Workshop Exercise no. 2. Wall type structural systems

Transcription

1 Subject: Building Constructions - Skeleton Frame Structures Lecturer: Dr. Zoltan Hunyadi, Dr. Zsuzsanna Fülöp Assistant: Dániel Bakonyi, MIhály Kanyó Year: 2014/2015 Semester: Fall Classroom /Dates: Tuesdays at 8:15 K393 Workshop Exercise no. 2. Wall type structural systems A summary for foreign students This workshop exercise aims to demonstrate the typical components of masonry wall systems and the steps involved in their selection. Note that some solutions are unique to this system and may not be used elsewhere. 1. Selection of the walled system: The example given is a weekend house with a cellular type floorplan. The need for the variability of the floorplan is negligible. The building has two stories (ground floor plus a first floor, no basement) and it is built on a slight incline with slightly shifted by-levels. The relatively small size of the rooms (spans) and the limited height of the building make the choice of a masonry system as the main vertical loadbearing component possible. The layout of the floorplan is compatible with both a longitudinal and a perpendicular wall direction, however the perpendicular system appears to be more economical, and it contributes to the bracing of the building as well. 2. The selection of the various materials for the structural components In masonry constructions the individual blocks are bound together with mortar (composed of sand with a maximum grain size of < 5 [mm], binder and water). In addition a monolithic RC structures the ring beam is used to reinforce these walls and to provide bend strength (or flexural strength) in their plane, to strengthen the bond within the components of the masonry and to help support the horizontal loads. In case of partially or entirely prefabricated slabs made with individual beams the ring beam is also utilized to create a fixed support for the beams, to distribute their punctual loads (both vertical loads and bending moment) more or less evenly along the length of the wall and to increase the rigidity of the slab in its plain (to better distribute horizontal loads). A short summary of the developmental stages of masonry units, their advantages and disadvantages was introduced during the lectures. As a remainder, the most important aspects of contemporary masonry systems are summarized here: So called hollow bricks (or aerated clay bricks) are the most commonly used today. These products were optimized to reduce their thermal conductivity which is achieved by an intricate system of cavities into the bricks (they are manufactured using an extrusion process) and by mixing various kinds of air pocked producing substances into the clay itself (e.g. saw-dust). This has the result that the strength, the sound insulation and the thermal mass of the finished product is severely reduced. A further measure to decrease their thermal transmittance is to completely eliminate the vertical mortar joint between the individual bricks in the masonry. This is made possible by using a tongue and groove type vertical joint. This poses further problems as the airtightness of the masonry itself (without any plastering) is basically eliminated, which can lead to severe moisture problems (e.g. condensation) and unwanted heat loss if necessary measures to avoid these are not taken (the airtightness must be provided by a dedicated completely continuous plaster layer in the construction). The lack of a vertical mortar joint also means that the masonry won t be able to support large shear forces. An even newer trend in reducing the thermal conductivity of these constructions is to use a very thin (< 5 [mm]) horizontal mortar bed or even just a polyurethane foam adhesive. This

2 makes the masonry stiff and brittle (like glass) and further reduces is capability to support bending forces. For all of these reasons such constructions are only able to support the weight of very small buildings with 1-2, in extreme cases perhaps 3 (usually only with some RC reinforcement) stories, with moderate spans and a favourable floorplan layout, and they must be properly sized. Therefore hollow bricks are usually used for small residential buildings. Lime silica bricks and aerated concrete (e.g. Ytong) bricks are more favourable in terms of their loadbearing capacity, but they usually share the same weaknesses with regards to the tongue and grove joints and the thin horizontal mortar beds. Lime silica bricks are very solid and are especially strong, but their thermal conductivity is as high as that of traditional solid brick walls. As and external construction for heated buildings they can only be used with a properly sized thermal insulation layer. They are mostly utilized for their excellent sound insulation properties and their strength. However they are quite expensive and hard to work with (cutting, drilling, etc.). Aerated concrete blocks come in different types depending their strength and porosity. P2 type blocks are good for up to 2, while P4 types even up to 4 stories (in extreme cases, but only with spans under 5,80 [m] and with less than 35% opening). Loadbearing walls must be at least 30 [cm] thick. Most companies producing masonry units today have developed so called comprehensive building systems with a complete and internally compatible product range from masonry units (bricks or blocks) to half and window reveal units, special sound insulation units, ring beam formwork units, different kinds of lintels and in some cases prefabricated or partially prefabricated slab systems. With masonry systems both horizontal and vertical module coordination is absolutely mandatory. It is usually recommended to stick to only one building system in a single building because different systems usually have a different module dimensions. 2.1 Internal load bearing walls For the building in this example a hollow core clay brick (e.g. Porotherm 30) was chosen. 2.2 External load bearing walls Sticking with a solution with a masonry system we could use a single brick wall, a brick wall with an external thermal insulation composite system or an external insulation with a ventilated cavity and an external cladding. Because the building at hand is only a weekend house we tried to choose the simplest solution possible that still complies with all the building energy regulations: the external wall s heat transfer coefficient mustn t be greater than 0.45 [W/m 2 K]. Considering all the unavoidable thermal bridges a more prudent goal is a 1D U value of 0.35 [W/m 2 K]. In this case our options are: Porotherm 44 (U = 0.34 [W/m 2 K]) or Ytong P (U = 0.32 [W/m 2 K]). Our choice of a Porotherm 30 wall for the internal loadbearing walls already betrayed that we chose the Porotherm system for this building, therefore we ll use a Pth 44 for the external walls. Because of the by-levels and the two floors of the two parts of the building are shifted from each other in the vertical direction, but the horizontal brick layers of the longitudinal walls must run continuously throughout the whole building. Therefore the difference between the shifted by-levels must correspond to vertical module of the masonry system (in case of Porotherm n*25 [cm], with n being a positive whole number). The height of the ring beam must also be equal to the vertical module (M = 25 [cm]). At the line where the by-levels meet the ring beams should be connected by a monolithic RC pillar (both in external and internal walls). It is further recommended to have the ring beams of the different by-levels extend around 1.5 [m] beyond the point where the levels meet, to create a good overlap and therefore a better transmission of horizontal forces. 2.3 Slabs

3 For both a longitudinal and a perpendicular walled system we could choose from the same vide variety of available slab constructions. For flat monolithic RC slab the necessary thickness can be estimated as: L/25 = 575/25 = 23 [cm] for the perpendicular and L/25 = 450/25 = 18 [cm] for the longitudinal system. In terms of prefabricated or partially prefabricated slab we could choose: - 20 [cm] Fert with 6 [cm] RC overlay - 23 [cm] Porotherm with 6 [cm] RC overlay - 24 [cm] E beam with 5 [cm] RC overlay - 22 [cm] PPB slab: 15,4 [cm] inlay with 6,6 [cm] RC overlay - 20 [cm] reinforced Ytong slab panels In all cases the RC overlay has a mesh reinforcement connected to the ring beam. The perpendicular walled solution is apparently the more favourable one: the spans are smaller and the solution of the staircase is easier. It is favourable to stick to the Porotherm system because their slab has the same horizontal module as the masonry. The Porotherm slab s beams are quite vulnerable, they can t be drilled at all, and the inlay blocks are very brittle, therefore only small loads can be hung from the ceiling and only with using special fixtures. The 23 [cm] thickness of the slab itself and a 2 [cm] setting and levelling mortar bed under the beams together corresponds to the same 25 [cm] vertical module as the rest of the system. The balcony slab can be supported in several ways: with RC beams fixed into the longitudinal walls supporting a simple RC slab and a complete thermal insulation envelope against the thermal bridges with a cantilever slab with an appropriate counterweight slab in the interior (leaving out the inlay blocks to create a sufficiently large balancing force) and with either a thermal envelope or a thermal brake against the thermal bridge with either solid brick or RC pillars in the exterior supporting a monolithic RC beam and a simply supported RC slab and a thermal insulation envelope. 2.4 Foundations Linear slabs are used under both load bearing and other walls, connected to a foundation ring beam. Min height 40 cm. Note solutions for pillars and partitions. Simple concrete strip foundations are used under both the longitudinal and the perpendicular loadbearing and external walls. The strip foundation is reinforced by a foundation ring beam, which must be at least 40 [cm] high to provide enough stiffness. The connection of the two by-levels is especially reinforced. The pillars are supported by pad foundations which are connected with foundation beams to the foundation ring beam. A sufficiently thick (12 [cm]) reinforce concrete floor slab provides sufficient support for the smaller partition walls. 2.5 Beams and transoms Possible solutions are: monolithic RC beam unified with the ring beam: height 25 [cm] under the slab (module!) with 12 [cm] external insulation against the thermal bridge 4 x Porotherm S ceramic shell module high (23.8 [cm]) prefabricated lintel beams (4*8 [cm]) with 12 [cm] external thermal insulation against the thermal bridge 3 x Porotherm A10 ceramic shell partially prefabricated lintel beams with a concrete compression zone and 12 [cm] external thermal insulation against the thermal bridge

4 in case external shading is required a wall-thick thermally insulated roller-shutter case is recommended, but the height of such products is at least 30 [cm] (e.g. HELLA, ROKA Lith, ROMA, Baltavári, ) therefore they can only be used with a monolithic RC beam and the height of the windows must be reduced by one vertical module. 2.6 Openings The parapet height must correspond to the vertical module of the masonry, so it must be determined by the multiples of the block unit height. In this building this corresponds to a 4*25 = 100 [cm] distance between the top of the slab and the top of the parapet wall and a final = 88 [cm] final parapet height considering a 12 [cm] thick floor. The height of the opening is also module coordinated: 5*25 = 125 [cm] or 6*25 = 150 [cm] respectively with and without and external shading. The horizontal position of the windows is determined by the shading (if one is present) or should be located according the thermal insulation at the lintels. If the window is located too deep in the wall the external window reveal must be thermally insulated! 2.7 Stairs R.c. is preferred since there is a pantry area under the stair. If the stair is non-linear, the casting is more difficult. Otherwise a triple flight stair is the simples, but takes too much space. Because a pantry is placed underneath it the staircase should be made out of RC. If the steps are not linear, the formwork becomes very complicated. A three flight staircase would require more space but it would be easier to build. 2.8 Roof construction A system of two monopitch roofs is selected to solve the problem of the by-levels, but a flat roof may also be considered or two simple pitched roofs with two separate ridge heights. The incline is relatively low, metal sheeting or similar sheet type coverings must be used. The thermal insulation is placed on the slab. 2.9 Chimneys Used for the hot water and heating combi-heater and for the fireplace. Pre fabricated elements outside of the walls to avoid problematic conflict with the crown beam. Possibly from stainless steel. A chimney is required for both the hating/hot water system and the fireplace. The two main possibilities are: a construction with prefabricated concrete chimney blocks placed in front of the masonry (if it were placed into the wall the ring beam couldn t be continuous and a crack would from on either side), or a lightweight stainless steels chimney system built in front of the wall or in a shaft Floor Insulation is required. Soundproofing is also required, multi-layer floor must be used. The floors lying on the ground must be thermally insulated: a maximum U value of 0.50 [W/m 2 K] is required. This is achievable with at least 7 [cm] of thermal insulation. The Hungarian standard MSZ :2007 (building acoustics) has no direct requirements against weekend houses, but it is recommended that the building should comply with all of the requirements for residential buildings (this greatly increases the value of the building). Even so, for a single detached family home an impact sound insulation is only required for high acoustic quality buildings: the impact sound insulation L nw (as measured in site) must be 55 db. The corresponding value for a Porotherm slab in itself is db, therefore a floating floor is required.

5 Only for general reference! This guide will not replace class attendance. Complete and comprehensive explanation that will be required for passing both midterm and final examinations is given only in lecture and practical classes.