Stephan Engelsmann, Stefan Peters, Valerie Spalding, Franz Forstlechner
B 4.1 Formwork plan B 4.2 Reinforcement plan
B 4.3 Three-dimensional planning and representation of reinforcement
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missioned during structural planning because, in this case, an analysis of the structure’s load-bearing behaviour in the event of fire will be required.
The architect prepares the construction docu-ments based on the results of the structural planner’s dimensioning. A structural engineer then draws up formwork plans that define the geometry of the building’s shell and form the basis of the execution of construction work (Fig. B 4.1). These must be checked and approved by the architect. Creating reinforce-ment plans (Fig. B 4.2) and formwork plans is a basic part of structural planning and is done only after the architect approves the form-work plans. Reinforcement plans are extremely complex and precise. Reinforcement is now often shown in three dimensions to prevent construction errors (Fig. B 4.3). The necessary planning periods involved are relevant for scheduling because the formwork plans and reinforcement plans of structural planning form the basis for subsequent construction.
Structural components with very complex geometries, such as doubly curved surfaces, are a particular challenge for concrete con-struction planners and builders because there are almost no tools available for representing and producing reinforcement curved in three dimensions. The preparation of plans of the building’s shell that do not have to be supple-mented by the plans of the architect on the building site, and of element plans in prefabri-cated construction, form additional services of structural design.
A failure to take this sequence into account in scheduling and insufficient coordination of different planning professions can signifi-cantly disrupt planning and construction operations, especially in concrete construc-tion, unfortunately a frequent occurrence in practice. It usually takes a considerable effort to subsequently incorporate even slight changes to the geometry of a struc-tural component once reinforcement plan-ning has begun. This often leads to unnec-essary discussions and can justify addi-tional planning fees. The architect is respon-sible for effective scheduling and timely coordination.
Due to the widespread notion that technical implementation of any design is now com-pletely unrestricted, architects usually prioritise the layout of floor plans and align the vertical support structure to the shape of the floor plans. Fundamental structural planning require-ments, especially the continuity of vertical load transfer, effective horizontal bracing of the building and the special requirements of mono-lithic construction particularly in large build-ings, are not always sufficiently taken into account in the early phases of the building design. It is not only the “logic” of the support structure and flow of forces that suffer from such inadequate planning, which is usually the result of insufficient experience and exper-tise. Inadequate calculations can restrict a structure’s serviceability if there are excessive deflections of the structure, for example. An increased material consumption also impairs the structure’s appearance and economy in many cases. It is generally better to avoid an indirect load transfer and superfluous load diversion unless there are particular reasons to do so. Earthquake-proof construction requires especially close coordination between archi-tect and structural engineers because regular load transfer is extremely important here. Intelli-gent structural engineering results in effective structures and an economic use of materials and complies with the fundamental principles of sustainable planning.
For these reasons, it is essential to regard planning processes as the joint achievement of various planning disciplines. Only a holistic planning process, in which all the technical disciplines work together towards a common goal from the outset, will produce a success-ful result.
It should perhaps be noted here that inspect-ing and approvinspect-ing reinforcement is not a standard service of the structural engineer’s scope of work. Given the vital importance of reinforcement for the load-bearing ability and serviceability of concrete structural compo-nents, it is advisable to commission a technical engineering inspection of the construction of the support structure for compliance with the verified structural documentation as an addi-tional service.
Design fundamentals
European standards, which developed out of efforts to standardise normative building regulations within the European Union, cur-rently encompass ten standards that deal mainly with designing load-bearing struc-tures. Eurocodes have now been incorporated into building codes in most European coun-tries, so the application of Eurocodes has become binding, while earlier national docu-ments are no longer valid.
Standards and design fundamentals
Eurocodes consist of two parts: the general document, which is the same in all European countries, and a national appendix, in which countries supplement the main document by setting national parameters and rules. In Germany, the Eurocodes are published under the title “DIN EN”. It is mainly Eurocodes 0 (DIN EN 1990), 1 (DIN EN 1991), 2 (DIN EN 1992), 4 (DIN EN 1994) and 8 (DIN EN 1998) that are of importance in the dimensioning of structural concrete components. Eurocode 0 prescribes general requirements as to the safety of load-bearing structures, serviceability and durability of structures as well as the required verification procedure. Eurocode 1 defines possible actions on structures and prescribes their size, distribution and duration of effect.
Eurocode 2 contains the material-specific stip-ulations for designing, calculating and dimen-sioning reinforced concrete structures. Euro-code 4 regulates the design, calculation and dimensioning of composite steel and concrete structures. Eurocode 8 deals with structures subjected to seismic action.
As well as European standards, the highly- recommended publications of the German Committee for Reinforced Concrete (Deutscher Ausschuss für Stahlbeton, DAfStb), which con-tain reports on research that deals equally with scientific fundamentals and practical issues, are available in German-speaking countries.
Relevant research results are included in the DAfStb guidelines, which in many cases are also incorporated into building inspection regu-lations. These are established codes of engi-neering practice.
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Action
One significant change from the previous generation of standards is the introduction of a safety concept, with partial safety factors for loads as well as for structural resistance.
The load-bearing ability of structural compo-nents is no longer verified by means of a global load safety factor set after the calcu-lations as an increase factor on the action side or as a reduction coefficient on the struc-tural resistance side. The Eurocodes’ semi-probabilistic safety concept means that the calculations are much more detailed, with uncertainties in the respective steps in the calculations directly and far more precisely taken into consideration with the help of separate partial load safety factors γ for both action and structural resistance. Increasing or reducing the nominal values with the respec-tive partial load safety factors gives you the essential design values. Once loads are identified, taking the partial load safety factors on the action side into account, it must be verified that the design values of the loads are less than that of the structural component’s resistance.
European standards clearly differentiate between the ultimate limit state and the limit state of serviceability in the dimensioning of structural components. The ultimate limit state describes the load-bearing behaviour immedi-ately before a structural component fails and is connected with large deformations and in concrete structures with large cracks, which clearly indicate imminent failure at an early stage. Load safety factors are used to ensure a sufficiently large margin against failure during design.
The limit states of serviceability describe load-bearing behaviour under actual expected imposed loads. Loads as well as stress levels on structures are generally much lower and deformations usually unobtrusive. In the verifi-cation process, the partial load safety factors γ are usually assumed to be 1.0. These verifi-cations in concrete construction ensure that deflections and especially cracks do not impair a structure’s use and durability.
B 4.4 Actions and their changes over time
B 4.5 Categories in accordance with DIN EN 1991-1-1 B 4.6 Imposed loads according to the usage categories
specified in DIN EN 1991-1-1
Actions and combinations of actions
DIN EN 1991 classifies actions on structures according to their probability of occurring into permanent, variable and exceptional (Fig. B 4.4).
Permanent actions “G” change only slightly during a structure’s service life. In multi-storey buildings, these actions include the structure’s self-weight and imposed loads and in build-ings with basements, the permanent earth pressure. The partial load safety factor γG is set at 1.35 because the magnitude and dis-tribution of permanent actions can be very accurately predicted. One particular form of permanent action in concrete construction is prestressing P. Because prestressing is very precisely monitored, the partial load safety factor γP can usually be assumed to be 1.0.
Transient actions Q, such as imposed loads, snow and wind loads and thermal actions, change frequently and /or significantly during a structure’s service life. The partial load safety factor γQ is set at 1.5 because of the greater unpredictability of these factors compared with permanent actions. If several variable loads are imposed simultaneously, the stan-dards allow for a reduction of partial load safety factors. A distinction is made between the main actions, which are crucial in planning and design, and secondary actions. Only the full extent of main actions need be taken into account in designing structural components.
Variable secondary actions can be reduced, depending on the verification, with so-called combination coefficients ψ.
Accidental actions A are events such as pres-sure from an explosion, impact of a colliding vehicle, fire or an earthquake. These can be very severe but are very unlikely to occur. Most partial load safety factors γ on the action and on the resistance side can be assumed to be 1.0.
The essential actions to be considered in designing normal multi-storey buildings are listed in DIN EN 1991-1-1 (self-weight and imposed loads), DIN EN 1991-1-3 (snow loads) and DIN EN 1991-1-4 (wind loads). Imposed loads depend on usage categories in accor-dance with DIN EN 1991-1-1. The categories of use for roofs are A (residential), B (office areas), C (areas where people may congre-gate), D (shopping areas), E (storage areas)
and F and G (traffic and parking areas). A dis-tinction is made in category of use A between roofs, stairs and balconies. For roofs, the cate-gories of use are H (roofs that are not accessi-ble, apart from normal maintenance and repair activities), I (accessible roofs used according to categories of use A to G) and K (accessible roofs with special uses, such as helicopter landing pads) (Figs. B 4.5 and B 4.6). Snow loads depend on the snow load zone, altitude of the site and pitch of the roof. Wind loads are applied perpendicular to building and roof sur-faces, and the position of the building, its form and height, and wind speeds and direction also play a role.
The verification of ultimate limit states takes into account basic load combinations for permanent and non-permanent actions, which are based on partial safety factors and combination coeffi-cients, as well as exceptional design situations.
In practice, this means that many different combinations of actions must be investigated because it is impossible to predict which action will be the most adverse main action without conducting calculations.
In verifying limit states of serviceability, a dis-tinction is made between quasi-permanent load combinations, frequent load combinations and rare load combinations (for which the partial load safety factors γF can be generally assumed to be 1.0, but the combination coef-ficients vary).
Designing concrete structures
Concrete structures are usually made of rein-forced concrete, with plain concrete now only used in secondary structural components.
The concrete in reinforced concrete struc-tures exposed to tensile loads will crack under relatively low loads due to concrete’s low compressive strength. This process is described as a transition from an uncracked state (I) to a cracked state (II). The reinforce-ment laid to absorb tensile loads enables a structural component to still absorb loads, even after cracking, although cracking greatly reduces a component’s stiffness (Fig. B 4.7).
Prestressed concrete structures behave
simi-72
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B 4.6 B 4.5
Usage category
Cate-gory
Specific use Example
A Areas for domestic and residential activities Rooms in residential buildings and houses;
bedrooms and wards in hospitals;
bedrooms in hotels and hostels, kitchens and toilets.
B Office areas
C Areas where people may congregate (with the exception of areas defined under category A, B, and D1)
C1: Areas with tables, etc., e.g. areas in schools, cafés, restaurants, dining halls, reading rooms, receptions.
C2: Areas with fixed seats, e.g. areas in churches, theatres or cinemas, conference rooms, lecture halls, assembly halls, waiting rooms, railway waiting rooms.
C3: Areas without obstacles for moving people, e.g. areas in museums, exhibition rooms, etc. and access areas in public and administration buildings, hotels, hospitals, railway station forecourts.
C4: Areas with possible physical activities, e.g. dance halls, gymnastic rooms, stages.
C5: Areas susceptible to large crowds, e.g. in buildings for public events like con-cert halls, sports halls including stands, terraces and access areas and railway platforms.
D Shopping areas D1: Areas in general retail shops
D2: Areas in department stores Areas for storage and
industrial activities
E1 Areas susceptible to accumulation of goods, including access areas
Areas for storage use including storage of books and other documents.
E2 Industrial use Garages and vehicle
traffic areas (excluding bridges)
F Traffic and parking areas for light vehicles (≤ 30 kN gross vehicle weight and ≤ 8 seats not including driver)
Garages;
parking areas, parking halls
G Traffic and parking areas for medium vehicles (> 30 kN, ≤ 160 kN gross vehicle weight, on 2 axles)
Access routes; delivery zones; zones accessible to fire engines (≤ 160 kN gross vehicle weight)
Categorization of roofs H Roofs not accessible except for normal maintenance and repair.
I Roofs accessible with occupancy according to categories A to D K Roofs accessible for special services, such as helicopter landing areas
Usage category Category q
k [kN/m2] Q
k [kN]
A Roofs
Stairs Balconies
1.5 – 2.0 2.0 – 4.0 2.5 – 4.0
2.0 – 3.0 2.0 – 4.0 2.0 – 3.0
B 2.0 – 3.0 1.5 – 4.5
C C1
C2 C3 C4 C5
2.0 – 3.0 3.0 – 4.0 3.0 – 5.0 4.5 – 5.0 5.0 – 7.5
3.0 – 4.0 2.5 – 7.0 (4.0) 4.0 – 7.0 3.5 – 7.0 3.5 – 4.5
D D1
D2
4.0 – 5.0 4.0 – 5.0
3.5 – 7.0 (4.0) 3.5 – 7.0 Imposed loads on
floors due to storage
E1 7.5 7.0
Imposed loads in garages and vehicle traffic areas
F 1, 3 Gross vehicle weight: ≤ 30 kN qk Qk
G 2, 3 30 kN < Gross vehicle weight
≤ 160 kN 5.0 Qk
1 For usage category F, a figure of between 1.5 und 2.5 kN/m2 can be chosen. The figure of Qk can be chosen in the 10 – 20 kN range.
2 For usage category G, a figure for qk of between 40 and 90 kN/m2 can be chosen.
3 For the areas specified in notes 1 and 2, the figure can be set in the National Appendix, so the underlined figures are recommended.
Imposed loads on roofs of category H
H 1, 2, 3, 4 qk [kN/m2] Qk [kN]
1 For usage category H, a figure for qk of between 0 and 1 kN/m2 can be chosen. The figure of Qk can be chosen in the 0.9 –1.5 kN range. The national appendix can set figures if areas are specified for the figures. The following figures are recommended: qk = 0.4 kN/m2; Qk = 1.0 kN.
2 The figure of qk can be made conditional on the pitch of the roof in the national appendix.
3 qk can be applied to a surface A, which can be specified in a national appendix. A size of 10 m2 is recommended for this surface.
4 Service loads on roofs (especially those in category H) do not have to be applied in combination with snow and /or wind loads.
Imposed loads on roofs of category K for helicopters
Class of helicopter Take-off load Q of helicopter Take-off load Qk Dimension of the loaded area (m x m) HC 1
HC 2
Q ≤ 20 kN 20 kN < Q ≤ 60 kN
Qk = 20 kN Qk = 60 kN
0.2 ≈ 0.2 0.3 ≈ 0.3
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Tension Compression
-d
Cross-section / longitudinal section Elongation Stress
Position of and reinforcement. In column-type structural components, whose slenderness can endanger their stability, imprecision in geometry resulting from the manufacturing process combined with second order effects can cause significant additional bending stress, which can greatly reduce their load-bearing capacity. A verifica-tion of stability, which takes these addiverifica-tional stress loads into account, is therefore vital for slender columns. The load-bearing ability of columns depends on buckling length, cross-sectional area, degree of reinforcement and the quality of the concrete, with a higher com-pressive strength of the concrete resulting in considerably reduced column cross-sections.
Aids, such as design programmes and design charts, are available for verifying the stability of compression members. Reinforced concrete columns are reinforced with longitudinal and shear or stirrup reinforcement.
Designing for bending and bending
The principal stress trajectories of a steel- reinforced concrete structural component under bending stress describe its load-bearing behaviour (Fig. B 4.8). Load-bearing capac-ity in structures subject to bending depends mainly on the structural component’s geometry and level of reinforcement. A distinction is made between the following two forms of failure at the ultimate limit state:
• Steel reinforcement in lightly reinforced struc-tural components can fail before the B 4.7 Moment deflection line of a steel reinforced
con-crete beam
B 4.8 Drawing of the main stress trajectories in a beam in an uncracked state (I)
B 4.9 Designing for bending: elongation, stresses and the position of the resulting forces
B 4.10 Reinforcement of a reinforced concrete beam B 4.11 Strut-and-tie model
a Designing for shear forces b Designing for torsion
larly, although pre-stressing means that the cracked state (II) occurs under much higher loads (see “Prestressed concrete”, p. 80ff).
Verification of the ultimate limit state
Designing structural components is an iterative process, the first step of which is to define the actions impacting components. The dimen-sions of components’ cross sections are then assumed and internal forces and moments cal-culated. Internal forces and moments are almost always calculated in concrete construction based on elasticity theory and the stiffness of structural components in their uncracked state I (first order effects). The calculation of internal forces and moments on the deformed system, taking second order effects into account, are unusual for concrete structural components because this calculation is only useful if the deformations can also be very precisely identi-fied at the same time. Precisely estimating or predicting deformations is, however, very com-plex due to the material’s non-linear behaviour, and only done in rare cases. The internal forces and moments calculated in this way form the basis for the component’s design.
Only the model of the cracked state (II) is used in the verification of the ultimate limit state, which describes a structural component’s behaviour just before failure. The concrete’s tensile strength is not directly taken into account here because it is subject to great variation, and its contribution to the structure’s
d statically effective height
x height of the com-pression zone z internal moment arm εc concrete elongation
εs steel elongation σc concrete stress σs steel stress Fc resulting concrete
compressive force Fs steel tensile force load-bearing capacity is slight. In areas where structural components are subject to tensile stress, the reinforcement required to absorb tensile forces results from the calculations. Rein-forcement must be positioned in the selected cross section, taking construction regulations into account. The minimum thickness of con-crete cover must also be taken into considera-tion in determining reinforcement. Adequate concrete cover is required to ensure the dura-bility and fire resistance of structural compo-nents, and it secures the bond between the reinforcement and the concrete. During de -sign and planning, a structural component’s chosen dimensions are also checked against the stresses and loads that will be imposed on it. In areas where structural components
compressive force Fs steel tensile force load-bearing capacity is slight. In areas where structural components are subject to tensile stress, the reinforcement required to absorb tensile forces results from the calculations. Rein-forcement must be positioned in the selected cross section, taking construction regulations into account. The minimum thickness of con-crete cover must also be taken into considera-tion in determining reinforcement. Adequate concrete cover is required to ensure the dura-bility and fire resistance of structural compo-nents, and it secures the bond between the reinforcement and the concrete. During de -sign and planning, a structural component’s chosen dimensions are also checked against the stresses and loads that will be imposed on it. In areas where structural components