Pathology of the framework, disorders

Pathology concerning the frame

Pathology Sheets

Details sheets

Glued laminated structures: from cracking to failure
Report
Fortunately, the risk of a glulam element breaking is rare. But we must nonetheless be concerned with the cracks observed in certain parts, in order to determine the cause (s) and to assess their possible seriousness.
Must be distinguished :
• checks of drying in part current and in the right of the assemblies;
• delamination;
• cracks, the start of future ruptures.
Diagnostic
Desiccation checks
Glued laminated timber remains a wood-based material sensitive to humidification and drying: placed in a dry environment, the wood will gradually lose its humidity and shrink until it presents "drying checks". These checks can occur either in part current, or in the right of large assemblies blocking the water withdrawal of the woods.
Delaminations
The delaminations are the consequence of a manufacturing defect in the factory, more precisely of a bad bonding between the lamellae constituting an element. The supervision, via the CE marking, of the self-checking procedures of the manufacture has made this phenomenon rare nowadays. Delamination can have several causes:
• poorly prepared coverslips;
• insufficient glue;
• insufficient tightening in terms of intensity or duration;
• uncontrolled temperature during manufacture.
Cracks are a sign of abnormal wood behavior
The mode of work of the element considered must already be able to inform on the possible cause of cracking. In particular, transverse tensile stress has long been an unfavorable element due to the now well-known overestimation of the corresponding admissible stress which existed in the CB71 rules.
This overestimation has since been revised: the definition of the strength classes and the associated admissible stresses are fixed by NF P 21-400 which provides, for transverse traction, a value of 0.15 to 0.30 MPA depending on the class of wood used.
The importance of these cracks and the modalities of their repair can be assessed by reference to the “Recommendations for the repair of structural LC wood with cracks and splits”, issued by the SNCCBLC.
Good practices
Properly design and prepare the framework
Choice of species and treatment of wood adapted to the classes of use.
Exposure to class H3 or H4 requires a preliminary treatment of the coverslips in a vacuum and pressure autoclave before manufacture.
Manufacture of the elements in a hygroscopic state close to the state of equilibrium that they will have in service.
Transport and assembly avoiding moisture absorption.
Joint design allowing wood to retract.
Pay particular attention to the stresses which generate transverse tensile and longitudinal shear forces in the bonding planes.
The essential
Adapt the woods to the intended uses.
Manufacture the elements at a hygroscopic temperature close to the conditions of end use.
To consult
DTU Rules CB 71: Timber frames - Calculation and design rules.
NF EN-14080: Timber structures - Glued laminated timber - Requirements.
NF EN 1194: Wooden structure - Glued laminated timber.
NF P21-400: Structural timber and wood-based products.

Deformation of industrial trusses
Report
The roofing can present visible deformations, with tiles which are no longer aligned: this is the sign of a deformation (deflection, spillage) of the industrialized framework.
This pathology is fortunately becoming rarer, thanks to the hardening of the rules of the art and the progress of the profession: more supervised fixing of trusses on the shell, systematic use of laying plans.
On the other hand, there are disasters in renovation, when the fitting out of the attic involves cutting the small farmhouses without discernment, without specific study of the principles of stability necessarily affected.
Diagnostic
Industrialized wooden frames assembled by metal connectors are used for residential houses, but also for larger buildings such as shops or sheds.
Most of the deformations affecting these trusses come from buckling or tilting of the compressed parts (crossbow beams, diagonals). In fact, the weight of the cover and the ceilings generate tensile or compressive forces in the bars constituting the trusses. However, the pieces of wood used are sometimes thin (36 mm) and yet several meters long. The compression that passes through these parts can cause buckling: a crossbow deforms, it drives its neighbors attached by the battens and it is the entire frame that is affected.
The phenomenon can lead to a collapse of the roof, in particular when the points of the gables are not solid and cannot therefore oppose the thrusts (spillage). The situation can become critical in windy weather.
To counteract these deformations, it is therefore necessary to install anti-buckling and bracing bars, in accordance with the installation plan provided by the manufacturer.
Deformation of wooden frames assembled on site
Report
The deformations of wooden structures are manifested more by their aesthetic nuisance (hollow in the roof, sagging of a ceiling) than by a real impairment of the solidity of the structures. In addition, the massive use of small farms in individual construction makes these disorders rarer.
Diagnostic
The causes may be different depending on the two main categories of structural work:
Flexed elements
Flexed elements
Purlins, rafters, floor joists, ...
Their dimensioning results from compliance with the admissible deformations dictated by the regulations (DTU Rules CB 71).
1/150 for parts of a structure in a console that does not support regular traffic (awning).
1/200 for parts directly supporting roofing elements (rafters, battens).
1/300 for purlins and parts supporting glass elements.
1/400 for flexed structures, other than brackets, supporting regular traffic or infill.
1/500 for the support parts of other load-bearing elements (recovery beam, etc.) and fragile elements determined by the DTU or specific market documents.
Again, we must not forget:
that the abnormal humidity of a wood during its installation is an aggravating factor with regard to future deformations and that any wood element is subject to creep under long-term load;
that a purlin does not work in the same way, whether it is placed vertically (simple bending) or according to the slope of the roof (deflected bending), even though nothing is planned to resume its transverse bending.
Triangulated systems (trusses of lost or convertible attics)
These are systems in which the bars work mainly according to axial forces, but in the design of which it is necessary to take account of the assembly slips necessary for their loading.
Their sizing and the approach to their deformation must take into account the assembly slips which accompany the loading of the assemblers (bolts, points, etc.).
Much rarer, a defect in the preservation of wood can lead to its progressive degradation and, as a consequence, to deformations.
Good practices
Ensure the correct sizing of structural works (and the compatibility of the standard with the chosen sizing reference, Eurocode 5 or CB 71) (diagram 3).
Choose the right wood material. This choice is essential in flexed systems. Structural timber must meet very specific characteristics (knot, grain of the wood, etc.) which allow its resistance to the stresses to which it will be exposed to be estimated.
Apply the appropriate preservative treatment. Depending on the use for which it is intended, it is necessary to determine its exposure class (1 to 5) and to draw the consequences in terms of choice of gasoline and treatment.
Attention to the termite risk must be made in certain regions where the use of wood resistant to termites, or treated accordingly, is an obligation.
Measure the consequences of the installation method. At equal span, certain installation methods can generate greater stresses in the wood (deflected bending).
Properly design and execute assembly devices, the precision of which is essential for the stability of trusses. In addition to the choice of device (type: bolts, crampons, nails, etc.), their protection (galvanizing, painting, etc.), their sizing and their positioning in relation to the edge of the part must be particularly studied according to the forces to be taken up.
The deformations of the trusses (totally triangulated or not) are largely a function of the assembly slips. It is therefore necessary to limit the "clearances" during installation, and in particular to adapt as accurately as possible the holes to receive the bolts.
Pay attention to the humidity of the wood. If we want to protect ourselves against unsightly "checks of desiccation" and deformations greater than those expected, it is essential to use wood whose humidity will be as close as possible to that which it will have in equilibrium.
Maintain the structures regularly and use them under normal conditions, for good durability.
Ensure good anchoring of the framework to the shell.
The essential
Properly size the structures.
Treat the woods.
Pay attention to the design and assembly of the work.
Maintain regularly.
To consult
Decree of June 27, 2006 relating to the application of articles R. 112-2 to R. 112-4 of the Construction and Housing Code (termites).
DTU Rules CB 71: Timber frames.
DTU 31-2: Construction of houses and buildings with wooden frames.
NF EN 335-1, 2 and 3: Durability of wood and wood-based materials - Definition of use classes.
NF EN 351-1 and 2: Durability of wood and wood-based products.
NF B52-001: Rules for the use of wood in construction.
NF P21-400: Structural timber and wood-based products.
Transformation of a frame for attic development
Report
Making lost attics habitable represents a heavy and high-risk intervention.
This work can lead to deformation of the frames, roofs, floors created, partitions on the ground floor, etc.
These disorders are generally serious and can go as far as the collapse of the floors created and the modified structures.
Diagnostic
The reasons for the disorders are ignorance of the new constraints which result from the modification of the original calculation assumptions, as well as various implementation faults.
Reinforcement defects
Supporting elements
The industrial trusses are interconnected by spacers, braces and anti-buckling bars which ensure the stability of the whole.
To make these lost attics habitable, it is necessary to remove all the bars encumbering the central volume and create a load-bearing floor.
Removing the bars in the central volume before reinforcing the trusses will cause their ruin or, failing that, serious deformations generally impossible to resume later. Certain bars stiffen in particular the crossbowman. Before removing them, it is necessary to strengthen the existing crossbowmen at the risk of seeing them bend.
At singular points
Care should also be taken when creating the necessary hoppers for the access stairs to attics, dormers or roof windows. To achieve them, it is necessary to cut the beam or crossbow of an existing farmhouse.
New headers then transfer the loads to the two neighboring farmhouses. These may sag if they are not sufficiently reinforced.
Various defects
Assemblies: threaded rods passing through holes that are too large, nails and screws in insufficient number, too short or badly implanted (too close to the edge), etc. The play thus caused in the assemblies risks causing deformations of structures which are added to the existing deformations of the trusses.
Fastening the longitudinal beams without penetrating the masonry gables is also particularly risky.
New undersized or non-braced joists. The new floor may then sag. This risk also exists if the joists are resting on deformable elements such as a poorly designed or poorly constructed longitudinal beam, for example. Floor sagging can lead to cracking and even collapse of the underlying ceiling, buckling and cracking of lower storey partitions, cracking of attic partitions, etc. Gables made of hollow concrete blocks can also crack by pulling.
Good practices
Design the transformation as if it were a new frame.
The transformation of a frame requires a very good knowledge of structures and frames. It requires checking all the key points by calculation, using a specialized BET integrating the seismic risk in certain regions.
Check the correct execution of each phase before moving on to the next.
The phasing of the works must make it possible to preserve the stability of the structure at all times of the site. For example, do not remove the original braces before putting in place the new ones or, where applicable, temporary bracing.
Use quality wood.
The wood and floor panels used must be dry and protected from moisture absorption during construction. The quality of the wood must guarantee their durability.
Pay close attention to the proper execution of the assemblies.
The assemblies are highly stressed. Their weakness leads to the weakening of the entire frame.
The essential
Systematically resort to a new calculation of resistance and stability by a specialized BET.
Preserve the stability of the structure by controlling each phase of the site.
Pay particular attention to assemblies, which are highly stressed.
To consult
DTU 31.3: Wooden frames assembled by metal connectors or gussets.
DTU Rules CB 71: Timber frames - Calculation and design rules.
DTU Rules N 84: Action of snow on constructions.
NF P21-400: Structural timber and wood-based products.

Details sheets

Defective assemblies of structural parts
Disorder
Significant deformation of the frame and the works it supports (roofing and floor on beams).
Diagnostic
In addition to the sections of the timber which are insufficient, no calculation having been carried out, the assemblies between preserved trusses and added reinforcement parts are defective:
• Loose bolts (nuts can be tightened by hand).
• Lack of wedging between parts which are not in contact.
• No washer between nut and wood.
• Insufficient number of fixing points between existing parts and add-on parts.
Recommendation
The transformation of a frame is a delicate job which requires a serious preliminary study. At the execution stage, take care of the connections between the preserved frame and the added reinforcement parts, so that the forces for which they were calculated are transmitted to them.

Framing attack by excessive condensation
Disorder
Development of mold and fungus on the traditional resinous wood frame and more particularly the purlins in the lost part of the roof next to the extraction box of the collective CMV.
Diagnostic
The ventilation box of the CMV is not connected to the roof outlet made up of a flat sheet metal cap, which causes the hot and humid air extracted from the apartments of the building to be returned to the interior of the attic. . Thus, in cold weather, significant condensation occurs on the underside of uninsulated steel tanks and continuous runoff on the structural purlins.
Recommendation
The VMC box discharges must always be connected in a sealed manner to roof outlets adapted to the dimensions of the discharge duct, and avoiding backflow under the action of external winds.

Wood / wood assembly defect in a wooden frame exposed to bad weather
Disorder
Development of wood-eating fungi at the level of a post / strut assembly exposed to bad weather.
Diagnostic
Cause: The lignivorous fungus develops in the presence of permanent humidity in the area of the assembly.
Origin: drainage defect in the assembly.
Recommendation
A hole in the bottom of the mortise would have allowed the water arriving from the struts to evacuate.
It is also necessary that the exterior paintings (carried out by the customer) have not blocked the discharge opening.

Wood / support assembly defect of a wooden frame exposed to bad weather
Disorder
Rotting of wood in direct contact with the concrete post.
Diagnostic
Cause: Persistent humidity in the area of the wood / concrete assembly leads to rotting of the wood parts.
Origin: assembly fault between the pieces of wood and the concrete post.
Recommendation
The post should be shorter.
A metal fitting must:
-Make the mechanical link between the concrete post and the timber farm,
-Allow ventilation of the underside of the pieces of wood.

Multiple truss assembly fault
Disorder
This type of disorder often has damaging consequences on the performance of the joints with the junction strips of the plasterboards of the suspended ceiling.
Unlike current trusses, doubled or tripled trusses are heavily loaded by:
- jibes supporting the covering of a rump,
- trusses of 5 to 6 m spans perpendicular to that of multiple trusses for L-shaped or Y-shaped houses.
Diagnostic
The assembly of the multiple trusses is faulty.
The most heavily loaded truss flexes and induces tensions in the hangers, rails and cardboard plasterboards of the suspended ceiling.
These tensions are detrimental to the durability of the assembly of the BA13 plates by strips with glued joints.
Recommendation
Marry multiple trusses by implementing one of the following effective assemblies:
- Fixings by notched nails of a length at least equal to 2/3 of the thicknesses of wood to be assembled at the rate of 1 every 10 cm in staggered rows.

Ceiling deformation
Disorder
The plasterboard ceiling, fixed under the beams by means of a secondary metal framework, has 2 lines of cracks on the joints of the plates perpendicular to the trusses. In the living room crossing the ceiling sagged more than 2 cm, and broke on the partition parallel to the facades.
Diagnostic
Only one type of farmhouse was used instead of two. To the right of the awning, the trusses designed to take support at their ends actually rest by the entry, between 2 nodes, on the recessed facade wall.
A strut is therefore missing to transfer the loads from the crossbowman directly to the support. This defect causes uncontrolled bending of the beams in this area, and therefore generalized deformation of the ceiling over the entire ground floor.
Recommendation
It was necessary to design and implement two types of trusses, one for each case of span between supports. It is not only the respect of the rules of calculation but also of simple common sense.

Farmhouses and differential arrows
Disorder
On either side of the return wall of the entrance, micro cracks reach the strips between BA13 plasterboards of the suspended ceiling.
Diagnostic
See the explanation by calculation in the graphic sheet.
Under normal wind and snow loads, the trusses of the frame have an allowable deflection of 1 / 400th of the span.
At the right of the masonry return, the joints between BA13 plasterboard cannot withstand the tensions in the plates due to the differential deflections of the W-shaped trusses on one side and truncated on the masonry return side.
Recommendation
Trusses of different rigidity must be "married":
As a preventive measure, the frame manufacturer provides vertical uprights on the W-shaped trusses in order to create a SAINT ANDRE cross between the trusses and bartered and the others.
In preventive and / or curative, 3 rafters sew the ties and take 2 to 3 trusses on either side of the masonry return.

Buckling of industrial trusses
Disorder
The crossbowmen of industrial farmhouses (span = 18 meters) present a more or less accentuated deformation perpendicular to their plane, leading to localized subsidence on the roof made of fiber cement slabs.
Diagnostic
The disorder is explained by the phenomenon of buckling generated by the compressive force passing through the crossbowmen. The anti-buckling beams under crossbow beams are insufficient, both in number and in size.
The crossbowmen have a thickness of 36 mm, while the DTU 31.3 envisages thicknesses of 47 mm or the twinning of two 35 mm trusses (for spans greater than 15 meters).
Recommendation
Comply with DTU 31.3 of May 1995 (Wooden frames assembled by metal connectors or gussets). Article 4.5 of DTU31.3 (Part 1 - Implementation rules) provides, for example, the minimum sections for anti-buckling beams according to the center distance of the trusses. (Recommended section of 36x72 instead of 30x70 set up in this case)

Frame movement
Disorder
During strong gusts of wind, squeaking noises are perceptible in the attic. The client, worried, reports the appearance of cracks in the ceilings and on the partitions of the attic spaces fitted out during the first year after acceptance of the work.
Diagnostic
All these cracks are due to a deformation of the frame which occurred after the completion of the construction. Indeed, the examination carried out in the lost attic reveals that the bracing and anti-buckling bars were not installed.
As a result, the trusses are deformed by buckling (lateral bending) and tend to overturn against the masonry gables.
Recommendation
All frames must be self-stable without the use of masonry gables. And the woods making up industrial trusses, with small sections, must be stiffened so as not to deform under load.
For this it is essential to comply with NF P 21-203-1 (DTU 31.3) Framework assembled by metal connectors, and the installation plan provided by the manufacturer of the framework. These documents specify the position of all the bars which must bind the trusses together and which are added on site.


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