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Design Optimization of Steel Structures from Conventional Steel Building to Pre-Engineered Building by Varying Loads
Saman Shahid*1, Shahid Ali2, Fazil Hussain2
1Department of Sciences & Humanities, National University of Computer & Emerging Sciences (NUCES), Foundation for Advancement of Science & Technology (FAST), Lahore Pakistan
2Department of Civil Engineering, National University of Computer & Emerging Sciences (NUCES), Foundation for Advancement of Science & Technology (FAST), Lahore Pakistan
AR T I C LE I N F O AB S T R AC T
*Corresponding Author:
[email protected] DOI:
10.24081/nijesr.2018.2.0002
Pre-engineered buildings’ (PEB) quality design, pre-fabrications and fast erection, it is now being replaced by conventional steel buildings (CSB). In this study, 16 different 2D Frames were selected for each pre-engineered building and conventional steel building. By varying the tributary width and wind speed, the frames were analyzed by a software of structural analysis i.e., STAAD pro (V8i). A comparison was conducted depending upon base reactions, moments at eave, horizontal defection at eave, vertical deflection at ridge and steel take off. A building with 25 m width, 100 m length and 10 m eave height was selected and AISI-ASD (American Iron &
Steel Institute-Allowable Stress Design) and MBMA-2006 (Metal Building Manufacturers Association-2006) protocols were adopted as design code and for load application respectively.
The results showed that PEBs gives low base reactions, horizontal deflection at eave, and steel take off as compared to the CSBs. The study proved that with increase in loading, the percentage of saving in steel increases in pre-engineered building as compared to conventional steel building. Hence, the performance and cost effectiveness of pre-Engineered building was much improved under heavy loading as compared to the conventional ones.
Keywords:
Pre-Engineered Buildings (PEB), Conventional Steel Buildings (CSB), Base Reactions, Bay Spacing, Moment at Eave, Steel Take off
I. INTRODUCTION
The advantages of steel as a construction material are universally accepted, and the concept of pre-engineering building is a relatively new concept as compared to conventional steel building (CSB). The advantage of pre- engineered buildings over conventional steel buildings is in debate currently. Pre-engineered building (PEB) refers to those steel buildings which are pre-fabricated before being transported to the project site. As the name indicates, it includes the pre-engineering of all structural components of the building taking into account the architectural and design requirements [1, 2]. The paper presented a comparison between pre-engineered building (PEB) and conventional steel building (CSB) design. In this study, 16 different 2D Frames were selected for each pre-engineered building and conventional steel building.
By varying the tributary width and wind speed, the frames were analyzed by a software of structural analysis i.e., STAAD pro (V8i). The design concept of PEB is to use only the required depth of member that is needed at that particular spot depending upon the bending moment. This results in the tapered sections throughout the span of the building. The tapered shape is obtained by the built up
members. Standard hot-rolled sections, cold-formed sections, corrugated sheets, etc. are also used along with the tapered sections, as described in different studies [2, 3]. The use of tapered sections results in reducing the cost of the building by cutting off unnecessary steel.
Conventional steel buildings (CSB) consist of a truss system supported by steel columns. The selection of a truss type depends on the span and pitch of the roof.
Generally, fink-truss is used for a large pitch, pratt-truss is used for medium pitch and howe-truss is used for smaller pitch. Lighting in steel buildings can be provided through skylights or wall lights and for more lighting, a north truss roof can be used [1]. The selection of the truss depends on the following, i.e., roof slope, transportation, fabrication, geometry of the building, climatic conditions. Trusses normally used standard hot rolled section connected together using gusset plates [1, 4].
The pre-engineered buildings (PEB) have been observed to be the most efficient economical and advantageous system particularly for the single story system as compared to convention construction systems. Steel is the basic material that offers low cost, flexible in design, ductile and adaptable in different conditions and recyclable. Steel comes in variety of different shapes and
104 colors, which makes it the most versatile and reliable construction material available. This means that we can achieve rapid installation of the structure with minimum energy, thus making a PEB sustainable. Infinitely recyclable, steel is a material that reflects the imperatives of the sustainable development. Steel is more common in the construction of single story industrial structures rather than in tall buildings because of economy and serviceability problems. The pre-engineered buildings (PEB) consist of main moment resisting frames connecting laterally secondary frames to the resist lateral forces. Secondary framing consist of purlins girts, eave struts, sag rods, flange braces and diagonal bracing. The purpose of secondary framing is to transfer the exterior loads to the main frame and eventually to the foundations.
Bracing are important component of PEB buildings, because they provide lateral stability to the buildings by transferring longitudinal wind pressure to the column bases. The majority of structures that made in steel are generally low rise structured and normally used as cold storage in ware house, steel plant, automobile industries, garages and large thermal power stations. Ordinary steel structures typically require large clear span which are not economically achievable using other constructions techniques [5]. In construction industry, long span and column-free structures are very essential and pre- engineered building have fulfilled these requirements through its diverse design related to pre-fabrication and pre-casting [6]. There are many advantages in using PEBs such as, flexibility of expansion, reduced cost, less construction time, large clear spans, best quality control, less maintenance, energy efficient wall and roof systems, architectural diversity, [7], good strength, corrosive resistance, no residual oils, reduced energy loads etc. [6].
II. MATERIALS & METHODS
A building having dimension 25x100x10 m was selected and analyzed for both type of systems i.e. PEB & CSB (Fig. 1). In this study, 16 different 2D Frames were selected for each pre-engineered building and conventional steel building. The software used was STAAD pro, which is universally accepted for such uses and purposes of the structural analysis program. Pinned supports were considered for both of the buildings. The Dead, Live, Wind-load were in according with MBMA- 2006 (Metal Building Manufacturers Association-2006) and Seismic load were in accordance with UBC-1997 (Uniform Building Code-1997). AISI-ASD (American Iron & Steel Institute-Allowable Stress Design) and MBMA-2006 (Metal Building Manufacturers Association-2006) protocols were adopted as design code and for load application respectively.
Following load combinations were taken: Dead + Live;
Dead + Live + Wind/Seismic and Dead + Wind/Seismic.
Figure 1. Pre-engineered building (PEB) versus Conventional steel building (CSB)
III. RESULTS
Different parameters were selected depending upon the structural configuration of both types of frames. The parameters included were: base reactions, moments at eave, horizontal displacement at eave, vertical displacement at ridge and steel take off.
A. Base Reactions Based on Bay Spacing
Both of the structures were analyzed for different parameters as mentioned above. The below graphs (Fig.
2) showed that the support reaction in PEB was on average 9% lesser as compared to CSB system. Lesser supports reaction means lighter foundations and hence reduction in the cost of footings.
Figure 2. Base Reactions at 9.1m Bay Spacing
B. Base Reactions Based on Wind Speed
The following graph (Fig. 3) shows the output of the support reactions based on the wind speed. The same trend has been observed in this study. The PEB support reaction were 8% lesser compare to CSB.
105 Figure 3. Base Reactions at 175 KPH Wind Speed
The vertical displacements at eave were also studied and plotted in a graphical form as shown in the graph below.
Vertical deflection was the important parameter to study.
Below graphs show that defection at ridge in PEB was more as compared to CSB frames. In CSB, the truss member was closely connected that made it more stable against vertical deflection at ridge. Deflection at mid span in both frames was low as compared to ridge.
C. Moments at Eave
The bending moments of both the PEB and CSB are summarized in the graph (Fig. 4) as shown. It has been observed that bending forces in PEB were lesser as compared to CSB that put an impact on the weight of required material.
Figure 4. Moment at Eave with 175 KPH Wind speed The graph depicted the difference in bending moment’s values at eave for PEB and CSB. On average the bending moments values in PEB were 57% greater compared to CSB. The steel in PEB was provided based on the bending moments along with the sections that made the PEB economical.
D. Horizontal Displacement at Eave
It has been observed that the horizontal deflection at eave in PEB was lesser than in CSB by 64%. At lower wind speed, with an increase in loading, the percentage difference in horizontal deflection increases. Significant difference in horizontal deflection made the PEB frame
more serviceable and safer with respect to the design point of view (see Fig. 5).
Figure 5. Horizontal Deflection at Eave with 175 KPH Wind Speed
E. Vertical Displacement at Ridge
Below graph shows (Fig. 6) that the vertical deflection at ridge in PEB at 175 KPH wind speed was 52% lesser with respect to CSB. On average, a vertical deflection in PEB was 49% lesser as compared to CSB. The deflection results show that the PEB frame was lighter in weight as compared to CSB.
Figure 6. Lateral Vertical Deflection at Eave with 175 KPH Wind Speed
F. Steel Take off by Varying Wind Speed
Fig. 7 shows that at wind speed 130 KPH, the steel reductions/saving in PEB was 16.7 %, at 145 KPH it was 26.3 %, at 160 KPH it was 34.2% and at 175 KPH it was 40.6 % as compared to CSB. On average (by weight), the steel saving was 29.8%, which shows a significant optimization of steel structure from CSB concept to PEB concept. The result trend showed that at low wind speed, the percentage difference in weight was low but with increase in wind speed, the percentage difference also increased. It can also be concluded that wind loading had a greater influence and governed the steel structures analysis and design results.
106 Figure 7. Steel take off at bay spacing 9.1 with different wind speeds
G. Steel Take off by Varying Bay Spacing
As shown in Fig. 8, by varying bay spacing, the percentage difference in weight reduction of PEB with respect to CSB remained same. At 7.1m bay spacing, the percentage weight decreased (weight saving) in PEB was 30.4%, at 7.7 m it was 28.0%, at 8.3m it was 30% and at 9.1m, it was almost 30.7%. On average, the saving in PEB was 29.8%, similar to as varying wind speed.
Figure 8. Steel take off at 175 KPH wind speed by varying bay spacing
IV. DISCUSSION
Currently, there is great progress in PEB designs which is more convenient and economical as compared to tedious and time consuming CSB design. The erection of pre- engineered building is considered faster and efficient as it follows the same procedure in every project. Whereas, in CSB design, the erection procedure used to be different in different projects, lengthy and tedious. It has been observed that pre-engineered building is a new concept with versatile and reliable designing properties including a better quality controlled fabrication and erection. This system not only saves the amount of steel used but it also lights in weight and easier to erect as compared to conventional steel building, which require a lot of technically skilled workmanship. The pre-engineered buildings have applications in storage warehouse, auditoriums, metro stations, gas stations, air craft hangers,
pedestrian bridges, parking sheds, show rooms, shelter warehouses, stadium roofs and swimming pool etc. The pre-engineered buildings structures are also designed as portable structures. The design of PEB structures include designing of main frame, column and rafters, purlin and girts, tie rod and sag rods, flange braces and tie braces etc.
[8]. The paper presented a comparison between a pre- engineered building (PEB) and conventional steel building (CSB) design. We selected 16 different 2D Frames for each pre-engineered building and conventional steel building. We analyzed the frames by varying the tributary width and wind speed through a structural analysis software STAAD pro. The results showed that pre-engineered buildings gave low base reactions, horizontal deflection at eave, and steel take off as compared to the conventional steel buildings. Pre- engineered design was found economical and durable as compared to conventional building design. The study proved that with the increase in loading, the percentage of saving in steel increases in pre-engineered building (PEB) as compared to conventional steel building (CSB). Hence, the performance and cost effectiveness of pre-engineered building was much improved under heavy loading as compared to the conventional ones. In current study, the steel takes off for pre-engineered building was 29% lesser as compared to CSB. This percentage increases with the increase in loading. Furthermore, the cost of PEB was much lower as compared to conventional steel buildings based on our analysis. The pre-engineered building gives less steel take off as compared to CSB and reinforced concrete structures more than the infinitely recyclability of steel makes these buildings as symbols of sustainable development. For larger column free spaces, the conventional building is no more economical, therefore now pre-engineered buildings will provide better solution.
With the advent of new software’s capabilities, it is now possible to achieve a clear span up to 90 meters and eave height up to 30 meters using pre-engineered building system. Main advantages of PEBs over CSBs is that we can cutoff the unnecessary steel in low stress areas in primary members which otherwise is not possible in CSB.
However, in small span structures, the CSBs may give more economical design as compared to PEB. Overall, the pre-engineered building system gives much more efficient, economical and reliable solution for large column free spaces [9].
We selected various parameters to compare pre- engineered building and conventional steel building such as base reactions, moments at eave, horizontal displacement at eave, vertical displacement at ridge and steel take off. We found 9% lesser support reaction in PEB as compared to CSB. The less support reactions
107 means foundations and hence reduction in the cost of footings. Regarding support reaction based on wind speed, 8% reduction in support reaction was achieved in PEB as compared to CSB. The deflection at mid span in both frames was low as compared to ridge. We observed that bending forces in PEB were lesser as compared to CSB that impacted the weight of required material. Overall, the bending moments values in PEB were 57% greater compared to CSB. The steel in PEB was provided based on the bending moments along with the sections that made the PEB economical. We observed that the horizontal deflection at eave in PEB was lesser than in CSB by 64%.
At lower wind speed, with an increase in loading, the percentage difference in horizontal deflection increases.
Significant difference in horizontal deflection made the PEB frame more serviceable and safer with respect to the design point of view. The vertical deflection at ridge in PEB at 175 KPH wind speed was 52% lesser with respect to CSB. On average, a vertical deflection in PEB was 49%
lesser as compared to CSB. The deflection results show that the PEB frame was lighter in weight as compared to CSB. On average (by weight), the steel saving was 29.8%, which shows a significant optimization of steel structure from CSB concept to PEB concept. The result trend showed that at low wind speed, the percentage difference in weight was low but with increase in wind speed, the percentage difference also increased. It can also be concluded that wind loading had a greater influence and governed the steel structures analysis and design results.
By varying bay spacing, the percentage difference in weight reduction of PEB with respect to CSB remained same. On average, the saving in PEB was 29.8%, similar to as varying wind speed.
In current study, in pre-engineered building system, the depth of members was calculated based on varying bending moment values. We not only optimized these values for the building, but also reduced the base reactions. A decrease in base reactions results in the reduction of footing sizes and this is not achievable in conventional steel buildings. On an average, a base reaction of PEB were more than 9% lighter than CSB. Our results showed that the bending moments at eave in PEB was about 57% more than CSB, because, the connection at eave was a fully moment connection in case of PEB, while in CSB the connection was pinned. The horizontal defection in PEB was lesser as compared to CSB. This means that PEB frame was more stable as compared to CSB frame. Thus conventional steel building design is more serviceable. The vertical deflection in CSB was less simple, because the members were braced together at regular interval, while in PEB this was not the case. It was observed previously that pre-engineered building
consumes less quantity of steel as compared to conventional steel buildings. This reduction in steel volume decreases the self - weight of the structures which correspond to decrease in footing sizes and amount of steel in footings thus proves to be more economical and efficient. Moreover, pre-engineered building possesses more aesthetical features as compared to conventional steel buildings [10, 6]. A study reported that the use of steel framed composite construction saved more than 55%
of time as compared to pre-cased frame with concrete slabs. This saving in project time eventually contributed to net saving in the project. The cast of steel frame with composite floor was more (23%) higher than the pre- casted frame and pre-casted concrete slab [11].
The concept of pre-engineered buildings in construction industry will enable us to optimize buildings to utilize full capacity of members which was not achievable with conventional steel buildings due to constant moment of inertia. In pre-engineered building designs, we can use small depths in less stress concentration region along with the main frame. Researchers have explored the advantages of PEBs over CSBs by analyzing industrial warehouse structure as per Indian standard IS 8001984, IS 800-2007 and MBMA-1996 [12]. Researchers have worked to finalize the suitable configuration of industrial building structure regarding strength, serviceability, cost- effectiveness. The trusses have been compared to design industrial shed by STAAD.pro software. They analyzed three trusses types i.e., saw-tooth, A-type and portal frame. The saw-tooth truss was found more economical as compared to others. Researchers have also analyzed the PEB frame and compared it with these three trusses and concluded that PEB was more economical [13]. The concept of pre-engineered building has been studied in detail for various construction benefits and applications in different frame types. Pre-engineered building is a combination of welded sections, hot rolled and cold form members, and gives the complete building structure with different insulating materials [14]. A study also recommended the use of engineered mixed construction materials for durable and strong structures which can further reduce steel erosion and chemical attacks [15].
Therefore, pre-engineered building design is considered the best construction system with weather tight, energy efficient, light weight and cast properties [14]. Moreover, the PEB design can easily fulfills the requirements of clients. A recent research has been conducted to compare different models of pre-engineered building spans from 10-50 meters were compared with other different models of CSBs. Both systems were modeled with STAAD pro software and evaluated with live, dead, wind and seismic loads. The researchers concluded that steel is quite
108 efficient in making PEBs and 30% lighter than CSBs [16].
We have found that the steel takes off for pre-engineered building was 29% lesser as compared to CSB.
Furthermore, the cost of PEB was much lower as compared to conventional steel buildings based on our analysis. A light weight PEB design is more durable against earthquakes. A study has concluded that a pre- engineered building design are most economical option, lighter weight and provides more resistance to seismic forces [16]. Another recent study conducted to prove the economic benefits of PEB frames by changing the bay- spacing designed for wind loads as the concept of PEBs is related to the reduction of steel quantity [17]. A study calculated various parameters such as, dynamic loads, wind loads, wind speed & pressure, shear forces, bending moments and calculation of steel in both designs (PEB and CSB) and reported a 37% material saving in pre- engineered buildings as compared to CSBs [18]. Another study concluded that steel structure of PEBs was 23%
lighter than CSBs with reduced steel amount, reduced cost of fabrication material, more strong, durable and low level wastages [6].
V. CONCLUSIONS
The study proved that with the increase in loading, the percentage of saving in steel increases in pre-engineered building (PEB) as compared to conventional steel building (CSB). Hence, the performance and cost effectiveness of pre-engineered building was much improved under heavy loading as compared to the conventional ones. We have found that the steel takes off for pre-engineered building was 29% lesser as compared to CSB. The cost of PEB was much lower as compared to conventional steel buildings. Pre-engineered steel based buildings will be extensively used in construction industries because of their diverse, fast and dynamic designs. Hence, due to the design evaluated the PEB must be preferred over conventional frame designs.
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