used to show detail on the roof construction, room heights, foundations etc. A “cross-section” is a section through the shorter axis of a building, whereas a “long section” is a section along the longer axis. Sometimes several sections will be needed, especially where different parts of the building are different heights, have different roof construction/pitch etc. Large scale sections are often used to provide construction detail on specific parts of the building.

Conventions and good practice: It is good practice to include scale bars on scaled drawings so that any distortions in scale (when printing, photocopying, converting between digital file types, etc) can be easily identified. The scale bars should be checked before measuring off drawings. Where sections are included they are normally marked on the plans and elevations with a line and arrows. The line shows where the building is “sliced”, the arrows point in the direction of view. Each section is typically allocated a letter, and this letter appears by the arrows at each end of the section line.

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 –TIPPS

001 Advantages of the Load-Bearing Technique

• Load-bearing construction uses less timber than other straw bale building methods, with straw carrying the structural loads. • Less timber means fewer thermal bridges compared to framed constructions. • Load-bearing is a very simple form of construction and for the most part uses only three basic skills: notching bales (around fixing posts for doors and windows), splitting bales and dressing bales. • The structural loading of the bales helps keeping the straw in place.

Load-bearing technique with big bales: • High insulation value (U-value) of the walls (esp. with bales on edge) • Fast montage of the walls (with crane)

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 –TIPPS

002 Disadvantages of the Load-Bearing Technique

• Unlike framed constructions (which can support a roof that works as weather protection), load-bearing is potentially more vulnerable to weather damage, unless some form of temporary roof or cover can be built. • Pre-fabrication off-site is not possible. • Because the straw is part of the buildings structure, repairs of any damage in it are more difficult compared with repairing damaged straw in timber frame constructions. •The lack of a frame means more care must be taken when building and compressing to ensure an even, level wall height. • Compressing the bales requires higher skill levels than placing them. • Load-bearing has no general building control approval in many countries. • Potential moisture issue with dew point between straw and external render. Load-bearing technique with big bales: • Relatively large floor areas necessary for walls • Montage only with crane

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 –TIPPS

003 Weather Protection in Load-Bearing Building Technique

In a wet climate like that of the U.K, protection against adverse weather needs to be factored into the build. On a small build it may simply be a case of covering over the works on a daily basis with temporary protection until the roof has been built and waterproofed. However, normally this approach is far from ideal and can lead to a stressful and vulnerable build process. Alternative solutions include: • Scaffolding with protective sheeting, (including a sheeted roof on “flyover” scaffold trusses). • The permanent roof structure can be built temporarily propped to provide protection. This can be done either by building the roof at height on a temporarily propped wall plate … • … or by building the roof on the floor below, craning it to height and propping in place.

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

SESSION S1 â&#x20AC;&#x201C;TIPPS

004 Alternative Solutions for Weather Protection

In either case, the propped roof should be approximately 1 50mm above its intended final position to allow the insertion of the uncompressed bales. Provision should be made for lowering the propped roof to allow for bale compression (e.g. by using Acrow props). Below: Temporary propping detail with prop screwed into side of wall plate. Note the chock screwed into the side of the prop to help support the wall plate. Whichever approach is taken, secondary bale protection should also be used (e.g. covering the tops of unroofed bale walls with secured tarpaulins, roofing felt etc).

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 –TIPPS

005 Organization of the Building Site: Planning Process

Some processes may take unexpectedly long periods of time and some materials need to be ordered well in advance of when they’re actually required on site. By working out a schedule in advance and identifying any potential “bottlenecks”, work on site is much more likely to run smoothly, minimising unexpected delays. The schedule should include the following items: Design work: This is very open-ended – time will depend on the approach. Designs can be arrived at very quickly or can be the result of years of careful consideration and gradual evolution of ideas. It is worth spending time at the design stage to get the concepts right – it’s much easier to work out ideas on paper (or the digital equivalent) rather than doing and redoing work on site. Planning permission: In some countries it will take at least eight weeks for a decision to be made even in the most straight-forward planning applications. Controversial ones may take much, much longer. Bear in mind that there is no guarantee an application will be approved. Some types of work do not require planning permission (e.g. Permitted Development), although in some cases a notification procedure must still be followed. Check with the Local Planning Authority (LPA) for clarification. In some cases, additional approvals may also be required (Conservation Area or Listed Building consents, ...). Building Regulations Approval: is usually required in addition to planning permission. Approval may take from 5–8 weeks, although sometimes site work is permitted before formal approval is issued.

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 –TIPPS

006 Organization of the Building Site: Preparatory Works

Time should be allowed for works that need to take place before straw work can begin. This could involve: • site clearance • excavation to level • excavation and construction of foundations • ground floor structure, including any services beneath (waste pipes, new gas, water and electricity connections) • sole plate and upright fixing posts • temporary weather protection (propped roof or similar) • security fencing • secure storage on site (tools & equipment, straw bales etc) • access, parking, delivery, covered working area etc.

U3 – LOAD-BEARING STRAW BALE BUILDING

SESSION S1 –TIPPS

007 Organization of the Building Site: Building Schedule

Many natural materials are seasonal and sourcing may need to be done a long time in advance or at a particular time of year. Straw is baled in August so to have bales made a certain way and available in May, it may be necessary to talk to a local farmer the summer before. Coppicing of hazel for stubs and pins stops at the end of March, so sourcing them can be difficult if left until the summer. Even for more “mainstream” materials, there can often be a considerable delay between ordering and arrival on site. Whilst many components are often available “offthe-shelf” or at short notice (e.g. regular softwood timber sizes like 100x50mm for sole plates or OSB sheets), it is good practice to source more unusual components, especially those required to seal the building from the elements (to make it “wind and watertight”) and check how long they will take to arrive so they can be ordered in plenty of time. Such materials would include doors and windows, materials for the roof structure (trusses or cut roof materials), the roof covering and external render or cladding. If a lime render is to be used externally, the build should ideally be scheduled so the lime is applied early enough in the year that it has time to carbonate well before the first frosts.

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

BULDING SCHEDULE

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

CUTTING LIST

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

TEMPLATE

SESSION S2

U3 – LOAD-BEARING STRAW BALE BUILDING

LOAD-BEARING CONSTRUCTION Objectives:

Trainees know… … different structural options for load-bearing construction and their characteristics and bale requirements … how to fix the bales, to stabilize the system and all the different techniques … different options for compressing the bales … details for how to connect with other building elements; foundations, corners, windows, doors, roof, etc. … how to prepare different surfaces for plastering

Competences and Skills: -

Building with load-bearing straw bales Using different compression and fixing techniques Organization and execution of the work within the schedule Executing good connection details Maintaining integrity of the insulation Preparing different surfaces for plastering

Methods:

Practice

Theory

Theory and practice of load-bearing construction Different structural options for load-bearing, their characteristics and bale requirements. Fixing the bales, and different techniques Compressing bales within different systems Details of connections: foundation, corners, windows and doors, roof, etc. Preparing different surfaces for plastering Study and develop a case-study comparing the results with other trainees. Fixing the bales, and different techniques Compressing bales within different systems Preparing different surfaces for plastering

Trainer:

Location:

Workshop or building-site and classroom

Duration: 1 6 hours

Equipment:

Projector Straw Bales Structure of model to work on Tools

Documents:

Info Sheets: I1 Dressing bales I2 Details Text sheets: X1 Load-bearing (Best building practice) Presentation: Examples Load-bearing

Evaluation:

Practical Test Final: Multiple Choice Oral Test

Organisation:

Prepare load-bearing build, prepare structure to demonstrate different construction details. Order bales, prepare tools for measuring, cutting, adjusting bales, fixing, compressing. Order all the material needed for preparing for plastering.

U3 – LOAD-BEARING STRAW BALE BUILDING

LOAD-BEARING CONSTRUCTION

SESSION S2 – INFO 1

U3 – LOAD-BEARING STRAW BALE BUILDING

LOAD-BEARING CONSTRUCTION

SESSION S2 – INFO 1

U3 – LOAD-BEARING STRAW BALE BUILDING

TIPPS S2

008 Preparing and Dressing Bales

Left; bale to be dressed and joint between undressed bales. Right; “dressed” bale and joint between dressed bales. It is good practice to inspect and dress bales before use; checking the bale has been kept dry and is free from seed heads etc. Bales tend to bulge up in the middle of the end face and drop away towards the corners. “Dressing” the bale involves flattening off the ends to allow bales to meet more snugly. Straw can be slid under the strings, moving straw from the high points in the centre to the lower parts at the sides and corners. Dressing bales gives a much better junction between bales, improving thermal performance and minimizing work later (reduces need to stuff gaps etc). Dressing bales can be most easily done by a pair of people working together as shown below. The bale can be held in place by the two people pushing against it from opposite sides with one lower leg each. Both hands should be made like garden claws; palms held flat, fingers splayed slightly apart and angled down and back towards them. Both ends of the bale should be dressed before the bales are laid or worked further (e.g. split, notched etc).

U3 – LOAD-BEARING STRAW BALE BUILDING

TIPPS S2

009 Notching Bales Where a straw bale wall meets a post (for example at window and door openings, at junctions with other materials etc) bales will need to be notched. This improves airtightness and minimizes draughts and also helps make the wall rigid. There are two sizes of notch that may be required. Where two bales will meet at the post, one on each side, each bale should be “half-notched”: the depth of the notch in each bale should be equivalent to half the diameter of the post (for a 100mm / 4” post this would mean a 50mm deep notch). This typically occurs beneath a window. Beside windows and doors, the post will be entirely recessed into the straw with a “full notch” – 100mm deep notch for a 100mm / 4” post. The process is very simple. For a 50mm notch a piece of 100×50mm / 4×2” timber is laid across the centre of the bale where the notch is to go. An old saw is used to cut the straw using the timber as a guide. After the first few strokes to mark the cut, the timber should be removed and the saw angled slightly outwards as it will naturally tend angle inwards, resulting in a “V”-shaped cut if unchecked. Once one side of the cut is done, the wood should be replaced before removing the saw blade so the timber can again be used as a guide for a cut the other side. The depth and size of the cut should be checked using the guide timber. When the notch is the right depth and shape, the timber should sit snugly inside it with the top of the timber flush with the surface of the bale. For a full notch, the process is identical but using a 100×100m / 4×4” guide timber. Care should be taken to avoid the strings either being accidentally cut or slipping into the notch. Notching can be carried out much quicker and more accurately using machines such as alligator or shark saws.

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

TIPPS S2

010 Straw Bale Walls Compression with Trucker Straps

Currently the most widely used method is to squeeze the straw using trucker straps with a ratchet fixing. These are typically passed under the sole plate, around the straw and over the wall plate, as shown in the picture. The straw can then be compressed between the two plates by tightening the straps. Straps are placed on each side of each door and window opening, at each corner and at regular intervals elsewhere. Straps are positioned so that alternate straps have their ratchets on opposite sides of the wall (the straps tend to compress more on the ratchet side of the wall so alternating helps to even out the compressive pressures).

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

TIPPS S2

011 Straw Bale Walls Compression with Car Jacks

This method can only be used where there is a suitably braced, robust surface to set the jacks against. The most common use is to compress bales under or above a window. A temporary 100x100mm beam can be securely fixed between the upright fixing posts as a jacking point. A permanent 100x100mm beam can be rebated into the top of the highest bale below the window (or the soffit of the lowest bale above) and pushed using the jacks to compress the straw. When fully compressed the permanent beam is fixed into place and the temporary one removed.

SESSION S3

U3 – LOAD-BEARING STRAW BALE BUILDING

LOADBEARING TOOLS & PRACTICE Objectives:

Trainees know … … the tools and machines used on the straw bale building site … specific problems of straw bale building site organization … how to solve any technical problem conveniently

Competences and Skills:

- To handle tools and machines which are used in load bearing straw bale construction

Methods:

Trainer:

Location:

Workshop or building-site

Duration: 8 hours

Equipment: Tools

Demonstration explanation practice

Theory

Specific problems of straw bale building site organization

Unterlagen:

Info Sheet: I1 Tools Text Sheet: X1 Building Site

Evaluation:

Practical Test

Practice

Handling the tools, visiting building site

Final: Multiple Choice Oral Test

Organisation: Looking for an appropriate building-site in the surroundings or preparing different tools and material for the demonstration

U3 – LOAD-BEARING STRAW BALE BUILDING

TIPPS S3

LOAD-BEARING TOOLS

01 2 Dividing Straw Bales with Bale Needles

Bale needles may be useful or even indispensable for load-bearing straw bale walls (in this case each new row of bales needs half-bales). In the infill-technique, the bales are compressed more easily with adding flakes between two plates. Bales can also be compressed to a certain density by simply rebinding it with strings, depending on the strength you tear, or with screw clamps. But as bale needles are connected to straw bale building like the halms to the bale, let's look at these tools. Actually a pointed wooden stick with two holes at the top is enough for their function (since there are two new cords needed –- one for the right, one for the left bale part). There are also combined bale needles (which are connected to a bar/handle, which pull all four cords through the bale at the same time (two upper and two lower – instead of the two existing cords). After you pulled your strings through the bale, the cords are knotted to each half-bale. There are special knots, as they are known from sailing. After you rebound your halfs, the original two cords are removed, thereby maintaining the density of the bale. If you took the right strings on the right half bale you can divide your bale (otherwise, if strings are crossed, you have two connected half-bales). So you can definitely say that some straw balers make a science out of a simple thing.

U3 – LOAD-BEARING STRAW BALE BUILDING

TIPPS S3

LOAD-BEARING TOOLS

01 3 Straw Bale Tools: Shaving Surfaces with Hedge Saws

We do not only want to squeeze a few protruding stalks with hedge saws, in most cases we want a plane surface to save plaster and to avoid different drying times, even if bales are slightly differently thick or bulged or not exactly installed plane. Even if there are customers who prefer a more organic-hunched surface, in most cases they mean slight bumps in the (hand-) plaster and not simply covering uneven bales with the plaster. The cutting of the hunches of straw bales is a long-term work, but it really becomes hard work with not really sharp hedge saws. Unfortunately sharpening is often as expensive as a new one. A tip to save energy is to pick hedge saws with a rather shorter sword (about 50–60 cm). The longer the sword, the more pressure we need to push to the bale surface, and the harder it is to shave a round wall or vault inside. In terms of strength: from 450 W, hedge trimmers are robust and strong enough. The sharpness of the knives is much more important. Hedge saws are among the very safe machines (since they cut only between the knives). Nevertheless, you can tweak neatly with hedge saws (a small cut with painful bruising is the result). It can happen when you sit on the scaffolding shaving the walls. More often, if you don't take care, you will cut the cable and cause a short circuit. Therefore, it is best to carry the cable over the shoulder. Or choose a model with battery.

U3 – LOAD-BEARING STRAW BALE BUILDING

LOAD-BEARING TOOLS

01 4 Straw Bale Tools: the Straw Bale Hammer or Persuader

This wooden hammer "persuades" each bale (at least as long as it is not fixed with battens). It is a must on each straw building site and also easy to manufacture with leftover construction wood. Only the handle should be sturdy (typically hardwood). Do not screw the handle to the wooden plates too often (twice should be enough), otherwise it will break more easily at this point. The persuader should be heavy, but it is used with a swing to move the bales back into the row, or to connect the upper bale with the underlying ones well.

TIPPS S3

U3 – LOAD-BEARING STRAW BALE BUILDING

TIPPS S3

LOAD-BEARING TOOLS

01 5 Straw Bale-Tools: Gap Filler in Two Sizes

No one can fill straw bales between posts/rafters (or stack them over one another in the load-bearing straw bale building) without leaving any holes at all. This is due to the fact that bales are often rounded at the edges rather than square; on the other hand, cavities can also develop when filling (densifying) with layers. And finally it happens again and again that, for structural reasons, there is a (diagonal) squared or round wood in the insulation area, such as the planks of an intermediate ceiling. In order to fill these cavities with (loose) straw and prevent convection in the insulation layer as well as thermal bridges, we use selfmade, simple stuffing tools. Of course, you can also stuff with your hands (fingers), but I wouldn’t recommend that for larger areas. After some days you cannot move your fingers any more. Especially if the wall is to be plastered, it is important to stuff the straw in the same density as the bales nearby, so that the weight of the plaster does not pull it out again. For these purpose we need two gap fillers or "stuff sticks", made of strips of 25×25 mm (1 ×1 ") for the small and 25×50 mm for the large one. So we are building a "T", which we screw together once or twice (6–8 cm screw, 5–6 mm hole pre-drill). The smaller handle has a length of 1 5 cm, the larger handle has a length of 25–30 cm, which we bevel at the top like a screwdriver (with a jigsaw or crosscut saw). Finally, we put a small notch in the middle of the "cutting edge", so that the straw to be stuffed is bundled together more effectively. Finally we take sandpaper (or a rasp) and round off all edges a little bit.

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

Cords and Knots

INFO 2

U3 – LOAD-BEARING STRAW BALE BUILDING

STRUCTURAL BEHAVIOUR

Virko Kade: Structural Behaviour Like any other building material, straw bales react in a very specific way to forces working in on single bales or on the whole building. Settlement is as much an issue as is known from traditional wood construction. The ductility of the straw bale results in (besides good thermal and acoustic insulation) an extremely good performance withstanding earth quakes. Tests conducted in the USA showed no failure even exposed to earth quakes of the highest level! Performing our own tests, we put a weight of 35 KN (3.5 metric tons) on a small bale, which did not destroy it. Only seconds after it was almost back to it’s original size, and after five minutes it was exactly like before the test. What loads can a straw bale wall bear without collapsing? The structural engineer Bruce King, who was part of the development of the straw bale building regulations in the USA, recommends not more than 500 kg per meter of straw bale wall as upper limit and calculation base. According to the building laws in the States however, the walls are supposed to bear the loads without being plastered. We put several tons per meter on a plastered sample straw bale wall without facing structural failure. Only the wall plate was forced between the two plastered surfaces. Massive snow loads on some of “my” buildings amounted to an actual load of 1.000 kg per meter wall and more without causing any cracks in the plaster. We can learn from this, that a heavy roof truss with roof tiles doesn’t work, at least with small bales. It makes sense to construct a roof as light as possible and reinforce it with a structural partition wall as “snow insurance”. Furthermore using small bales one-storey buildings are advisable, at least with the usual Alpine snow loads in mind. The Swiss architect Werner Schmidt proved that with big bales even 3.5 storeys and substantially higher snow loads can work.

What height of a straw bale wall is legally possible? In countries with specific straw bale building regulations the condition is being upheld, that the ratio of height and width of the wall should be < 6:1. This means a maximum of eight small bales on top of each other. The actual height of the room can be varied to some extent by adjusting the height of the wooden construction at the base and of the ring beam. This very regulations recommend for small bale constructions, to keep the length of a straight wall without bracing (e.g. with partitions) below six metres. A realistic limit according to my personal experiences!

INFO 3

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

STRUCTURAL BEHAVIOUR

INFO 3

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

STRUCTURAL BEHAVIOUR

INFO 3

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

STRUCTURAL BEHAVIOUR

INFO 3

U3 – LOAD-BEARING STRAW BALE BUILDING

COMPRESSING & SETTLEMENT

Virko Kade: Compressing and Settlement IIn load-bearing straw bale building, the bales are usually installed “lying flat”. Because the orientation of the straw is more or less uniform thanks to the pressing process, the stems are lying on the floor or parallel to it, respectively. In principle, thus the settlement can be bigger compared to an upright installment of the bales (which is common in certain timber frame constructions), but “standing” bales are running the risk of snapping under pressure, which would lead to the wall collapsing. If the bale/stem orientation is lying flat, the stalks are pressed into one another and wedged tight. The higher the pressure, the better the stability. Soon a single bale as part of a wall construction can no longer be moved or put into a better position. Corrections should therefore be done before the compression with straps. Generally speaking, straw bales should already be compressed as much as possible during the “brick”-laying; e.g. you could chose the heaviest person at site and let him walk on the wall crest every two or three layers. Nevertheless: Be careful when fitting bales in openings that are actually too small! A bale that is too long and is nonetheless forced into place with all strength moves other bales to the side, the corners are bulging outwards, the point load on window frames can get very high and so forth. When using small bales, the wall height is already below the theoretical measure of 36 cm × 8 bales only by compressing it with a persons weight. Ideally the compression should be done mostly with straps and ratchets rather than by the weight of the roof truss. Therefore the strap-compressing should be exhausted before mounting the roof truss. The amount of settlement is the result of so many variables, that it can hardly be calculated. With small bales, the difference between the actual and the theoretical measurements can be between 5 and 7 %, with jumbo bales it’s 0 to 0.5 %. The compressing of big bales yields almost no measurable results, it’s importance lies in the enabling of an even settlement. Furthermore the resistance of the bales towards roof loads is increased. And: The straps which remain below the plaster are the only statically effective bracing against wind load.

INFO 4

U3 â&#x20AC;&#x201C; LOAD-BEARING STRAW BALE BUILDING

COMPRESSING & SETTLEMENT

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U3 – LOAD-BEARING STRAW BALE BUILDING

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U3 BUILDING LAW (IN AUSTRIA)

Virko Kade: Building Law: Legislation in Austria

The straw bale building regulations differ very much from country to country. Many countries don’t have any specific regulation at all, others only recommendations. The most precise laws are to be found in the USA, which comes with advantages and disadvantages. E.g. the so-called chicken wire has to be used flat-spread, which can cause problems in wet climates, not to speak of electromagnetic radiation etc. On the plus side the regulations provide some sort of planning security, especially with regard to load-bearing constructions. Consequently the majority of straw bale buildings in the USA are load-bearing in the making, like in several other countries, btw. In Austria there is no general building control approval including a structural calculation. The certified bales are only approved with regard to insulation properties, fire resistance and humidity. When using them for insulation purposes only, it is usually no problem to get a building approval, even insurance companies nowadays have listed straw bales as building material. If you’d like to build with structurally loaded straw bales, however, an individual authorization is required. That means, that the building control can simply give its approval “one house at a time”, ideally without the need of any further proof. It helps the cause to refer to existing buildings as well as to the ecological advantages and the high level of innovation of this building technique. Written documentation (books, brochures), that shows how far the professionalization of straw bale building has come in Europe, can also help the authorities to rethink. But it is still possible that one has to provide a structural report. In this case it’s usually of little help to quote the building laws and structural surveys of other countries; you’ll have to deliver. Which can be quite time- and money-consuming, if you find a structural engineer at all. In the past it was often enough to convince the construction site management (master builder, carpenter) to incur liability for this construction method to satisfy the building control as well. Extensive structural tests aiming for a general building control approval according to the Austrian norms would unfortunately be very expensive and require a huge effort – too big a price considering the public and trade interest, which is not overwhelmingly high.

U3 – LOAD-BEARING STRAW BALE BUILDING

DESIGN

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Characteristics of Planning SBBs

Each planning should take into account the specifics of the wall constructions (with regard to materials used and special features). This is particularly true in straw bale building, since the building material is not familiar to many planners and yet it requires a different way of thinking. First, of course, we should consider the bale dimensions – which are not precise and not following any industrial standard. A small bale has a fairly exact width (between 45 and 50 cm, according to the baler); the height may also vary according to the baler, but the most regular size is 36 cm. But: The bales become smaller by compression, so this height vanishes after the second or third layer. How much settlement you actually have in the end is not exactly calculable, so always calculate a bigger settlement to prevent the top beam from touching or resting on the window sill and endangering the entire statics. Stiff components such as intermediate walls or additional supports should be installed at a fixed height when the setting has been completed. Before that, work with scaffolding, e.g. threaded rods. Window sockets are designed ideally 20–30cm below the top beam and infilled later. For smaller windows that are walled in, calculate the total height of the window frame a few centimeters smaller than the theoretical bale dimension. The length has the greatest variance in straw bales. It can be set as desired in the baler, but still depends on the swath (width and stroke length of the tracks in the field) and the driving method. Even well-made small balls can have a variance on average of up to 10 cm or more. For a load-bearing building I calculate with a length of 100cm (which is regular for proper-density-bale dealers). That said: the length of small bales is not that big an issue, many of them have to be resized anyway, which can easily be done. In the case of big bales, however, it can take 20–30 min per piece (assuming crane/forklift is available). Theoretically, the length of most jumbo bales is 240cm. Although big bales are far more precise than small ones, some are still longer than others, gaps between bales occur … suddenly the wall is 20cm longer! Therefore I calculate in the planning with 245cm length. Filling a gap later is definitely the least effort. Every second row, however, we still need half-bales to build properly bonded. At best you order the halfbales from the straw merchant before harvesting (some more than you need). I order with my desired length minus 5–10cm. In order to be able to work exactly and to facilitate window installation and plaster connections, all wooden components of the wall should be planned level with the straw surface, so both can be plastered in one layer. We should definitely focus on the following points regarding the statics: • no point loads on the top beam • avoid thrust forces on the wall crown (ring beam / top beam) • do not put loads on a blunt end wall, always arrange wooden posts here • use only the same bales for a building, preferably from the same harvest • enough additional bracing by intermediate walls (snow loads) • bearing walls (usually two): as few and as small windows as possible • max. wall length without bracing (for small bales with 50cm width): 6 metres • roof loads should be as small as possible (small bales)

U3 – LOAD-BEARING STRAW BALE BUILDING

BUILDING PHYSICS

Virko Kade: Remarks on Building Physics In central and northern Europe we should take into account a number of points that are less important in other parts of the world: • avoid metal parts in the straw, especially in the outer third (threaded rods, etc.) • avoid thermal bridges to ensure good insulation; furthermore, thermal bridges increase the effect of weak points • never place bales directly on concrete, always use a thermal separation • place the first row of bales always on wooden base plates (at least 5cm) in order to avoid water damage • do not use plasters which contain cement on the outside, only pure lime plasters; use clay plaster inside • interior constructive changeovers with straw walls such as partition walls, ceilings, etc. should always be sealed well, for example with plaster or clay fleece or similar • first bale layer should be above the splash water area (> 30cm)

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U3 – LOAD-BEARING STRAW BALE BUILDING

QUESTIONS ABOUT DETAILS

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Virko Kade: Questions about Details Wall bushings (pipes, cables, etc.) • Electricity and water (interior) • Chimney • Plaster preparations • Weather • Scheduling • Costs

STEP – Straw Bale Training for European Professionals UNIT 3 – Loadbearing Straw Bale Construction (201 7) Editors/Tipps: Herbert Gruber, Helmuth Santler (ASBN) Virko Kade, Michael Howlett; Coworks: BuildStrawProTeam (Erasmus+ Projekt) Design & Photos: Herbert Gruber; Virko Kade, Werner Schmidt; more Photos: Werner Schmidt, provided by Architects. Illustrations/Icons: Michael Howlett (SBUK) This Handbook bases on the Handbook by LeonardoGroup STEP (201 5)