Partially restrained timber-concrete composite beam with reinforced ash glulam

A new quadruple sports hall is currently being built in Sargans. This will replace the existing triple hall from 1981 and will meet today's requirements in terms of operational, structural and energy efficiency. The sports facility is being built as a sustainable timber construction in the interests of regional added value. The finely structured load-bearing structure is particularly interesting. This characterizes the building and leads to spectacular spatial and lighting moods. The main part is the quadruple hall. Attached to it on one side is the structure of the equipment store, on the other a two-storey building, whose rooms serve as changing and gymnastics areas.

Client: Building Department of the Canton of St. Gallen, St. Gallen
Architects: blue architects Rupret Architekten, Zurich
Structural engineer: Walt & Galmarini AG, Zurich
Planning facades and interior fittings: Pirmin Jung, Rain
Timber construction: Blumer-Lehmann, Gossau
Supply of frames and HBV beams: neue Holzbau AG, Lungern

The actual hall consists of 40 filigree timber frames with an axial grid of 1.65 and 1.84 meters. The frames stiffen the building in the transverse direction. In the longitudinal direction, a roof slab made of large-format cross-laminated timber panels stabilizes the frames and transfers the horizontal loads (wind, earthquake) to the respective wall panels. The roof and ceiling beams of the outbuildings are attached to the frame. The roof beams are connected to the frames on both sides with Sherpa connectors. They are made of glulam and are clad with large-format cross laminated timber panels, as in the central wing. The ceiling beams are made of reinforced ash glulam and are connected to the frames as partially clamped wood-concrete composite elements. From a structural point of view, the very delicate frames for a span of 28.8 m and the wood-concrete composite ceiling spanning 10.65 m are particularly interesting.

Frames
The frames span 28.8 m and have a height of approx. 10 m in the middle. Despite the length of the beam of almost 30 m, it is only 140 mm wide but 1440 mm high. This contrasts with the standards, which are only 140/800 mm wide. As a result of the different cross-sections between the transom and the standard, the corner moment is small for the transom dimension but too large for the standard dimension. The frame corner must be relieved. This is achieved by a global pre-tensioning of the frame. The frame is produced in such a way that a positive moment is imposed on it during assembly. This counteracts the high stresses caused by superimposed loads and snow in the frame corner area. Of course, the moments subtracted in the frame corners must be added up unfavorably in the middle of the field.

Such a global prestressing of the frames is only possible if all the parameters of the connections are known very precisely. An extremely rigid connection with very little slip is required. A simple control system makes it easy to check the parameters theoretically defined in the structural analysis on the construction site. The frame width of just 140 mm made it impossible to assemble the frames horizontally and then pull them up. The frames therefore had to be assembled upright using auxiliary scaffolding. The dimensions of all the frames are the same despite the different axis grid (1.65 and 1.84 m). This was only possible by specifically adjusting the timber strength. While all studs were executed in glulam GL36, the transoms could be executed in glulam GL28 for the girder spacing of 1.65 m. At a girder spacing of 1.84 m, both ledgers and standards are in glulam GL36.

Wood-concrete composite floor
The intermediate floor beams are designed as unevenly tensioned 2-span beams with spans of 10.65 m and 4.80 m respectively. They are connected directly to the frames on one side. The upper floor cantilevers by approx. 2.5 m from the ground floor. The suspended ceiling is therefore also subjected to the loads and snow loads of the roof (in addition to its own loads and a live load of 5.00 kN/m2). Despite the unfavorable static system, a timber cross-section of only 140/500 mm was used. The shear above the center bearing, caused by the large span and the additional load from the roof (cantilever), was decisive for the structural safety. The shear load is so great that it was necessary to switch to ash glulam. Its shear strength is approx. 1.5 times greater than that of spruce.

Various measures were also required to maintain the serviceability of the ceiling beams. In addition to being designed as timber-concrete composite beams, the ash beams were reinforced and partially clamped to the frames. Naturally, the girders were elevated in the area of the large spans. The timber-concrete composite was created using reinforcing bars glued crosswise to each other at an angle of 45°. As there is only a beam width of 140 mm, the bars are also arranged at an angle to each other in the transverse direction.

In collaboration with Professor Ernst Gehri, neue Holzbau AG in Lungern has been working for over 10 years on the further development of connections and new products in timber engineering. Thanks to optimized connection technology, ever larger and more sophisticated timber constructions are becoming possible. This requires and enables the use of high-quality timber construction materials.

Glulam made from hardwood
By using boards made of hardwood (strength class T40, ash and beech), beams of class GL48 are possible. The decisive factor when switching from softwood to hardwood is, in addition to the higher bending strength, the significantly higher shear strength and shear stiffness of around 60%. When designing high-strength bending beams, shear problems are more likely to occur (higher bending strength and therefore smaller beam cross-sections, but also smaller shear cross-sections).

Reinforcements with steel
Reinforcements with steel have been used for over 60 years. They are easy to combine, as wood and steel have compatible elastic stress-strain ranges. The aim is usually to achieve higher bending performance and rigidity with a low overall height. In the past, the basic material was glulam. Reinforcement only makes sense (due to the shear problem) in combination with glulam made of hardwood. Tests in Lungern have shown that strengths of up to glulam GL60 are possible with reinforced hardwood glulam.

Wood reinforcement using the example of RSA
In our opinion, wood as a material is not used efficiently enough. For example, the new Holzbau AG in Lungern still produces almost 90% in glulam GL24. Below is a comparison of different cross-sections in relation to the field moment of the RSA beams. Without the composite and in GL24, the glulam beams would have been almost twice as high, so in Sargans they would certainly have been replaced by a steel or concrete structure! Changing the ribs to glulam GL36 with a composite with concrete already results in almost the desired cross-section. Compared with a steel-concrete composite girder or a concrete ribbed slab, it is noticeable that the other materials usually offer the better solutions from a structural point of view. This is because their cross-sectional profile corresponds to an I-beam, not a T-beam as with timber. With simple reinforcement, an I-beam can also be produced from a timber-concrete composite beam, which, in addition to higher strength, also has a massively higher stiffness (approx. 30% in the RSA example).

The beams of the RSA were made of ash in strength class GL40. The ash was necessary due to the high shear stresses. In addition to the higher strength, hardwood also has advantages in the area of connections. The pull-out performance of a GS anchor in ash can be increased by a factor of 1.5 compared to spruce. This means that costs can be saved in the connections, despite somewhat greater processing effort.

In theory, partial restraint is an extremely efficient means of positively influencing a bending moment curve. Using the example of the HBV beam RSA, the field moment could be massively reduced by partial restraint without any negative effects from the connection moment at the stem. Hinged or fully clamped connections are common. As they are easier to implement, they are preferable to partially clamped connections.

Connections with GSA technology
GSA technology (connection system with bonded threaded rods) enables extremely efficient, ductile connections. Over time, our in-house development has resulted in high-performance standard parts that are used by neue Holzbau AG whenever possible. All our standard parts have been tested using test specimens and optimized where necessary. As a result, we know the behavior of our parts very precisely, even in extreme load situations.

In buildings such as the RSA Sargans sports hall, where many components are manufactured in the highest strength classes and redistributions of the bending moments (part clamping, frame pretensioning) are also required for the desired cross-section, the connection technology is crucial in addition to appropriate quality management. If the connection stiffness and the slip of the connection are not known, the theoretically determined forces definitely do not correspond to the existing forces. This in turn leads to problems, particularly in the connections (usually the weak point of a timber construction despite all further developments).

As much of the development work in the field of connections is carried out by timber companies and they represent their connections exclusively on the market, the engineer has insufficient access to such technologies until the contract is awarded.

Ceiling-stem connection
The clamping must be applied at a clearly defined construction stage (in the case of the RSA suspended ceiling, only the live load is clamped). For the connection, this means many different construction phases with completely different loads. When unrestrained, the connection must function as a joint. Both the loads and the respective rotations must be absorbed. The construction stages vary from a beam with multiple supports to a beam with a span of 10.65 m and a load of 5.25 kN/m2 (due to HBV concrete floor and anhydrite subfloor). The connection part consists of a lower joint and an upper tension connection.

The joint is a modified n'H standard part. In addition to the GSA anchors (for transferring vertical and horizontal loads), the timber reinforcement is also connected to this part. The GSA-G consists of two half shells that are connected to each other via two conical rings. GSA anchors can be connected at different angles in the respective half shells as required. During assembly, the prefabricated parts can be connected very easily with just one screw.

Next to the joint is the tension connection at the top. This component also consists of two parts. One part is connected to the concrete using angled threaded rods. The other part is attached to the girder stem with GSA anchors. The main problem is the connection of the two parts. This is because the elements may only be bolted together after the subfloor has been laid, which means that all construction stages must function without obstructing the joint. In addition, the entire connecting part is cast into the concrete and must not be visible at the end. The screw connection is only accessible from the front side. A simple screw connection of two slabs therefore suddenly becomes complex due to the offset of 800 mm (stem height). Every effort must be made to prevent concrete from penetrating into the area of the screw connection.

The RSA Sargans shows how clever rearrangements can be used to create statically optimized and extremely filigree load-bearing structures. Naturally, high-quality, quality-tested materials (glulam GL36; glulam in hardwood GL40/48) are essential for this. However, the most important thing is the connections. In addition to ductile behavior with high performance, precise knowledge of the connection is important.