ANALYSIS OF THE MAIN FRAME AND DESIGN OF THE CROSS- CROSS-SECTIONS
Annex 9 illustrates the program application, step by step
4.2 Load model
4.2.1 Load groups and load cases
The load cases are set into the following groups:
- dead loads - snow loads - wind loads
The load cases in the snow and wind load groups disclose each other in any load combination.
4.2.1.1 Load cases in dead load group
Self weights of structural members are taken into consideration automatically by the software.
Effect of roof slope is neglected. The loads of wall covering system are neglected.
Dead load of roof covering system - load on surface [kN/m2]
q g.c q tr.ext q tr.int q iso q iso.other q purlin
e 4 q g.c 0.451=
- load for beam [kN/m]
p g.c c q g.c. p g.c 2.706=
Installation loads
- load acting on roof [kN/m2]
q g.i q light q equip q other q g.i 0.45=
- load on beam [kN/m]
p g.i c q g.i. p g.i 2.7=
4.2.1.2 Load cases of snow load group
Unsymmetric snow load cases are neglected.
Accidental snow load (accidental design state) is not examined.
Effect of roof slope is neglected for safe.
- Totally distributed snow load on beam [kN/m]
p s c s. p s 6=
4.2.1.3 Load cases of wind load group
Wind effect is not considerable because of the given geometry of the building.
After discussion of the load system the above five wind load cases are considered:
- external cross wind effect (0 degree): wind sucking on zones F-G-H - external cross wind effect (0 degree): wind pressure on zones F-G-H - external and internal cross wind effects (0 degree)
- external longitudinal wind effect (90 degrees)
- external and internal longitudinal wind effects (90 degrees)
Wind loads are calculated with csds=1, because the height of the building is less than 15 meters.
4.2.1.3.1 Two load cases due to external cross wind effect (0 degree)
Distributed wind pressure on walls [kN/m] p w.e.D c w D.0.10. p w.e.D 1.801= Distributed wind sucking on walls [kN/m] p w.e.E c w E.0.10. p w.e.E= 0.876 Distributed wind sucking on the roof [kN/m]
p w.e.F c w F.0.10. p w.e.F= 3.222 Zones F-G-H (i) wind sucking
p w.e.G c w G.0.10. p w.e.G= 2.479 p w.e.H c w H.0.10. p w.e.H= 1.115 Zones I-J (ii) wind pressure p w.e.FGH c w FGH.0. p w.e.FGH 0.248=
p w.e.I c w I.0.10. p w.e.I= 1.239 p w.e.J c w J.0.10. p w.e.J= 0.991 Examined load area (second main frame) is covered by zones F and G, but for safe effect on zone F is considered on the load area.
Width of the zone F [m] e 0.10.F e 0.10 e 0.10.F 1.804= Width of the zone J [m] e 0.10.J e 0.10 e 0.10.J 1.804= Models of load cases
4.2.1.3.2 External and internal cross wind effects (0 degree)
Internal wind sucking effect reinforces the second case of external cross wind effect, therefore first case is neglected.
Distributed wind pressure on wall [kN/m]
p w.e.D c w D.0.10 w i.0. p w.e.D 2.301= Distributed wind sucking on wall [kN/m]
p w.e.E c w E.0.10 w i.0. p w.e.E= 0.377
Distributed wind sucking and pressure on the roof [kN/m]
Zones F-G-H p w.e.FGH c w FGH.0 w i.0. p w.e.FGH 0.748=
Zones I-J p w.e.I c w I.0.10 w i.0. p w.e.I= 0.739
p w.e.J c w J.0.10 w i.0. p w.e.J= 0.492
4.2.1.3.3 Load case of the longitudinal external wind effect (90 degrees)
On examined load area (second main frame) zone B is dominant for walls, while zone H for roof.
- Distributed wind sucking load on walls [kN/m]
p w.e.DE c w B.90.10. p w.e.DE= 1.983 - Distrubuted wind sucking effect on roof [kN/m]
p w.e.H c w H.90.10. p w.e.H= 1.611
4.2.1.3.4 Load case of longitudinal external and internal wind effects
Internal wind effect is sucking effect which weaknings the external wind sucking, therefore this load case may not be relevant.
4.2.2 Load combinations
Load combinations shown below were generated by the ConSteel software. These load combinations belong to persitent design situation. Dead load in combination 12 may be favorable, therefore the partial factors were modified to 1.00.
5.1.4 Static model
Static model may be generated by software. The generation is based on the structural model and the basic setting. The professional user may change the basic setting and by this way the main properties of the static model can be determined. One of the main properties of a beam-column model is the number of finite elements (FEs). The ConSteel software uses a general beam-column FE which has uniform cross-section. Following a simple method (so called segment method), the program distributes the tapered (or haunch) members into a set of uniform elements. Figure 41 shows the FE model of a haunch member where eccentric uniform FEs are used. The optimal number of FEs depends on the ratio of the length of the haunch and the depth of the cross-section. The model with 4 FEs leads usually acceptable results. The model with 8 FEs may be ‘exact’. Tapered members can be modeled on the same way but at least 8 FEs should be used. The segments may be placed centrically (see Figure 42a) or eccentrically (see Figure 42b). The centric model may be more exact from static point of view. The eccentric model may be easier to build up and it is closer to the world of the CAD/CAM systems but it is less exact from static point of view.
Fig.41 Static model for haunch beam using uniform segments (FEs)
5.2 Analysis 5.2.1 General
During the design process analysis should be executed on the following structural models:
• Conceptual model
• Detailed model
• Final model
Fig.42 Static models for tapered frame:
(a) using centric segments (b) using eccentric segments
Analysis on conceptual model is performed in the tender phase when the architectural design office launches the structural solution. The sharp race in the market requires very fast procedure for preliminary design, therefore instead of classical “hand” made calculations the more accurate computer calculation is used. In this phase it is not needed to deal with all the structural details, and only first order analysis is executed on simplified structural and load models. In this design project the preliminary draw may satisfy the requirements of the conceptual design. The structure was simple, therefore the initial cross-sections were able to determine without any analysis. It is noted that in case of more sophisticated structures the preliminary analysis and design could not have been neglected.
Analysis on detailed model should be performed in the realization phase of the structure, where there are direct economical and criminal consequences of any design mistake. In this phase all the details should be examined which have any effect on the safe of the structure.
The static analysis is normally executed by computer program which is usually based on finite element method. Usually second order theory is applied. It may be assumed that the structural joints are initially rigid or pinned. If any joint of the structure is semi-rigid, the effect of the initial stiffness of the joint should be built into the model. If any column base is pinned, it should be modeled by appropriate external point support. Design of joints follows the analysis and design of the cross-sections, therefore the design procedure is recursive, if there are semi-rigid joints in the structure. In this design project semi-semi-rigid joints are not suggested designing.
(a) centric model (b) eccentric model
The properties of the whole structure (for example displacements and column base reactions, should be determined on the final model. The final 3D structural model should contain all the structural members which have any effect on the complex structural behavior and response.
5.2.2 Main steps of analysis
The structural model is normally analyzed by computer software. Displacements are the primary result of the analysis. The displacements are shown on the 3D window as the deformation of the structural model. The displacements should be considered as fictive structural properties, unless the analysis was executed with the final cross-sections. The computer analysis may be performed in the following main steps: