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Dr Youssef Hammida

Seismic Design Special Moment Frames

Strong Column Weak Beam

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Reinforced concrete special moment frames

are used as part of seismic force-resisting systems in buildings that

are designed to resist earthquakes. Beams, columns, and beam-column joints

in moment frames are proportioned and detailed to resist flexural, axial, and shearing actions that result as a building sways through multiple displacement cycles during strong earthquake ground shaking.

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Special proportioning and detailing requirements result in a frame capable of resisting strong earthquake shaking without significant loss of stiffness or strength.

These moment-resisting frames are called “Special Moment Frames” because of these additional requirements, which improve the seismic resistance in comparison with less stringently detailed Intermediate and Ordinary Moment Frames.

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columns weak beam behavior, shear failures of-strong columnTo ensure M interaction diagram shows -, moment capacity. The Pmust be precluded

.this range of axial loads for an example column

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The important aspects of joint design are ensuring proper bar development and precluding shear failures in the joint. This can be accomplished through proper detailing of hoop reinforcement and bar ho

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انھیار العامود باجھاد القص

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he important aspects of joint design are ensuring proper bar development and precluding shear failures in the join

) اطارات مقاومة للعزومRالعامل(

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(R) جدران قصیة حمالة

جدران قصیة بناء ھبكلي

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تفاعل مشترك اطارات+جدران

الزلزالیة قشدة المناطو اطارات عزمیة

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three types of frames: ordinary, intermediate, and special. Ordinary moment frames have very few requirements in ACI 318

وع الجدار القصي وشدة المنطق الزلزالیةن

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جدرا ن قصیة مسیقة الصنع

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طارات خاصة مقاومة للعزما

العامود القوي والجائز الضعیف

strong column-weak beam design is required for special moment frames For a system with strong columns and weak beams, a mechanism is created when ALL beams on ALL stories yield (much more seismic energy dissipated prior to collapse).

Special Moment Frames

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To ensure that the beams develop plastic hinges before the columns. the sum of the flexural strengths of the columns at a joint must

exceed 120% of the sum of the flexural strengths of the beams. This requirement protects against premature development of a story

mechanism

تحقیقات عقدة اتصال الجائز مع العامود تشكل المفصل اللدن في الجوائز - 1 مجموع عزوم 1.2مجموع عزوم األعمدة = - 2

الحوائز في عقدة التالقي

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متطلبات تشكل المفصل اللدن في الجوائز

to ensure proper hinge development. The hinges must be able to form and then undergo large rotations and load reversals without significant reduction in strength

1 - تحقیق المطاوعة والممطولیة واستمرار عمل العناصر في المنطقة اللدنة من طع الخرسانةاللدن مع دورانات كبیرة وتشقق مق تشكل المفصل - 2

ومقاطع التسلیح تأثیر الحموالت المتناوبة دون الزيادة في مقاومة التصمیم

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تشكل المفصل اللدن وتشتیت وامتصاص جزء من طاقة الزلزال

العمل في المنطقة اللدنة يشتت اللدن واستمرار حیث تشكل المفصل

دون ل زيادة االنتقاالت والتشوھات اللدنةبكبیر من طاقة الزلزال مقا قسم زيادة في مقاومة التصمیم او زيادة في مقطع وتسلیح العناصر

قیمة الطاقة المشتتة من قوة الزلزال تعادل نفس قیمة ان ونقريبا

) Rالتصعید من عامل زيادة المقاومة ( R =(4في حال اطارات عادية لدينا قیمة ( وكمیثال حیث

R = (8وفي حال اطار خاص مقاوم للعزم تصبح (

=2=8/4وعلى المفصل اللدن تشتیت وامتصاص ما قیمنه القص القاعدي يجب ان يشتتھا المفصلاي ضعف قوة

بالنظريةوعلى ھذا سمیت طريقة تطبیق المفصل اللدن اللدن االقتصادية الزلزالیة

حیث نصمم البناء تقريبا على نصف قوة القص القاعدى

ولیس كامل قوة الزلزال المقطع والتسلیح في المنطقة المرنة عيصمم من قوة الزلزال النصف األول و

اي و ال يضاف وتشتیته شكل المفصل اللدن بمقاومتهاآلخر يقوم توالنصف مقابل زيادة الحمولة الزلزالیة زيادة في المقطع او لتسلیح

بیتون الخرسانةالمفصل اللدن معناه تشقق مقطع نشكل يبقى لكن

واعادة تشققات انشائیة كبیرة وضارة قد تجبر السكان للنزوح خارج البناء تدعیمه وتأھیله انشائیا.

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parabolic distribution in the concrete, and some compressive stress in the top steel. Upon spalling, the stress distribution changes, The compression block of the concrete moves lower in the cross section, and the stresses in the compression steel are greatly increased.

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Under reverse load applications, hinge development affects both the top and bottom faces of beams. This leads to bidirectional cracking and spalling of cover on the top and bottom of the beam

beam longitudinal reinforcement requirements per ACI 318. The reinforcement ratio limits insure a tension controlled failure mode

in bending and reduce congestion of reinforcing steel. Continuous bars in the top and bottom are required due to reversal

of seismic motions and variable live load. Splice locations and transverse reinforcement are specified because

lap splices are unreliable and cover concrete will spall.

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At low loads the section is uncracked and an analysis using uncracked

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Shear Design

Seismic induced energy in special moment resisting frames is expected to be dissipated through flexural yielding of members. During inelastic response, however, the members should be protected against premature brittle shear failure. This is ensured by providing sufficient shear capacity to resist seismic design shear forces. Seismic design shear Ve in plastic hinge regions is associated with maximum inelastic moments that can develop at the ends of members when the longitudinal tension reinforcement is in the strain hardening range (assumed to develop 1.25 fy)

Fig. 6-1 Internal forces in a reinforced concrete section at probable moment resistance

Figure 6-1 illustrates the internal forces of a sectM pr for a rectangular section with tension reinforcement can be obtained from Seismic 3.

This design aid provides values for coefficient Kpr, which is used to solve the following equation:ion that develop at probable moment resistance.

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Once Mpr is obtained, the seismic design shear can be computed from the equilibrium of forces shown in Seismic 4.

within the plastic hinge region (length c The contribution of concrete to shear, Vequal to twice the member depth at each end) may be negligibly small upon the

.ion of hinge due to the deterioration of concreteformat

half or more of -to one within the hinging region is equale Therefore, when V the maximum required shear strength, and the factored axial compression

should be ignored c V/ 20, c f’g including earthquake effects is less than A)= 0c completely in design (V

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Strong-Column Weak-Beam Concept

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تخفیض العطالة ھو خاص في حال تشكل مفاصل لدنة uncracked یكون المقطع غیر متشقق في حال اطارات عادیة

0.8 – 0.7ویمكن اعتبار لكل العناصر = في مجال المرونة

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ASCE 7 - 12.2.5.5 outlines requirements where special moment frames extend through below-grade floors, as shown in Figure 4-2. The restraint and stiffness of the below-grade diaphragms and basement walls needs to be considered. In this condition the columns would be modeled as continuous elements downto the footing. Thetypeof rotational restraint at the column base will not have a significant effect on the behavior of the moment frame. Large forces are transferred through the grade level diaphragm to the basement walls, which are generally very stiff relative to the special moment frame

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Design Strong colum Weak Beam

When a building sways during an earthquake, the distribution of damage over height depends on the distribution of lateraldrift. If the building has weak columns, drift tends to concentrate in one or a few stories (Figure 3-1a), and may exceed the drift capacity of the columns. On the other hand, if columns provide a stiff and strong spine over the building height, drift will be more uniformly distributed (Figure 3-1c), and localized damage will be reduced. Additionally, it is important to recognize that the columns in a given story support the weight of the entire building above those columns, whereas the beams only support the gravity loads of the floor of which they form a part; therefore, failure of a column is of greater consequence than failure of a beam.

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Recognizing this behavior, building codes specify that columns be stronger than the beams that frame into them. This strong-column/weak-beam principle is fundamental to achieving safe behavior of frames during strong earthquake ground shaking.

Figure 3-1 - Design of special moment frames aims to avoid the story mechanism (a) and instead achieve either an intermediate mechanism (b) or a beam mechanism (c).

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Seismic Design Aids

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- أكبر من عرض الجائزعرض العامود بالطة السقف قبل صب یوضع تسلیح العامود - وصل القضبان في العامود مابعد منطقة المفصل اللدن

M =0ویفضل الوصل في منتصف العامود حبث

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elevation and plan views of cast-in-place concrete frame

buildings

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Plan views of steel moment-frame buildings

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-sheet Excel -design beams frame تصمیم كمرات اطار مقاوم للعزوم, والزالزل

beams-design.xls

https://usc.academia.edu/

design beams frame - sheet Excel- تصمیم كمرات اطار مقاوم للعزوم, والزالزلReinforced concrete special moment frames are used as part of seismic force-resisting systems in buildings that are designed to resist earthquakes. Beams, columns, and beam-column joints in moment frames are proportio... more abstract.

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Dr youssef Hammida