Alternative design procedure for calculation of compressive resistance Pc for columns

As an alternative procedure to that described in subclause 5.4.1 (g) the compressive resistance Pc of a column may be obtained from where Ag is the gross sectional area of the trial section, and pc is the compressive strength. (a) choose a trial section avoiding slender UB sections and obtain the design strength py from Table 2 according to the thickness of the flanges and grade of steel of the chosen section (b) calculate the slenderness A by dividing the effective length LE obtained as in...

Alternative procedure for calculation of Mbs the buckling resistance moments for columns for simple construction

Calculate Mbs the buckling resistance moment capacity about the major axis from the equation where Sx is the plastic modulus of the section about the major axis, and pb is the bending strength obtained from Table 10 using an equivalent where L is the distance between levels at which both axes are restrained in position, and ry is the radius of gyration about the minor axis.

Angles and hollow sections

Angles and hollow section purlins may be designed in accordance with the empirical method, provided that they comply with the following rules claddings and fixing thereof to be capable of providing lateral restraints to the purlins unfactored loads to be considered, and loading to be basically uniformly distributed imposed load used in design to be not less than 0.75 kN m2 span not to exceed 6.5m, and roof pitch not to excced 30 purlins to be connected at each end by at least two fixings. If...

Angles channels and Tsections

In simple tension members composed of angles, channels, and T-sections any eccentricity may be ignored, and the members may be treated as axially loaded provided that the effective areas, At are taken as follows (a) Single angles connected through one leg only, channels connected through web only, or T-sections connected through flange only where ax net area of connected leg, web or flange a2 gross area of unconnected legs or flanges as illustrated in Fig. 6. T- section connected through flange...

Braced multistorey buildings robustness

Multistorey construction that has been framed in accordance with the recommendations given in clause 3.2.4 and designed in accordance with the rest of the Manual, should produce a robust construction subject to the connections also being designed in accordance with the Manual. However in order to demonstrate that the requirements for robustness are met, the following checks should be carried out 1. Beams, ties and their connections at each column and beam in two orthogonal directions at each...

Bracing

Steelwork Structural Member

Choose the location and form of bracing in accordance with the recommendations in subsection 9.4 and clause 2.2.3. Typical locations are shown on Figs. 10 and 11 for single-storey buildings. Wind loads on the structure should be assessed for the appropriate load combinations and divided into the number of bracing bays resisting the horizontal forces in each direction. * Wall adequatley tied and permanently positioned * Wall adequatley tied and permanently positioned Valley bracing members wall...

Bracing members in compression only

For members in compression only, the compressive resistance Pc should be obtained in accordance with the design procedure in clause 5.4.2. For discontinuous struts the eccentricities arising from the connections may be ignored, and members shown in Table 17 may be designed as axially loaded only. The slenderness should be obtained from Table 17, where the Length L should be taken as the distance between the intersection of centroidal axes or the intersections of the setting out lines of the...

Bracing members in tension only

The tension capacity of a member should be calculated from where Ae the effective area of the member, and py the design strength obtained from Table 2 according to the grade of steel and thickness of the flange. Table 17 Slenderness for discontinuous angle, channel and T-section struts Slenderness ratios see notes 1 and 2 ) vv axis 0-85Lvv rvv but sO-7Lvv rvv +15 aa axis 10Laa raa but 0-7LQa raa +30 bb axis 0-85Lbt) rbb but s 0-7Lbb rbb+30 vv axis 10Lvv rvv but5 0-7Lvv rvv+15 aa axis 10Laa raa...

Case I Columns braced in both directions simple construction

For simple multistorey construction braced in both directions the columns should be designed by applying nominal moments only at the beam-to-column connections. The following conditions should be met (a) columns should be effectively continuous at their splices (b) pattern loading may be ignored (c) all beams framing into the columns are assumed to be fully loaded (d) nominal moments are applied to the columns about the two axis (e) nominal moments may be proportioned between the length above...

Coefficient of linear expansion

The coefficient of linear expansion, a, should be taken as 12 x 10 6 per C. Table 3 Limits to thickness to avoid brittle fracture (sections other than hollow sections) Table 3 Limits to thickness to avoid brittle fracture (sections other than hollow sections) These limiting values do not apply to baseplates designed in accordance with clause 15.1.2 however the values may be increased to 50mm when baseplates transmit moments. These limiting values do not apply to baseplates designed in...

Column selection

Before selecting a trial section it is necessary to note that elements and cross-sections have been classified as plastic, compact, semi-compact or slender in combined compression and bending according to the limiting width thickness ratios stated in Table 7 of BS 5950. In this Manual slender sections are not considered for use in Case I. Slender sections have been identified (for axial compression only) in the blue book. In order to assist the selection of suitable sections as columns for...

Columntocolumn splices

Column splices located should be adjacent to the floor and designed to meet the (a) they should hold the connected members in place (b) the centroidal axis of the splice should coincide with the centroidal axis of the connected members (c) they should provide continuity of stiffness about both axes and should also resist any tension. (d) they should provide the tensile forces to comply with the robustness requirements of Section 7.

Composite beams

The design of composite beams is a lengthy iterative process and is thus ideally suited to computer analysis. For grade 50 steel and slab depths in the range of 110- 140 mm, approximate span overall depths of construction ratios of L 19-L 23 may be used for UB sections and L 22-L 29 for UC sections, where L is the span of the beam. Section 1 of Steel framed multistorey buildings design recommendations for composite floors and beams using steel decks20 contains tables that may be used for the...

Condition I Full lateral restraint provided Design procedure

(a) Calculate the factored load 1.6 x imposed + 1.4 x dead, and then calculate the maximum factored bending moment (M), and the factored shear forces (b) Calculate the second moment of area (7) required to satisfy the deflection limitations described in clause 2.6.2. For simply supported beams where is the second moment of area required in cm4. W is the total unfactored imposed distributed or point load in kN L is the span in metres and C is the deflection coefficient obtained for each loading...

Corrosion protection

Structural steelwork should be protected from corrosion. For different parts of the steelwork in a single-storey building this may be achieved as follows (a) Steelwork integral with external cladding and that which is not readily accessible for inspection and maintenance Concrete encasement, or high-quality corrosion protection (i.e. galvanizing and bituminous paint, etc). IStructE ICE Steelwork manual 59 post-and-pitched truss post-and-lattice girder A protective system commensurate with the...

Design of base plates

Base plates transmitting concentric loads may be designed by the empirical method using the following formulae For 1-, H-, channel, box and RHS sections, the minimum thickness should be not less than In no case should it be less than the thickness of the column flanges For solid or hollow round columns the minimum should be not less than where a is the greater projection of the plate ignoring any oversizing beyond the column b is the lesser projection of the plate ignoring any oversizing beyond...

Design procedure

(a) Calculate the factored load 1.6 x imposed + 1.4 x dead and then calculate the maximum factored bending moments (Mx) and the factored shear forces (Fv). (b) Calculate the second moment of area (I) required to satisfy the deflection limitations described in clause 2.6.2. For simply supported beams, use the method described in clause 4.2 (b). (c) Determine the effective length LB as described in clause 4.3 (c). (d) Choose a trial section and grade of steel and check that the equivalent uniform...

Determination of effective length of columns

For braced multistorey buildings the columns are held in position, so that the effective length Le to be used in design depends on the degree of restraint in direction (i.e. rotational restraint) afforded by the beams attached to the columns at each floor level or the foundations. Fig. 4 illustrates typical joint and foundation restraint conditions. Substantial base provides restraint about both axes Restrained or partially restrained about both axis Substantial base provides restraint about...

Fin plates

Shop welded to column Site welded to beam (a) Choose fin plate size, number and grade 8.8 bolts (b) Calculate force in outermost bolts from reaction and eccentricity moment (c) Check bolt strength yi single shear (reduce permissible shear values by 20 ) (d) Check bearing stress in web and the fin plate (e) Check the shear stress in the plate across the net area after deducting hole areas (g) Check size of weld in shear and bending and choose a fillet weld size to suit double length of weld...

Finalization of design

Before the design of the structure can be finalized, it is necessary to obtain approval of the preliminary drawings from the other members of the design team. The drawings may require further amendment, and it may be necessary to repeat this process until approval is given by all parties. When all the comments have been received, it is then important to marshall all the information received into a logical format ready for use in the final design. This may be carried out in the following...

Fire resistance

In the absence of specific information, choose a fire-resistance period of lh for the superstructure and 2h for ground floor construction over a basement and the basement structure. This may be achieved by choosing one of the alternatives in Table 4. period of fire resistance thickness in mm for intumescent paint (normally up to lh) reinforced concrete casing loadbearing reinforced concrete casing (1 2 4 mix) non-loadbearing Guidelines for the construction of fire resisting structural elements9...

Forms of bracing

Bracing may consist of any of the following triangulated steel members concrete floors or roofs adequately designed and fixed profiled steel decking reinforced concrete walls preferably not less than 180 mm in thickness, masonry walls preferably not less than 150mm in thickness adequately pinned and tied to the steel frames. Precautions should be taken to prevent such walls being removed at a later stage, and temporary bracing provided during erection before such masonry walls are constructed.

General

Connections may be designed on the basis of a realistic assumption of the distribution of internal forces, provided that they are in equilibrium with the externally applied loads. The analysis of the forces on the connections, which can be either elastic or plastic, should be carried out using factored forces and moments, noting the following (a) The centroidal axes of the connected members should meet at a point otherwise the effect of the eccentricity of the connection should be taken into...

Hollow sections Design procedure

The procedure given in subsection 4.2 for condition I (full lateral restraint provided) may be followed provided that A (i.e. LE ry) is within the limits shown below. where D and B are overall depth and breadth of box section, respectively. For a circular hollow section D B 1.

Local capacity check

This should be carried out at the locations of the greatest bending moment and axial load (usually at the ends) by checking that AtPy Mcx Mcy The procedure to be followed is (a) determine the design strength py from Table 2 according to the grade of steel and the flange thickness. (c) calculate the b t ratio for the flange outstand and the d t ratio for the web where b is the width of the flange outstand d is the depth of the web t is the thickness of the element concerned. If the b t ratio...

Multistorey braced structures

Lateral stability in two directions approximately at right-angles to each other should be provided by a system of vertical and horizontal bracing within the structure so that the columns will not be subject to sway moments. Bracing can generally be provided in the walls enclosing the stairs, lifts, service ducts, etc. Additional stiffness can also be provided by bracing within other external or internal walls. The bracing should preferably be distributed throughout the structure so that the...

Other sections

The tension capacity of sections, other than those given in clause 6.4.1, may be found from where, Ae, the effective area is found multiplying the net area at a connection by the factor Ke< which for BS 4360 steels can be taken as Kc 1.2 for grade 43 steel Kt 1.1 for grade 50 steel In no case should the effective area At be taken as greater than the gross area Ag of the section under consideration. Effective area e An< t a, + a. 9 x smaller leg length Solid Samzdia. 6.5 Bracing member in...

Overall buckling check

This should be carried out by checking that where Fc is the compressive force due to axial load Pc is the compression resistance obtained as in clause 5.4.2 Ag is the gross cross-sectional area Mx is the equivalent uniform moment about the major axis Mbx is the buckling resistance moment Mh obtained as in subsection 4 4 or 4 5 as appropriate, but should not be taken as greater than p Z My is the equivalent uniform moment about the minor axis py is the design strength obtained from Table 2 Zy is...

Portal frame ridge

Bolts at levels 1 and 2 resist moment Bolts at level 3 resist shear 28 Portal frame ridge (a) Assume the number and type of bolts required at 1 and 2 (see Fig. 28) to resist the bending moment and locate them to obtain maximum lever arm. (b) Using the distribution of force shown in Fig. 19, calculate the resisting moment. If it is less than the applied moment increase the number or size of bolts. (c) Check the thickness of end plates to resist bending moments caused by the bolt tension provide...

Roof and wall cladding

Although this Manual is concerned with the design of structural steelwork, it is essential at the start of the design to consider the details of the roof and cladding systems to be used, since these have a significant effect on the design of steelwork. The choice of cladding material largely depends on whether the roof is flat or pitched. For the purposes of this Manual, a roof will be considered flat if the roof pitch is less than 6 . It should be noted, however, that roofs with pitches...

Sequence for finalizing design

When all the above checks, design information, data lists and preparation of the preliminary drawings have been carried out, the design of the structure should be finally checked. This should be carried out in the same logical sequence, as in the preceeding sections, e.g. The redesign of any steel members that may be necessary should be carried out as described for each member in the preceeding sections. 9 Single-storey buildings general

Serviceability check deflection

The horizontal deflection at the eaves may be estimated for unfactored loads by obtaining the deflection factor D from Fig. 16 using L h and the angle of the roof slope 0. The estimated horizontal deflection of one side stanchion at the eaves, dE, is then obtained from where h height to eaves in mm L span in mm dr depth of rafter in mm py design strength in N mm2, and gp the load factor on the frame, which may be taken as 1.5 for this check. This horizontal deflection dE should not normally...

Singlestorey portals sizing of rafters and stanchions

The plastic design of a portal with pinned bases is carried out in this Manual by the selection of members from graphs. This method is based on the following assumptions (a) plastic hinges are formed at the bottom of the haunch in the stanchion and near the apex in the rafter, the exact position being determined by the frame geometry (b) the depth of the haunch below the rafter is approximately the same as the depth of the rafter (c) the haunch length is not more than 10 of the span of the...

Snapthrough stability check

This should be carried out for frames of 3 or more spans, as in each internal bay snap-through instability may occur because of the spread of the stanchions and inversion of the rafter. To prevent this the rafter slenderness should be such that 22(4 + (1+M(275) D 0(12-1) ,) (Pyr) where 0r for the symmetrical ridged frame is the rafter slope. For any other roof shape where hx is the height of the apex above the top of the stanchions. No limit need be placed on Lb D when fi < 1 and the other...

Spacing and edge distances

A summary of the requirements is given in Table 19. maximum spacing in unstiffened plate in direction of stress in any environment exposed to corrosion in any direction rolled, machine flame cut or planed edge any end in the direction that the fastener bears In Table 19 t is the thickness of the thinner part d is the nominal bolt diameter D is the hole diameter e is f (215 py)

Stability checks

The following stability checks should be carried out restraint of plastic hinges stability of rafter stability of haunch stability of stanchion. Distance x from column to point of max moment span 90 7-5 6-5 5-5 45 3-5 2-5 10 0 I 8 0 I 7 0 6-0 I 5 0 I 4 0 I 3 0 I 2 0 1-5 10 90 7-5 6-5 5-5 45 3-5 2-5 10 0 I 8 0 I 7 0 6-0 I 5 0 I 4 0 I 3 0 I 2 0 1-5 10 Distance x from column to point of max moment span 18 Distance x from column to point of maximum moment span

Strength and stability limit states

The load combinations and load factors to be used in design for the limit states of strength and stability are shown in Table 1. The factored loads to be used for each load combination should be obtained by multiplying the unfactored loads by the appropriate load factor yf from Table 1. Table 1 Load combinations and load factors yf adverse beneficial adverse beneficial

Strength checks

The strength of ordinary bolts to carry the forces should be checked using the formulae in Table 20 In Table 20 ps is the shear strength obtained from Table 21 pbb is the bearing strength of the bolt obtained from Table 21 pbs is the bearing strength of the ply obtained from Table 22 e is the end distance but not greater than 2d p, is the tension strength of the bolt Tg is the thickness of the grip (mm) Yf is the specified minimum yield strength of the fastener. U is the specified minimum...

Structural form and framing

The method for 'simple construction' as defined in BS 5950 should be used and the following measures adopted (a) provide braced construction by arranging suitable braced bays or cores deployed symmetrically wherever possible to provide stability against lateral forces in two directions approximately at right-angles to each other (b) adopt a simple arrangement of slabs, beams and columns so that loads are carried to the foundations by the shortest and most direct routes using UC sections for the...

Sway and snapthrough stability

Two modes of failure have been identified for portal frames, the first may occur in any frame and is called 'sway stability'. The mode of failure is caused by the change in frame geometry due to applied loading which gives rise to the PA effect, when axial loads on compression members displaced from their normal positions give moments that reduce the frame's capacity. The second mode can take place when the rafter frames of 3 or more bays have their sections reduced because full advantage has...

The next step

Preliminary general arrangement drawings should be prepared when the design of the structural members has been completed, and sent to other members of the design team for comments. It is important to establish the general form and type of connections assumed in the design of the members and to check that they are practicable. Reference should be made to subsection 8.2 and Section 15 as the items described therein also apply to single-storey buildings The details to be shown, checking of...

Column splices ends prepared for contact in bearing

Column Flange

25 Column splice ends prepared for contact in bearing Splices should be designed for full contact bearing to resist the vertical loads. In addition, the following recommendations should be followed a the projection of the flange cover plates beyond the ends of the column members should be equal to the width of the flange of the upper column or 225 mm, whichever is greater b the thickness of the flange cover plates should be half the thickness of the flange of the upper column or 10 mm,...

Butt welds

Throat thickness for full penetration welds should be taken as the thickness of member. For partial penetration welds, it should be taken as the minimum depth of weld penetration, except that it should be taken as the actual depth less 3mm for V- or bevel welds. The depth should not be less than 2 f t, where t is the thickness in mm of the thinner connected part. The design strength should be taken as that of the parent metal, provided that the weld is made with a suitable electrode. Any...

Top and bottom cleats

Cleats Structures

a Choose size of seating cleat angles b Calculate the number of bolts required in shear and bearing on the lower cleat, which is assumed to support the whole of the vertical loading c Alternatively, calculate the weld size to suit maximum length available d Check buckling strength of beam web e Check bearing strength at the root of the beam web f Check bearing strength of angle cleat area of bearing x design strength g Check bearing strength of column due to bolt loads where appropriate....

Web buckling and bearing

Web Buckling Resistance

This check should be carried out when heavy loads or reactions are applied to unstiffened webs, e.g. it applies to beams supported on the bottom flange with the load applied to the top flange to a column supported by a beam to a beam continuous over a column and to web resisting compression forces from haunches in portals. Web buckling and bearing may be checked as described below, the dimensions being shown in Fig. 29. where Z gt , is the length of stiff bearing is as shown on Fig. 29 t is the...

Uncased columns

This Section describes the design of uncased columns for braced multistorey construction which are subject to compression and bending. Two cases are considered Case I columns braced in both directions and subject only to nominal moments Case II columns braced in both directions and subject to applied moments other than nominal moments. For both of these cases an iterative process is used requiring selection and subsequent checking of a trial section. The first step is to determine the effective...

Stability of stanchion

Near the top of the stanchion a restraint should be provided at the location of the plastic hinge, together with a further restraint at a distance lm below the position of the hinge restraint. If the stanchion is restrained on the tension flange as described in subclause 11.7.3 d then the distance to the nearest restraint on the compression flange may be taken as lt as calculated for the stability of the haunch. The stanchion should then be checked in accordance with the overall buckling check...

Portal frame connections Portal frame haunch

Portal Frame

a Assume the number and type of bolts required at 1 and 2 see Fig. 27 to resist the factored bending moment, and locate them to obtain the maximum lever arm. b Using the force distribution shown in Fig. 19, calculate the resistance moment. If this is less than the applied moment increase the number and or size of bolts. c Check the thickness of the end plate required to resist the bending moments caused by the bolt tension. Double-curvature bending of the plates may be assumed since bolts...