Design Steel Structures Heavy Load

8/17/2019 Design Steel Structures Heavy Load

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Design for Steel Structure of Heavy Load and Large

Span Roof

SHUANGLING WEN HAIJUN ZHANG SHENGHUA ZHANG

(Central Research Institute of Building and Construction, Beijing, PRC, 100088)

XONG YING

(Jiuyuan Construction Design Ltd. Co., Beijing, PRC, 100031)

Shuangling Wen is a Professor of Civil Engineering at Central Research

Institute of Building and Construction, PRC. She has undertaken designing work

and fabricating management on steel structure for over 15 years. She has designed

several large-spans steel structures for example Xi’an International Exhibition

Center, Century Hall of Renmin University of China and so on. Design and

Calculation of Steel Structures has being used widely .She is one of authors on the

book. She received Master degree of Engineering from Xi'an University of

Architecture & Technology in 1989. She is Class A Registered Structural Engineer from 1999.

ABSTRACT

A roof garden is a round shape with diameter of 42.2m and with the total loads of 20kN/m2. After

comprehensive comparison, decision was made to use steel cross-grid beam structure of large span which the

section was H shape being welded. The paper has introduced the comparison of steel structure proposals, support

and assembling joints structures, which could be as a reference to designers.

The roof area of roof garden of a science and technology center is 1,400 m2 (see photo 1:bird view of roof

garden), in the center is a square shape swimming pool surrounded by flowers, grass, bushes, and also stone

tables and seats are arranged for people to rest, recreation and barbecue. The space under the roof is a large

conference center for international seminars, equipped with simultaneous interpretation room. The structure span

Shuangling Wen


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of roof garden is 36.8m, the normal value of permanent loads is

20kN/m2, cross-grid beam structure with solid H section beam is

used in the roof design. See Fig. 1.

STRUCTURE PROPOSAL

Construction Requirement

Since the center part of the roof garden is a swimming pool, so, the

roof is designed as the forms of flat at center and slop at its

surroundings. Roof is structurally slopping method. First, the support structure design of roof panel was to begin

the slop at upper surface, and the same thickness insulation layer and waterproof layer was laid on the 150mm

thick reinforced concrete slab, which did not only avoiding the heavy loads caused by normal roof insulation slop

structure, and also meeting the basic requirement of heat insulation, drainage and waterproof. Furthermore, both

the roof structure height and loads bearing capacity had been improved, the allowable clear height under the roof

slab structure is 2.2m.

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Fig. 1, Steel Structure Plane Arrangement Fig.2, Vertical Section Drawing of Steel Beam

Proposals Comparison

The larger space structure under the roof garden was reinforced concrete frame, the reinforced concrete column

position is shown in Fig. 1. The roof is polygon (nearly as a round shape), which is suitable for the arrangement

of space structure system. Considering the facts of heavy loads on roof, larger impact from earthquake, the

requirement of construction functions and clear space, two proposals of steel cross-grid girder structure with

solid-section beam and space truss were compared.

A, Steel cross-grid beam structure

Steel cross-grid beam structure normally has been used for large space concrete structure, but, steel cross-grid

Photo 1, Bird View of Roof Garden


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beam structure with solid-section girder has been hardly used for large span roof. In China, the first steel

cross-grid girder structure with solid-section beam of large span under heavy load was used in the lobby roof

garden of Building B, Beijing Xiyuan Hotel. Its structure span is 28m×28m, the internal distance between beams

is 4m, steel beam cross section is H1800×450×20×30. The preliminary proposal of this project was 36.8m span,

double directions orthogonal, steel cross-grid beam structure with H welded solid-section girder. Calculation

results had shown that if the steel used was grade Q345, height of steel beam was 2.2m, pre-arching measure was

taken for the large span beam during construction, both its structure strength and deformation met the demands

of application.

B, Space network structure

At present, most of the large span space structure still is space network structure, the advantage is lower steel

consumption and easy passing through the pipeline facilities, but, when the span is larger, it takes up more space.

Calculation indicated that when space truss was applied, the height of central line of upper and lower chords

required 3.0m; meanwhile, the joint ball required more than 0.8m in diameter. The roof panel support beams of

space frame upper chord were H300×300×10×6, unfortunately, the space height of steel structure under

concrete panel was as high as 4.7m, which was too large and exceeded the construction allowable space

requirement.

C, Comparison on the two proposals

Table 1 shows the comparison calculations and analysis on these two proposals. These two proposals had

similar steel consumptions. Since steel cross-grid beam structure with solid-section girder could meet the

requirement of construction space and also had the properties of quick and easy construction, so, at last, the steel

cross-grid beam structure with variable cross-section H girder hade been applied (Cross- Section see Fig. 2).

DESIGN AND CALCULATION OF CROSS-GRID BEAM STRUCTURE

Roof garden was supported on reinforced concrete frame, which was un-equilateral octagon, top 800mm

concrete ring beam was supported on 12 reinforced concrete columns, the plane position of columns was shown

in Fig. 1.

Loads

Considering comprehensively the function requirement of roof garden and garden design proposal, the loads

selection and effective combination were done according to “Load Code for the Design Structure” (GBJ 9-87)


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and “Code for Seismic Design of Building” (GBJ 11-89). Standard value of roof loads were:

A, Standard value of permanent loads: 20kN/m2, including the weight of concrete slab, covering soil of roof,

pebbles, swimming pool, ventilation ducts, water supply and heating pipes and structures.

B, Standard value of active loads: 2.0 kN/m2;

C, Snow loads: 0.5 kN/m2;

There were three kinds of loads combination had been considered in this project:

A, 1.2 permanent loads + 1.4 active loads;

B, 1.2 permanent loads + 1.4 active loads for half roof span and

C, 1.2 permanent loads + 1.3 vertical seismic loads

Table 1, Comparison on Two Structural Proposals

Proposal

PropertyCross-grid beam structure Space truss

SteelBeam: Q345 B.F.

Truss: Q235 B.F.

Member: Q345 B.F.

Roof Support Beam: Q235 B.F.

Height of main loads support structure 2400 mm300mm (central line distance

between upper and lower chords)

Max. cross-section member H2400×500×30×18Member: φ325×16

Ball: φ1200×60

Max. height of space taken by structure

(support beam under roof panel included)2400mm 4650mm

Slab support beam No. H300X300X10X6

Vertical deformation 133mm 135mm

Steel consumption 200kg/m2 192kg/m

2

Structure space applicationSmall holes can made,

cannot directly use

Pipes can pass through the space

between members

Construction requirementHeight requirement of

2.4m can be met

Height requirement of 2.6m can

not be met

Construction Schedule Site erection is quick Site erection is slow

Structure Arrangement

Steel structure plane arrangement for roof garden is shown in Fig. 1. Maximum structure span was 36.8mm;

internal distances between beams were 4.5m or 4.9m. Steel beams inside the square area of 19.2m ×19.2m in the

center part of roof (shadow area in Fig. 1) are equal cross-section girders. The height of roof beam middle parts

was 2.4m; height of support base was 2.2m, which meeting the requirement of 2.2m clear height of construction.

Beams were become variable cross-section members, and rest of the beams were equal cross-section; its cross


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section was symmetrical along the longitudinal. Fig. 2 shows the half beam cross-section.

Structure Calculation

First to presume the calculation model:

A, Steel beam support base was ideal splice connection, turning around the main axis of steel beam

cross-section and no lateral displacement outside the plane;

B, Main load bearing steel beams of roof structure were all double direction continuous beams, the

connection between main load bearing steel beams and the end of two ring beams outside (LL) was hinge joint.

SAP93 finite element program was used in the calculation. After the space calculation model was established,

based on the Grade Q345B steel and the different loads to calculate out the internal force and deformation of

structures. According to the result analysis, the vertical seismic combination (1.2 permanent loads + 1.3 vertical

seismic loads) played the key role, which showed that for heavy load roof structure, the working condition of

vertical seismic effect should be considered carefully (since the γRE value was 1.0 in this case). After repeatedly

calculation and optimization, the largest steel beam cross-section at last was H(2200 – 2400) ×500×18×30 (see

Fig. 2). Under the permanent loads effect, the maximum deflection of steel beam was 118.4mm; under the basic

combination effect controlled by permanent load effect, the maximum deflection of steel beam was 130.1mm.

Considering the center part of garden proposal was a 6.4m×6.4m square and 600mm deep swimming pool,

actual loads could not reach the calculation value, and also the maximum vertical deformation of beams could

not reach the calculation value, the design arching value could be less than the deflection brought by permanent

load, the arching value finally selected was 100mm.

Structure Connection

A, Support

Support (connection between steel and RC beams)

was important parts to guarantee the actual loads

situation to meet the requirement of calculation

model. Since each piece of steel beam on the concrete

structure support joint was single direction splice and

turning around the main axis of steel beam

cross-section, there was no lateral displacement outside the plane. 60mm thick single side arc supporting panel

Fig. 3, Support (Joint A)


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was selected, 4 pieces of M36 anchor bolts were installed, and pressure springs were added on the upper of

anchor bolts to allow the support having little turning and linear displacement, so that to guarantee the supporting

loads property of large span steel beams caused by deflection and temperature stress impact. The joints structure

are shown in Fig. 3

B, Beam-to-beam moment connection

To guarantee the beam to beam rigid connection in the double direction of H shape steel beam structure

proposal, the assembly joints were put at the position far away from the intersection (1500mm or 500 from the

axis intersection) by the design, which making the assembly position as the moment connection joints with beam.

The joints were connected by bolt and weld structure method, that is, welding the wing edge, and bolts

connecting the web plate (see Fig. 4). The high strength bolts connection of web plate benefited the construction,

and also could guarantee erection precision. The fully melted welding on wing edge could guarantee the

continuity of upper and lower edge plates. Beam and web plate assembly structure applied the double plates

connection form. According to precise calculation, high strength bolts could resist part of the bending moment,

so that, the last decision was to use 120 sets of M24 high strength bolts for the maximum bolts on beam and web

plate assembly.

CONCLUSION

Steel Structure members were fabricated precisely based on the design drawings, and during the installation of

high strength bolts, no hole-enlarged phenomenon was found. The erection of 280 tons steel structure took only

20 days, the construction was quick. Excellent quality standard has been rewarded in the project acceptance.

Fig. 4, Welded H Shape Steel Beam Connection Joints (Joint B)

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Design Steel Structures Heavy Load